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                                 UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                            ------------------------
 
                                   FORM 10-K
 

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<C>        <S>
   /X/     ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
           SECURITIES AND EXCHANGE ACT OF 1934 FOR THE FISCAL YEAR
           ENDED JUNE 30, 2001
</Table>

 
                                       OR
 

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<C>        <S>
   / /     TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
           SECURITIES EXCHANGE ACT OF 1934
</Table>

 
                         COMMISSION FILE NUMBER 0-17999
                            ------------------------
 
                                IMMUNOGEN, INC.
             (Exact name of registrant as specified in its charter)
 

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<S>                                              <C>
                 MASSACHUSETTS                                     04-2726691
        (State or other jurisdiction of               (I.R.S. Employer Identification No.)
        incorporation or organization)
</Table>

 
                     128 SIDNEY STREET, CAMBRIDGE, MA 02139
          (Address of principal executive offices, including zip code)
 
                                 (617) 995-2500
              (Registrant's telephone number, including area code)
 
          SECURITIES REGISTERED PURSUANT TO SECTION 12(B) OF THE ACT:
                                      None
 
          SECURITIES REGISTERED PURSUANT TO SECTION 12(G) OF THE ACT:
                          COMMON STOCK, $.01 PAR VALUE
                                (Title of class)
                            ------------------------
 
    Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports,) and (2) has been subject to such
filing requirements for the past 90 days. Yes /X/  No / /
 
    Aggregate market value, based upon the closing sale price of the shares as
reported by the Nasdaq National Market, of voting stock held by non-affiliates
at July 31, 2001: $561,241,621 (excludes shares held by executive officers,
directors, and beneficial owners of more than 10% of the Company's Common
Stock). Exclusion of shares held by any person should not be construed to
indicate that such person possesses the power, direct or indirect, to direct or
cause the direction of management or policies of the registrant, or that such
person is controlled by or under common control with the registrant. Common
Stock outstanding at July 31, 2001: 38,552,902 shares.
 
    Indicate by check mark if disclosure of delinquent filers pursuant to Item
405 of Regulation S-K is not contained herein, and will not be contained, to the
best of the registrant's knowledge, in definitive proxy or information
statements incorporated by reference in Part III of this Form 10-K or any
amendment to this Form 10-K. / /
 
                      DOCUMENTS INCORPORATED BY REFERENCE
 
    Portions of the Registrant's Definitive Proxy Statement for its 2001 Annual
Meeting of Shareholders are incorporated by reference into Part III of this
Report.
 
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ITEM 1.  DESCRIPTION OF BUSINESS
 
    In this Annual Report on Form 10-K, ImmunoGen, Inc. (together with its
subsidiaries, we, us, or the Company), incorporates by reference certain
information from parts of other documents filed with the Securities and Exchange
Commission. The SEC allows us to disclose important information by referring to
it in that manner. Please refer to all such information when reading this Annual
Report on Form 10-K.
 
THE COMPANY
 
    We are a leading developer of antibody-based cancer therapeutics. Our
proprietary, tumor-activated prodrug, or TAP, technology, combines extremely
potent, small-molecule drugs with monoclonal antibodies that recognize and bind
directly to tumor cells. Our targeted delivery technology increases the potency
and specificity of these cancer-specific antibodies, which allow our drugs to
kill cancer cells with minimal harm to healthy tissue.
 
    We believe that our TAP technology will enable us to become a leader in the
development of innovative biopharmaceutical treatments for cancer and other
debilitating human diseases. We plan to achieve this goal by carrying out a
business model that leverages our proprietary methods of targeting cancer as
well as our broad scientific capabilities and drug development expertise. We are
focused on developing our own proprietary products that we will take through
later stages of clinical development in an effort to maximize shareholder
return. We pay for the development of this product pipeline by selectively
out-licensing our technology in exchange for cash that we use to feed the
pipeline with new targets and fund clinical development. Currently, we have
out-license agreements with GlaxoSmithKline plc, British Biotech plc,
Genentech, Inc., Millennium Pharmaceuticals, Inc. and Abgenix, Inc. We also have
technology in-license agreements to acquire therapeutic targets with Avalon
Pharmaceuticals, Inc. and Raven Biotechnology, Inc.
 
    We are testing our two most advanced product candidates,
huC242-DM1/SB-408075 and huN901-DM1/BB-10901, as single agents in patients with
colon, pancreatic and non-small-cell lung cancer and small-cell lung cancer,
respectively. huC242-DM1/SB-408075 has already demonstrated safety in one phase
I clinical trial and is currently being evaluated in two other phase I clinical
trials. In published pre-clinical studies, an unhumanized version of this drug
completely eliminated transplanted human colorectal tumors in mice with no
detectable toxicity. Along with our partner, British Biotech, we are conducting
a Phase I/II trial with our second product candidate, huN901-DM1/BB-10901, for
the treatment of small-cell lung cancer at two clinical sites in the United
States. We retain worldwide manufacturing rights to huN901-DM1/BB-10901 and
commercialization rights in North America and the rest of the world, excluding
the European Union and Japan.
 
    ImmunoGen was organized as a Massachusetts corporation in March 1981. Our
principal executive offices are located at 128 Sidney Street, Cambridge,
Massachusetts 02139, and our telephone number is (617) 995-2500. We maintain a
web site at www.immunogen.com.
 
OUR MARKET OPPORTUNITY
 
    Cancer is a leading cause of death worldwide and the second leading cause of
death in the United States with approximately 1.2 million new cases and over
550,000 deaths expected this year. Existing cancer therapies, including surgery,
radiation therapy and chemotherapy, frequently prove to be incomplete or
ineffective, and are often toxic to the patient. We have developed our TAP
technology to address this unmet therapeutic need.
 
    Monoclonal antibodies have been widely tested as a potential cancer
therapeutic. While some of these antibodies have demonstrated anti-tumor
activity as a single agent, most are not potent enough on their own to kill
cancer cells. We believe that the potency and efficacy of a monoclonal antibody
can
 
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be significantly improved by attaching to it a toxic payload. When using the
right components and engineered properly, the antibody acts as delivery vehicle,
carrying our powerful small molecule drugs specifically to cancer cells while
minimizing the effect on healthy tissue.
 
TUMOR-ACTIVATED PRODRUGS
 
    We call our products tumor-activated prodrugs, or TAPs. Each TAP consists of
an antibody that is chemically linked, or conjugated, to a small molecule drug
that serves as an effector molecule. The antibodies we use target and bind
specifically to antigens that are primarily found on certain types of cancer
cells. Once bound to the cell surface, the cell internalizes our TAP, triggering
the release of the effector molecules that then kill the cancer cell.
 
    Because TAPs are inactive until the drug component is released from the
antibody component inside the target cell, each TAP acts as a prodrug. This
means that the effector molecule remains inactive while circulating in the body
and is only activated once inside the target tumor cell, thereby causing minimal
harm to healthy tissue. This prodrug design allows us to deliver significantly
more drug to the patient than would be the case if it were administered detached
from the antibody.
 
    The small molecule drug we currently use in all of our TAPs is a
maytansinoid, which is a chemical derivative of a naturally occurring substance
called maytansine. This agent, which we refer to as DM1, is a potent inhibitor
of cell division and can kill cancer cells at exceedingly low concentrations.
 
    In addition to DM1, we have tested several other classes of small-molecule
drugs. Laboratory and pre-clinical tests lead us to believe that some of these
small-molecule drugs offer great promise for use as effector molecules in TAPs.
We are in the process of developing derivatives of some of these drugs that
allow them to be attached to antibodies as inactive agents, but allow for their
release in a fully active form at the target site.
 
    We believe our TAP product candidates will offer advantages over other
cancer treatments because we design them to have all of the following
attributes:
 
    - HIGH SPECIFICITY. We develop our TAPs with antibodies that bind to
      specific markers primarily expressed on certain types of cancer cells to
      pinpoint treatment only to the targeted cell.
 
    - HIGH POTENCY. We use highly potent small molecule effector drugs which are
      at least 100 to 1000 times more cytotoxic than traditional
      chemotherapeutics.
 
    - STABLE LINKAGE AND RELEASE. We design our TAPs with a highly stable link
      between the antibody and the effector molecule, allowing the effector
      molecule in its active form to be released only after the TAP is inside
      the cell.
 
    - MINIMAL TOXICITY. We expect our TAPs will offer the potential for an
      improved quality of life for patients due to reduced toxicity and more
      tolerable side effects.
 
    - NON-IMMUNOGENIC. We use fully-humanized antibodies and non-protein-based
      small molecule effector drugs in our TAP products. This reduces the risk
      that our TAPs will elicit an attack by the body's immune system, which
      could render them ineffective before they reach the targeted cancer cells.
 
BUSINESS GOALS AND STRATEGY
 
    Our goal is to become a leader in the development of innovative
biopharmaceutical treatments for cancer and other debilitating human diseases.
We plan to achieve this goal by carrying out a business model that is designed
to leverage our proprietary TAP technology as well as our broad scientific and
technological capabilities. Specifically, we license our TAP technology to third
parties to generate cash
 
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flow that we use to fund the development of our own proprietary products,
reducing the amount of operating cash we spend developing our internal pipeline.
 
    We refer to these arrangements as "autopilot" deals because they do not
significantly burden our internal resources. We have entered into such deals
with leading biotechnology companies including Genentech, Millennium and
Abgenix. The arrangements are structured to provide us with up-front development
fees, milestone payments and royalties if our collaborators are successful in
developing and commercializing products. Under each of these arrangements, we
work cooperatively with the other party to enhance the development of
commercially viable products.
 
    We apply the cash flows from our "autopilot" deals to the development of our
own product pipeline. We feed our pipeline through a combination of both
internal targets and acquired technologies. Specifically, we acquire potential
therapeutic targets and drug discovery technology through in-license agreements
with third parties. Our in-license agreements with companies including Avalon
and Raven offer a rich source of potential therapeutic targets while our
collaboration with Morphosys AG, provides us with access to technology that
enables us to identify fully human antibodies against a specific cell surface
marker that we have identified. We also conduct our own, in-house discovery and
development efforts. To date, our internal development efforts have been at
least partially responsible for our huC242 and huN901 antibodies, as well as for
several research and development stage antibody candidates.
 
    The key initiatives to carrying out our business model are:
 
    - EXPAND OUR PRODUCT PIPELINE. We intend to grow our pipeline of product
      candidates based on our proprietary TAP technology. We currently have two
      TAP candidates, huC242-DM1/SB-408075 and huN901-DM1/BB-10901, in human
      clinical trials. We partnered these product candidates to expedite their
      development. We are developing additional TAP products and antibodies
      in-house for the treatment of cancer. We are also working hard to discover
      additional cancer markers that we will use to develop new cancer
      therapeutics.
 
    - ESTABLISH AND EXPAND STRATEGIC ALLIANCES. We intend to continue to
      out-license our TAP technology to third party collaborators. We anticipate
      that these arrangements will generate cash flow through up-front fees,
      milestone payments and royalties on the sales of any resulting products.
      We already have a strong base of established strategic alliances with
      major pharmaceutical and biotechnology companies and, in the future, we
      expect to enter into additional similar collaborations. These alliances
      provide us with substantial cash flow, furnish us with access to important
      technology, broaden our product development pipeline and reduce our
      product development risks. These alliances also enhance our ability to
      bring products to market because of our collaborators' substantial
      resources and expertise in research, pre-clinical and clinical
      development, regulatory issues, manufacturing and marketing.
 
    - RETAIN SIGNIFICANT PRODUCT RIGHTS. We intend to further develop new
      product candidates prior to entering into collaborations in order to
      obtain greater long-term returns from our product candidates. In addition,
      we intend to enter into collaborations in which we can retain marketing
      and/or manufacturing rights. For example, in the case of
      huN901-DM1/BB-10901, we have retained commercial rights in all territories
      outside of the European Union and Japan, as well as worldwide
      manufacturing rights.
 
    - BROADEN OUR TECHNOLOGY BASE. We will continue to enhance our TAP
      technology platform by identifying and developing potential target
      candidates using the latest technological advances. Our target
      identification and product development activities take advantage of our
      own internal development efforts as well as identifying, evaluating and
      integrating technologies licensed from third parties. We also believe that
      no single effector molecule will be applicable to all clinical needs. At
      present, we have a portfolio of effector molecules under development. We
 
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      are using our experience to develop additional effector molecules and will
      continue to select and design new effector molecules with different
      mechanisms of cell destruction. Finally, we are pursuing, both internally
      and with third parties, innovative methods of manufacturing and process
      development.
 
PRODUCT CANDIDATES
 
    We currently have two products in human clinical trials. In addition, we
have several other products, our own as well as those that are being developed
in conjunction with our collaborators, in pre-clinical and research stages of
development.
 
    The following table summarizes the primary indications, development stage
and collaborative partner for our product candidates. This table is qualified in
its entirety by reference to the more detailed descriptions of these product
candidates appearing elsewhere in this Form 10-K. The results from pre-clinical
testing and early clinical trials may not be predictive of results obtained in
subsequent clinical trials and there can be no assurance that our or our
collaborators' clinical trials will demonstrate the level of safety and efficacy
of any product candidates that is necessary to obtain regulatory approval.
 

<Table>
<Caption>
PRODUCT CANDIDATE                  CANCER INDICATION         STATUS(1)         PARTNER
-----------------              --------------------------   ------------   ---------------
<S>                            <C>                          <C>            <C>
huC242-DM1/SB-408075.........  Colorectal cancer            Phase I        GlaxoSmithKline
                               Pancreatic cancer
                               Non-small-cell lung cancer
 
huN901-DM1/BB-10901..........  Small-cell lung cancer       Phase I/II     British Biotech
 
Herceptin-Registered Trademark--DM1... Multiple cancers     Pre-clinical   Genentech
 
Anti-PSMA-DM1................  Multiple cancers             Pre-clinical   Millennium
 
MAb-DM1 Conjugates...........  Multiple cancers             Research       ImmunoGen
 
MAb-DM1 Conjugates...........  Multiple cancers             Research       Genentech
 
MAb-DM1 Conjugates...........  Multiple cancers             Research       Abgenix
 
MAb-DM1 Conjugates...........  Multiple cancers             Research       Millennium
</Table>

 
------------------------
 
(1) See "Regulatory Matters," below, for definitions of "Phase I" and "Phase
    I/II" clinical trials. Pre-clinical status indicates that we, or our
    partners, are conducting formulation, efficacy, pharmacology and/or
    toxicology testing of a compound in pre-clinical models or biochemical
    assays. Research status indicates that we, or our partners, are conducting
    research studies to determine the product candidates' viability as a
    potential therapeutic.
 
huC242-DM1/SB-408075
 
    Our most advanced TAP product candidate, huC242-DM1/SB-408075, consists of
the humanized C242 monoclonal antibody linked to our small drug effector
molecule DM1. We are developing this TAP with GlaxoSmithKline for the treatment
of colorectal, pancreatic and certain non-small-cell lung cancers. We believe
the C242 antibody possesses the specificity needed for use as a targeting agent
in a TAP. It binds to all colorectal cancers, binds strongly to approximately
70% of colorectal cancers, and has minimal cross-reactivity with normal human
tissues. In addition, laboratory tests indicate that the marker targeted by C242
is found on all pancreatic tumors and a majority of non-small-cell lung tumors.
 
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    huC242-DM1/SB-408075 is currently in two Phase I clinical trials. The
initial Phase I human clinical study, which began in December 1999, was a
dose-escalating study designed to evaluate the pharmacokinetics, maximum
tolerated dose and dose-limiting toxicities of huC242-DM1/SB-408075 when
administered as a single infusion once every three weeks. The study was
conducted at the Institute for Drug Development of the Cancer Therapy and
Research Center, or CTRC, in San Antonio, Texas, under the direction of Anthony
W. Tolcher, M.D. and Eric K. Rowinsky, M.D.
 
    This study has completed enrollment and results of this study were presented
at the 2001 Annual Meeting of the American Society of Clinical Oncology. The
reported results were from patients with either colorectal (32 patients),
pancreatic (4 patients) or non-small-cell lung cancer (1 patient). All patients
treated had advanced solid malignancies refractory to standard therapy. The
study results demonstrate a dose at which the TAP is well tolerated when given
as a single bolus every three weeks. Further, no evidence of immunogenicity was
observed. Two patients demonstrated minor responses (reduction of tumor size by
approximately one-third) and four additional patients had persistent stable
disease for greater than three months. A total of nine patients showed decreases
in carcinoembryonic antigen levels. This antigen can be used by physicians to
follow the course of colon cancer, monitor the effect of treatment, and detect
recurrence.
 
    A second Phase I human clinical study, designed to evaluate the safety of
huC242-DM1/SB-408075 when administered in a weekly regimen, is ongoing at the
University of Chicago Cancer Research Center under the direction of Richard L.
Schilsky, M.D. This study began in September 2000 and is designed to test the
safety and tolerability of the drug on a more frequent dosing schedule.
 
    Finally, in May 2001, we began enrollment for a third Phase I human clinical
study of huC242-DM1/SB-408075. This study is designed to evaluate
huC242-DM1/SB-408075 when administered in a more dose-intensive regimen where
patients are dosed three times weekly. The study is being conducted at the CTRC
in San Antonio, Texas, under the direction of Anthony W. Tolcher, M.D. and Eric
K. Rowinsky, M.D.
 
huN901-DM1/BB-10901
 
    Our second TAP product in human clinical trials is huN901-DM1/BB-10901. We
are developing this TAP, which was discovered and developed by ImmunoGen prior
to entering into an agreement with British Biotech, for the treatment of
small-cell lung cancer, or SCLC. We retain worldwide manufacturing rights to
huN901-DM1/BB-10901 and commercialization rights in North America and the rest
of the world, excluding the European Union and Japan.
 
    huN901-DM1/BB-10901 was created by conjugating our effector molecule, DM1,
with the humanized monoclonal antibody, huN901, which binds to a protein found
on the surface of SCLC cells. In pre-clinical studies, huN901-DM1/BB-10901
eradicated SCLC tumors. Under the same experimental conditions, other
chemotherapies used to treat SCLC, such as cisplatin and etoposide, produced
only a temporary interruption of tumor growth.
 
    In May 2001, we initiated a Phase I/II trial for this product at two
clinical sites in the United States. This open-label, dose-ranging study marks
the first use of huN901-DM1/BB-10901 in cancer patients. The first phase of the
study will test increasing doses of huN901-DM1/BB-10901 to evaluate the safety
and maximum tolerated dose of the drug. Once the maximum tolerated dose has been
defined, the Phase II portion of the study, designed to assess the drug's
biological activity, will begin. Patients will receive a once-weekly,
intravenous dose of huN901-DM1/BB-10901 for four weeks, followed by two weeks
off, which is defined as one cycle of treatment. Patients may be eligible to
receive repeat cycles. Approximately 80 patients who have failed other treatment
options are expected to participate in this study. The study is being conducted
by Frank V. Fossella, M.D., at the University of Texas M. D. Anderson Cancer
Center in Houston, and by Anthony W. Tolcher, M.D., at the CTRC in San Antonio.
 
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    SCLC is a serious and rapidly progressive form of lung cancer, most common
in middle-aged and elderly patients, accounting for approximately a quarter of
all lung cancer cases. Existing treatments for SCLC include chemotherapy and
radiotherapy, and although initial responses to therapy are often obtained,
patients commonly relapse and most die from their disease. Median survival for
such patients is less than a year. The overall 5-year survival rate is estimated
to be less than five percent.
 
HERCEPTIN-REGISTERED TRADEMARK--DM1
 
    We have licensed our maytansinoid technology, including DM1, to Genentech
for the development of TAPs for cancers expressing the HER2 antigen.
Herceptin-Registered Trademark--DM1 combines DM1 with Genentech's monoclonal
antibody Herceptin-Registered Trademark-. As a naked antibody,
Herceptin-Registered Trademark- is currently approved for use as first-line
therapy in combination with Taxol-Registered Trademark- and as a single agent in
second- and third-line therapy in patients with metastatic breast cancer who
have tumors that overexpress the HER2 protein.
 
OTHER PRODUCTS
 
    In addition to Herceptin-Registered Trademark--DM1, we also have licensed
our maytansinoid technology to Genentech for use in a collaborative research
project directed toward the development of TAPs linked to other antibodies owned
by Genentech.
 
    We also have licensed our maytansinoid technology to Abgenix for use with
its fully-human antibodies to develop a succession of TAP products. Finally, we
have a collaboration with Millennium that provides them access to our TAP
technology for use with Millennium's proprietary antibodies. They have declared
Prostate Specific Membrane Antigen, or PSMA, as the first antibody target in
this collaboration.
 
    We also have two collaboration agreements with MorphoSys. Pursuant to the
terms of the first agreement, MorphoSys will attempt to identify fully-human
antibodies against one of our cell surface targets that we may then develop as
an anti-cancer therapeutic. We intend to develop products using antibodies
generated by MorphoSys against this marker. Under the second agreement, we have
licensed MorphoSys' HuCAL-Registered Trademark-, or Human Combinatorial Antibody
Library, technology for the generation of research antibodies. We believe that
access to the HuCAL-Registered Trademark- technology will facilitate and
accelerate our internal research efforts.
 
LICENSES AND COLLABORATIONS
 
    As part of our business model, we enter into license agreements with third
parties. In some cases, we out-license certain rights to our TAP technology to
companies with product development and commercialization capabilities we wish to
access, in exchange for up-front fees, milestone payments, and royalties on
product sales. In other cases, we in-license certain rights to targets or
technologies in exchange for up-front fees, milestone payments and royalties on
product sales. Our principal licenses and collaborative agreements are listed
below.
 
GLAXOSMITHKLINE PLC
 
    In February 1999, we entered into an exclusive license agreement with
SmithKline Beecham plc, London and SmithKline Beecham, Philadelphia,
wholly-owned subsidiaries of GlaxoSmithKline, to develop and commercialize our
lead TAP, huC242-DM1/SB-408075, for the treatment of colorectal, pancreatic and
certain non-small-cell lung cancers. Under the terms of this agreement, we could
receive payments totaling $41.5 million, subject to our achievement of certain
development milestones. As of June 30, 2001, we have received one up-front and
four milestone payments totaling $11.5 million under the GlaxoSmithKline
agreement. We are also entitled to receive royalty payments on future product
sales, if and when they commence. Finally, at our option and subject to certain
conditions, GlaxoSmithKline will purchase up to $5.0 million of our common
stock. Between the signing of the
 
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agreement and June 30, 2001, GlaxoSmithKline had purchased, pursuant to our put
option, $2.5 million of our common stock.
 
    We expect the GlaxoSmithKline agreement to provide us with sufficient cash
funding to carry out our responsibilities in developing huC242-DM1/SB-408075.
All costs subsequent to the Phase I clinical studies will be the responsibility
of GlaxoSmithKline.
 
GENENTECH, INC.
 
    In May 2000, we entered into two separate licensing agreements with
Genentech. The first agreement grants Genentech an exclusive license to our TAP
technology for use with antibodies such as Herceptin-Registered Trademark-.
Under the terms of this agreement, Genentech will receive exclusive worldwide
rights to commercialize anti-HER2 targeting products using our TAP platform.
Genentech will be responsible for manufacturing, product development and
marketing of any products resulting from the agreement; we will be reimbursed
for any pre-clinical and clinical materials that we manufacture under the
agreement. We received a $2.0 million non-refundable payment upon execution of
the agreement. In addition to royalties on net sales, the terms of the agreement
include other payments based upon Genentech's achievement of milestones.
Assuming all benchmarks are met, we would receive approximately $40.0 million in
payments under this agreement.
 
    In addition to the Herceptin-Registered Trademark- agreement described
above, we entered into an additional agreement with Genentech. This second
collaboration provides Genentech with broad access to our TAP technology for use
with Genentech's other proprietary antibodies. This agreement provides Genentech
with a license to utilize our TAP platform in its antibody product research
efforts and an option to obtain product licenses for a limited number of antigen
targets over the agreement's five-year term. Genentech will be responsible for
manufacturing, product development and marketing of any products developed
through this collaboration; we will be reimbursed for any pre-clinical and
clinical materials that we manufacture under the agreement. Under this
agreement, we received a non-refundable technology access fee of $3.0 million in
May 2000. This agreement also provides for other payments based on Genentech's
achievement of milestones per antigen target, and royalties on net sales of any
resulting products. Assuming all milestones are met, we would receive
approximately $40.0 million in payments per antigen target under this agreement.
The agreement can be renewed for one subsequent three-year period, for an
additional technology access fee.
 
BRITISH BIOTECH PLC
 
    In May 2000, we entered into a collaboration with British Biotech to develop
and commercialize our huN901-DM1/BB-10901 TAP for the treatment of small-cell
lung cancer. We granted British Biotech exclusive rights to develop and
commercialize huN901-DM1/BB-10901 in the European Union and Japan. We retain the
rights to commercialize huN901-DM1/BB-10901 in the United States and the rest of
the world, as well as the right to manufacture the product worldwide. Under the
terms of the agreement, British Biotech will be responsible for conducting the
clinical trials necessary to achieve marketing approval in the United States,
European Union and Japan. We will be reimbursed for manufacturing the product
for clinical trials. British Biotech paid us a fee of $1.5 million for its
territorial rights to huN901-DM1/BB-10901. Upon approval of the product for
marketing in the United States, we will pay to British Biotech a one-time
milestone payment of $3.0 million. We will receive royalties on sales of
huN901-DM1/BB-10901 in the European Union and Japan.
 
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ABGENIX, INC.
 
    In September 2000, we entered into a collaboration agreement with Abgenix.
The agreement provides Abgenix with access to our TAP technology for use with
Abgenix's antibodies along with options to obtain product licenses for antigen
targets. We received a total of $5.0 million in technology access fee payments
from Abgenix and are entitled to potential milestone payments and royalties on
net sales of any resulting products. In addition, on September 7, 2000, Abgenix
purchased $15.0 million of our common stock in accordance with the agreement.
Abgenix has the right to extend its options under this agreement to obtain
product licenses for a specified period of time for an extension fee. Our
agreement with Abgenix will terminate upon expiration of a specified time period
during which we have given Abgenix access to our technology. Either party can
terminate the agreement for any material breach by the other party that remains
uncured for a certain period of time.
 
MORPHOSYS AG
 
    In September 2000, we entered into a collaboration agreement with MorphoSys.
Pursuant to this agreement, MorphoSys will identify fully human antibodies
against a specific cell surface marker that we have identified through our
apoptosis research. This cell marker is associated with a number of forms of
cancer. We intend to develop products using antibodies generated by MorphoSys
against this marker. We paid MorphoSys an $825,000 technology access payment and
will pay development-related milestone payments and royalties on net sales of
any resulting products. We reimburse MorphoSys for its research and development
efforts related to identifying these antibodies. We can terminate this agreement
unilaterally at any time and either party can terminate the agreement for any
material breach by the other party that remains uncured for a certain period of
time.
 
    In June 2001, we entered into a second collaboration agreement with
MorphoSys. Under this second agreement, we will license MorphoSys'
HuCAL-Registered Trademark- technology for the generation of research
antibodies. We believe that access to the HuCAL-Registered Trademark- technology
will facilitate and accelerate our internal research efforts. Under this new
agreement, ImmunoGen will pay MorphoSys technology access, license and annual
subscription fees during a four-year term.
 
GENZYME TRANSGENICS CORPORATION
 
    In November 2000, we entered into a collaboration agreement with Genzyme
Transgenics Corporation. Pursuant to this agreement, Genzyme Transgenics will
produce our humanized monoclonal antibody, huN901. huN901 is the antibody
component of huN901-DM1/BB-10901. We paid Genzyme Transgenics a $500,000 project
start-up fee and will pay development-related milestone payments and royalties
on net sales of any resulting products. We can terminate this agreement
unilaterally at any time and either party can terminate the agreement for any
material breach by the other party that remains uncured for a certain period of
time.
 
AVALON, INC.
 
    In January 2001, we entered into a collaboration agreement with Avalon.
Pursuant to the agreement, Avalon will provide us with gene targets. We will be
responsible for the development, manufacture and commercialization of any
resulting products. We paid Avalon an up-front fee. Either party can terminate
the agreement for any material breach by the other party that remains uncured
for a certain period of time.
 
MILLENNIUM PHARMACEUTICALS, INC.
 
    In March 2001, we entered into a five-year collaboration agreement with
Millennium. The agreement provides Millennium access to our TAP technology for
use with Millennium's proprietary antibodies. Millennium acquired a license to
utilize our TAP technology in its antibody product
 
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research efforts and an option to obtain product licenses for a restricted
number of antigen targets during the collaboration. We received an up-front fee
of $2.0 million in the third quarter of 2001. This agreement also provides for
certain other payments based on Millennium's achievement of milestones. Assuming
all benchmarks are met, we could receive more than $40.0 million per antigen
target. We will also receive royalties on net sales of any resulting products.
 
    Millennium will be responsible for product development, manufacturing and
marketing of any products developed through the collaboration. We will be
reimbursed for any pre-clinical and clinical materials that we make under the
agreement. The agreement can be renewed for one subsequent three-year period,
for an additional technology access fee. Millennium recently declared PSMA as
the first antibody target in this collaboration.
 
RAVEN BIOTECHNOLOGIES, INC.
 
    Also in March 2001, we entered into a collaboration with Raven aimed at
identifying targets and therapeutic antibodies with the potential to treat
ovarian cancer. Raven will discover and provide us with cell surface targets and
monoclonal antibodies. We will use these targets and antibodies to develop
therapeutic products. We will have the development, manufacturing and
commercialization rights to these products in North America and Europe in
exchange for an up-front licensing fee, research support, milestones and
royalties.
 
OTHER LICENSES
 
    We also have licenses with third parties, including other companies and
academic institutions, to gain access to techniques and materials for drug
discovery and product development and the rights to use those techniques and
materials to make our products. These licenses include rights to certain
antibodies, software used in antibody development, and apoptosis technology.
 
PATENTS, TRADEMARKS AND TRADE SECRETS
 
    We seek patent protection for our proprietary technologies and products in
the United States, Europe, Japan and elsewhere. Among others, we have received
patents in the United States and Europe claiming the use of maytansinoids in
conjugated form as an invention, United States patents claiming use of DC1 and
its analogs in immunoconjugates, and patents claiming apoptosis technology.
 
    We have also submitted additional patent applications in the United States,
Europe, Japan, and elsewhere covering proprietary small-drug derivatives, TAPs,
apoptosis technology and use of some of these products and inventions for
certain diseases. We expect that our work will also lead to other patent
applications. In all such cases, we will either be the assignee or owner of such
patents or have an exclusive license to the technology covered by the patents.
We cannot assure you, however, that the patent applications will issue as
patents or that any patents, if issued, will provide us with adequate protection
against competitors with respect to the covered products, technologies or
processes.
 
    In addition, many of the processes and much of the know-how that are
important to us depend upon the skills, knowledge and experience of our key
scientific and technical personnel, which skills, knowledge and experience are
not patentable. To protect our rights in these areas, we require that all
employees, consultants, advisors and collaborators enter into confidentiality
agreements with us. We cannot assure you, however, that these agreements will
provide meaningful protection for our trade secrets, know-how or other
proprietary information in the event of any unauthorized use or disclosure of
such trade secrets, know-how or proprietary information. Further, in the absence
of patent protection, we may be exposed to competitors who independently develop
substantially equivalent technology or otherwise gain access to our trade
secrets, know-how or other proprietary information.
 
                                       10
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COMPETITION
 
    We focus on highly competitive areas of product development. Our competitors
include:
 
    - major pharmaceutical and chemical companies;
 
    - specialized biotechnology firms; and
 
    - universities and research institutions.
 
    Many of these companies and institutions also compete with us in recruiting
and retaining highly qualified scientific personnel. Many competitors and
potential competitors have substantially greater scientific research and product
development capabilities, as well as greater financial, marketing and human
resources than we do. In addition, many specialized biotechnology firms have
formed collaborations with large, established companies to support the research,
development and commercialization of products that may be competitive with ours.
 
    In particular, competitive factors within the cancer therapeutic market
include:
 
    - the safety and efficacy of products;
 
    - the timing of regulatory approval and commercial introduction;
 
    - special regulatory designation of products, such as Orphan Drug
      designation; and
 
    - the effectiveness of marketing and sales efforts.
 
    Our competitive position also depends on our ability to develop effective
proprietary products, implement production and marketing plans, including
collaborations with other companies with greater marketing resources than ours,
obtain patent protection and secure sufficient capital resources.
 
    Continuing development of conventional and targeted chemotherapeutics by
large pharmaceutical companies may result in the identification of new compounds
that may compete with our product candidates. In addition, monoclonal antibodies
developed by certain of these companies have been approved for use as cancer
therapeutics. In the future, additional monoclonal antibodies may compete with
our product candidates.
 
    Because of the prevalence of combination therapy in cancer and the variety
of genes and targets implicated in cancer progression, we believe that products
resulting from applications of new technologies may be complementary to our own.
 
    Such new technologies include, but are not limited to:
 
    - the use of genomics technology to identify new gene-based targets for the
      development of anti-cancer drugs;
 
    - the use of high-throughput screening to identify and optimize lead
      compounds;
 
    - the use of gene therapy to deliver genes to regulate gene function; and
 
    - the use of therapeutic vaccines.
 
REGULATORY MATTERS
 
    Our products are regulated in the United States by the Food and Drug
Administration, or FDA, in accordance with the United States Federal Food, Drug,
and Cosmetic Act, as well as the Public Health Service Act. Therapeutic
monoclonal antibody products are most often considered biologicals and therefore
subject to regulation by the Center for Biologics Evaluation and Research within
the FDA, while new chemical entities are regulated under the FDA's Center for
Drug Evaluation and Research, or CDER. We expect that huC242-DM1/SB-408075,
huN901-DM1/SB-10901 and other of our TAPs will
 
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be reviewed by CDER. In addition, each drug manufacturer in the United States
must be registered with the FDA.
 
    The steps required before a new drug may be marketed in the US include:
 
    1)  Performance of pre-clinical laboratory, animal, and formulation studies;
 
    2)  The submission to the FDA of an Investigational New Drug Application,
       which must become effective before clinical trials may commence;
 
    3)  The completion of adequate and well-controlled human clinical trials to
       establish the safety and efficacy of the drug;
 
    4)  The submission of a New Drug Application to the FDA; and
 
    5)  FDA approval of the New Drug Application, including approval of all
       product labeling and advertising.
 
    Even if we, or our partners, obtain regulatory approvals for our product
candidates, the Company, our products, and the facilities in which our products
are manufactured are subject to continual review and periodic inspection. The
FDA will require post-marketing reporting to monitor our products' safety.
Manufacturing establishments are subject to periodic inspections by the FDA and
must comply with the FDA's Current Good Manufacturing Practices, or cGMP. In
complying with cGMP, manufacturers must expend funds, time and effort in the
areas of production, quality control and record keeping to ensure full technical
compliance. The FDA stringently applies regulatory standards for manufacturing.
 
    The regulatory issues that have potential impact on the future marketing of
our products are summarized below:
 
    CLINICAL TRIALS PROCESS
 
    Before a new drug may be sold in the United States and other countries,
clinical trials of the product must be conducted and the results submitted to
the appropriate regulatory agencies for approval.
 
    In the United States, these clinical trial programs generally involve a
three-phase process. Typically, Phase I trials are conducted in healthy
volunteers to determine the early side-effect profile and the pattern of drug
distribution and metabolism. In Phase II, trials are conducted in groups of
patients afflicted with the target disease to determine preliminary efficacy and
optimal dosages and to expand the safety profile. In Phase III, large-scale
comparative trials are conducted in patients with the target disease to provide
sufficient data for the proof of efficacy and safety required by federal
regulatory agencies. In the case of drugs for cancer and other life-threatening
diseases, Phase I human testing usually is performed in patients with advanced
disease rather than in healthy volunteers. Because these patients are already
afflicted with the target disease, it is possible for such studies to provide
results traditionally obtained in Phase II trials and they often are referred to
as Phase I/II studies.
 
    We intend to conduct clinical trials not only in accordance with FDA
regulations, but also within guidelines established by other applicable agencies
and committees. Whether or not FDA approval has been obtained, approval of a
product by the comparable regulatory authorities of foreign countries must be
obtained prior to the commencement of marketing of the product in those
countries. Regulatory approval in other countries is obtained through the
various regulatory bodies governing pharmaceutical sales in those individual
countries. We intend to rely on foreign licensees to obtain regulatory approvals
to market our products in foreign countries.
 
    Regulatory approval often takes a number of years and involves the
expenditure of substantial resources. Approval times also depend on a number of
factors including, but not limited to, the severity
 
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of the disease in question, the availability of alternative treatments and the
risks and benefits demonstrated in clinical trials.
 
    ORPHAN DRUG DESIGNATION
 
    The Orphan Drug Act of 1983 generally provides incentives to biotechnology
and pharmaceutical companies to undertake development and marketing of products
to treat relatively rare diseases or diseases affecting fewer than 200,000
persons in the United States at the time of application for Orphan Drug
designation.
 
    We may pursue this designation with respect to products intended for
qualifying patient populations. A drug that receives Orphan Drug designation and
is the first product of its kind to receive FDA marketing approval for its
product claim is entitled to a seven-year exclusive marketing period in the
United States for that product claim.
 
    NEW DRUGS FOR SERIOUS OR LIFE-THREATENING ILLNESSES
 
    The FDA Modernization Act allows the designation of "Fast Track" status to
expedite development of new drugs, including review and approvals, and is
intended to speed the availability of new therapies to desperately ill patients.
"Fast Track" procedures permit early consultation and commitment from the FDA
regarding pre-clinical and clinical studies necessary to gain marketing
approval. We may seek "Fast Track" status for some, or all, of our products.
 
    "Fast Track" status also incorporates initiatives announced by the President
of the United States and the FDA Commissioner in March 1996, intended to provide
cancer patients with faster access to new cancer therapies. One of these
initiatives states that the initial basis for approval of anti-cancer agents to
treat refractory, hard-to-treat cancer may be objective evidence of response,
rather than statistically improved disease-free and/or overall survival, as has
been common practice. The sponsor of a product approved under this accelerated
mechanism is required to follow up with further studies on clinical safety and
effectiveness in larger groups of patients.
 
RESEARCH AND DEVELOPMENT SPENDING
 
    During each of the three years ended June 30, 2001, 2000 and 1999, we spent
approximately $15.2 million, $8.9 million and $6.1 million, respectively, on
research and development activities. Most of these expenditures were for
Company-sponsored research and development.
 
EMPLOYEES
 
    As of June 30, 2001, we had 76 full-time employees, of whom 60 were engaged
in research and development activities. Twenty-two employees hold post-graduate
degrees, including eighteen Ph.D. degrees. We consider our relations with our
employees to be good. None of our employees is covered by a collective
bargaining agreement.
 
    We have entered into confidentiality agreements with all of our employees,
members of the Board of Directors and other consultants.
 
SCIENTIFIC ADVISORY BOARD
 
    As of August 1, 2000, ImmunoGen, Inc. formed a Scientific Advisory Board
consisting of the following individuals:
 
        Gerard I. Evan, Ph.D., FMedSci., Gerson and Barbara Bass Bakar
    Distinguished Professor of Cancer Biology, UCSF Comprehensive Cancer Center
    and Cancer Research Institute. Dr. Evan is
 
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    a cancer biologist and an authority on the control of cellular proliferation
    and programmed cell death in mammalian cells.
 
        Stuart F. Schlossman, M.D., Professor of Medicine, Harvard University
    Medical School; member of the National Academy of Sciences; Head of the
    Division of Tumor Immunology, Dana-Farber Cancer Institute.
 
RISK FACTORS
 
    THE RISKS AND UNCERTAINTIES DESCRIBED BELOW ARE THOSE THAT WE CURRENTLY
BELIEVE MAY MATERIALLY AFFECT OUR COMPANY. ADDITIONAL RISKS AND UNCERTAINTIES
THAT WE ARE UNAWARE OF OR THAT WE CURRENTLY DEEM IMMATERIAL ALSO MAY BECOME
IMPORTANT FACTORS THAT AFFECT OUR COMPANY.
 
IF OUR TAP TECHNOLOGY DOES NOT PRODUCE SAFE, EFFECTIVE AND COMMERCIALLY VIABLE
PRODUCTS, OUR BUSINESS WILL BE SEVERELY HARMED.
 
    Our TAP technology is a novel approach to the treatment of cancer. None of
our TAP product candidates has obtained regulatory approval and all of them are
in early stages of development. Our TAP product candidates may not prove to be
safe, effective or commercially viable treatments for cancer and our TAP
technology may not result in any meaningful benefits to our current or potential
collaborative partners. Furthermore, we are aware of only one chemotherapeutic
product that has obtained FDA approval and is based on technology similar to our
TAP technology. If our TAP technology fails to generate product candidates that
are safe, effective and commercially viable treatments for cancer, and obtain
FDA approval, our business will be severely harmed.
 
CLINICAL TRIALS FOR OUR PRODUCT CANDIDATES WILL BE LENGTHY AND EXPENSIVE AND
THEIR OUTCOME IS UNCERTAIN.
 
    Before obtaining regulatory approval for the commercial sale of any product
candidates, we and our collaborative partners must demonstrate through
pre-clinical testing and clinical trials that our product candidates are safe
and effective for use in humans. Conducting clinical trials is a time consuming
and expensive process and may take years to complete. Our most advanced product
candidates, huC242-DM1/SB-408075 and huN901-DM1/BB-10901, are only in the Phase
I and Phase I/ II stages of clinical trials. Historically, the results from
pre-clinical testing and early clinical trials have often not been predictive of
results obtained in later clinical trials. Frequently, drugs that have shown
promising results in pre-clinical or early clinical trials subsequently fail to
establish sufficient safety and effectiveness data necessary to obtain
regulatory approval. At any time during the clinical trials, we, our
collaborative partners or the FDA might delay or halt any clinical trials for
our product candidates for various reasons, including:
 
    - ineffectiveness of the product candidate;
 
    - discovery of unacceptable toxicities or side effects;
 
    - development of disease resistance or other physiological factors; or
 
    - delays in patient enrollment.
 
    The results of the clinical trials may fail to demonstrate the safety or
effectiveness of our product candidates to the extent necessary to obtain
regulatory approval or that commercialization of our product candidates is
worthwhile. Any failure or substantial delay in successfully completing clinical
trials and obtaining regulatory approval for our product candidates could
severely harm our business.
 
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IF OUR COLLABORATIVE PARTNERS FAIL TO PERFORM THEIR OBLIGATIONS UNDER OUR
AGREEMENTS, OUR ABILITY TO DEVELOP AND MARKET POTENTIAL PRODUCTS COULD BE
SEVERELY LIMITED.
 
    Our strategy for the development and commercialization of our product
candidates depends, in large part, upon the formation of collaborative
arrangements. Collaborations allow us to:
 
    - fund our internal research and development, pre-clinical testing, clinical
      trials and manufacturing;
 
    - seek and obtain regulatory approvals;
 
    - successfully commercialize existing and future product candidates; and
 
    - develop antibodies for additional product candidates, and discover
      additional cell surface markers for antibody development.
 
    If we fail to secure or maintain successful collaborative arrangements, our
development and marketing activities may be delayed or scaled back. We may also
be unable to negotiate additional collaborative arrangements or, if necessary,
modify our existing arrangements on acceptable terms. We have entered into
collaboration agreements with GlaxoSmithKline and British Biotech with respect
to our two most advanced product candidates, huC242-DM1/SB-408075 and
huN901-DM1/BB-10901, respectively. The development, regulatory approval and
commercialization of these two product candidates depend primarily on the
efforts of these collaborative partners. We have also entered into
collaborations with Genentech, Abgenix and Millenium. We cannot control the
amount and timing of resources our partners may devote to our products. Our
partners may separately pursue competing products, therapeutic approaches or
technologies to develop treatments for the diseases targeted by us or our
collaborative efforts. Even if our partners continue their contributions to the
collaborative arrangements, they may nevertheless determine not to actively
pursue the development or commercialization of any resulting products. Also, our
partners may fail to perform their obligations under the collaboration
agreements or may be slow in performing their obligations. Our partners can
terminate our collaborative agreements under certain conditions. If any
collaborative partner were to terminate or breach our agreement, or otherwise
fail to complete its obligations in a timely manner, our anticipated revenue
from the agreement and development and commercialization of our products could
be severely limited. If we are not able to establish additional collaborations
or any or all of our existing collaborations are terminated and we are not able
to enter into alternative collaborations on acceptable terms, we may be required
to undertake product development, manufacture and commercialization and we may
not have the funds or capability to do this.
 
WE DEPEND ON A SMALL NUMBER OF COLLABORATORS FOR A SUBSTANTIAL PORTION OF OUR
REVENUE. THE LOSS OF ANY ONE OF THESE COLLABORATORS COULD RESULT IN A
SUBSTANTIAL DECLINE IN REVENUE.
 
    We have and will continue to have collaborations with a limited number of
companies. As a result, our financial performance depends on the efforts and
overall success of these companies. The failure of any one of our collaboration
partners to perform its obligations under its agreement with us, including
making any royalty, milestone or other payments to us, could have a material
adverse effect on our financial condition. Also, if consolidation trends in the
healthcare industry continue, the number of our potential collaborators could
decrease, which could have an adverse impact on our development efforts.
 
WE HAVE A HISTORY OF OPERATING LOSSES AND EXPECT TO INCUR SIGNIFICANT ADDITIONAL
OPERATING LOSSES.
 
    We have generated operating losses since our inception. As of June 30, 2001,
we had an accumulated deficit of $169.2 million. We may never be profitable. We
expect to incur substantial additional operating expenses over the next several
years as our research, development, pre-clinical testing and clinical trial
activities increase. We intend to invest significantly in our products and bring
more of the product development process in-house prior to entering into
collaborative arrangements.
 
                                       15
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We may also incur substantial marketing and other costs in the future if we
decide to establish marketing and sales capabilities to commercialize certain of
our products. None of our product candidates has generated any commercial
revenue and our only revenues to date have been primarily from up-front and
milestone payments from our collaboration partners. We do not expect to generate
revenues from the commercial sale of our products in the foreseeable future, and
we may never generate revenues from the commercial sale of products. Even if we
do successfully develop products that can be marketed and sold commercially, we
will need to generate significant revenues from those products to achieve and
maintain profitability. Even if we do become profitable, we may not be able to
sustain or increase profitability on a quarterly or annual basis.
 
WE ARE SUBJECT TO EXTENSIVE GOVERNMENT REGULATIONS AND WE MAY NOT BE ABLE TO
OBTAIN REGULATORY APPROVALS.
 
    We or our collaborative partners may not receive the regulatory approvals
necessary to commercialize our product candidates, which could cause our
business to fail. Our product candidates are subject to extensive and rigorous
government regulation. The FDA regulates, among other things, the development,
testing, manufacture, safety, record-keeping, labeling, storage, approval,
advertising, promotion, sale and distribution of pharmaceutical products. If our
potential products are marketed abroad, they will also be subject to extensive
regulation by foreign governments. None of our product candidates has been
approved for sale in the United States or any foreign market. The regulatory
review and approval process, which includes pre-clinical studies and clinical
trials of each product candidate, is lengthy, expensive and uncertain. Securing
FDA approval requires the submission of extensive pre-clinical and clinical data
and supporting information to the FDA for each indication to establish the
product candidates' safety and efficacy. Data obtained from pre-clinical and
clinical trials are susceptible to varying interpretation, which may delay,
limit or prevent regulatory approval. The approval process may take many years
to complete and may involve ongoing requirements for post-marketing studies. In
light of the limited regulatory history of monoclonal antibody-based
therapeutics, we cannot assure you that regulatory approvals for our products
will be obtained without lengthy delays, if at all. Any FDA or other regulatory
approvals of our product candidates, once obtained, may be withdrawn. The effect
of government regulation may be to:
 
    - delay marketing of potential products for a considerable period of time;
 
    - limit the indicated uses for which potential products may be marketed;
 
    - impose costly requirements on our activities; and
 
    - provide competitive advantage to other pharmaceutical and biotechnology
      companies.
 
    We may encounter delays or rejections in the regulatory approval process
because of additional government regulation from future legislation or
administrative action or changes in FDA policy during the period of product
development, clinical trials and FDA regulatory review. Failure to comply with
applicable FDA or other applicable regulatory requirements may result in
criminal prosecution, civil penalties, recall or seizure of products, total or
partial suspension of production or injunction, as well as other regulatory
action against our product candidates or us. Outside the United States, our
ability to market a product is contingent upon receiving clearances from the
appropriate regulatory authorities. This foreign regulatory approval process
includes all of the risks associated with the FDA approval process. In addition,
we are, or may become, subject to various federal, state and local laws,
regulations and recommendations relating to safe working conditions, laboratory
and manufacturing practices, the experimental use of animals and the use and
disposal of hazardous substances, including radioactive compounds and infectious
disease agents, used in connection with our research work. If we fail to comply
with the laws and regulations pertaining to our business, we may be subject to
sanctions, including the temporary or permanent suspension of operations,
product recalls, marketing restrictions and civil and criminal penalties.
 
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WE MAY BE UNABLE TO ESTABLISH THE MANUFACTURING CAPABILITIES NECESSARY TO
DEVELOP AND COMMERCIALIZE OUR POTENTIAL PRODUCTS.
 
    Currently, we only have one pilot manufacturing facility for the manufacture
of products necessary for clinical testing. We do not have sufficient
manufacturing capacity to manufacture our product candidates in quantities
necessary for commercial sale. In addition, our manufacturing capacity may be
inadequate to complete all clinical trials contemplated by us over time. We
intend to rely in part on third-party contract manufacturers to produce large
quantities of drug materials needed for clinical trials and commercialization of
our potential products. Third-party manufacturers may not be able to meet our
needs with respect to timing, quantity or quality of materials. If we are unable
to contract for a sufficient supply of needed materials on acceptable terms, or
if we should encounter delays or difficulties in our relationships with
manufacturers, our clinical trials may be delayed, thereby delaying the
submission of product candidates for regulatory approval and the market
introduction and subsequent commercialization of our potential products. Any
such delays may lower our revenues and potential profitability. We may develop
our manufacturing capacity in part by expanding our current facilities or
building new facilities. Either of these activities would require substantial
additional funds and we would need to hire and train significant numbers of
employees to staff these facilities. We may not be able to develop manufacturing
facilities that are sufficient to produce drug materials for clinical trials or
commercial use. We and any third-party manufacturers that we may use must
continually adhere to Current Good Manufacturing Practices regulations enforced
by the FDA through its facilities inspection program. If our facilities or the
facilities of third-party manufacturers cannot pass a pre-approval plant
inspection, the FDA approval of our product candidates will not be granted. In
complying with these regulations and foreign regulatory requirements, we and any
of our third-party manufacturers will be obligated to expend time, money and
effort on production, record-keeping and quality control to assure that our
potential products meet applicable specifications and other requirements. If we
or any third-party manufacturer with whom we may contract fail to maintain
regulatory compliance, we or the third party may be subject to fines and
manufacturing operations may be suspended.
 
OUR INABILITY TO LICENSE FROM THIRD PARTIES THEIR PROPRIETARY TECHNOLOGIES OR
PROCESSES WHICH WE USE IN CONNECTION WITH THE DEVELOPMENT AND MANUFACTURE OF OUR
TAP PRODUCT CANDIDATES MAY IMPAIR OUR BUSINESS.
 
    Other companies, universities and research institutions have or may obtain
patents that could limit our ability to use, manufacture, market or sell our
product candidates or impair our competitive position. As a result, we will have
to obtain licenses from other parties before we could continue using,
manufacturing, marketing or selling our potential products. Any such licenses
may not be available on commercially acceptable terms, if at all. If we do not
obtain required licenses, we may not be able to market our potential products at
all or we may encounter significant delays in product development while we
redesign potentially infringing products or methods.
 
WE RELY ON ONE SUPPLIER FOR THE PRIMARY COMPONENT TO MANUFACTURE OUR SMALL
MOLECULE EFFECTOR DRUG, DM1. ANY PROBLEMS EXPERIENCED BY THIS SUPPLIER COULD
NEGATIVELY AFFECT OUR OPERATIONS.
 
    We rely on third-party suppliers for some of the materials used in the
manufacturing of our TAP product candidates and small molecule effector drugs.
Our most advanced small molecule effector drug is DM1. DM1 is the cytotoxic
agent used in all of our current TAP product candidates and the subject of most
of our collaborations. One of the primary components required to manufacture DM1
is its precursor, ansamitocin P3. Currently, only one vendor manufactures and is
able to supply us with this material. Any problems experienced by this vendor
could result in a delay or interruption in the supply of ansamitocin P3 to us
until this vendor cures the problem or until we locate an alternative source of
supply. Any delay or interruption in our supply of ansamitocin P3 would likely
lead to a delay or
 
                                       17
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interruption in our manufacturing operations and pre-clinical and clinical
trials of our product candidates, which could negatively affect our business.
 
WE MAY BE UNABLE TO ESTABLISH SALES AND MARKETING CAPABILITIES NECESSARY TO
SUCCESSFULLY COMMERCIALIZE OUR POTENTIAL PRODUCTS.
 
    We currently have no direct sales or marketing capabilities. We anticipate
relying on third parties to market and sell most of our primary product
candidates. If we decide to market our potential products through a direct sales
force, we would need to either hire a sales force with expertise in
pharmaceutical sales or contract with a third party to provide a sales force to
meet our needs. We may be unable to establish marketing, sales and distribution
capabilities necessary to commercialize and gain market acceptance for our
potential products and be competitive. In addition, co-promotion or other
marketing arrangements with third parties to commercialize potential products
could significantly limit the revenues we derive from these potential products,
and these third parties may fail to commercialize our potential products
successfully.
 
IF OUR PRODUCT CANDIDATES DO NOT GAIN MARKET ACCEPTANCE, OUR BUSINESS WILL
SUFFER.
 
    Even if clinical trials demonstrate safety and efficacy of our product
candidates and the necessary regulatory approvals are obtained, our product
candidates may not gain market acceptance among physicians, patients, healthcare
payors and the medical community. The degree of market acceptance of any product
candidates that we develop will depend on a number of factors, including:
 
    - the degree of clinical efficacy and safety;
 
    - cost-effectiveness of our product candidates;
 
    - their advantage over alternative treatment methods;
 
    - reimbursement policies of government and third-party payors; and
 
    - the quality of our or our collaborative partners' marketing and
      distribution capabilities for our product candidates.
 
    Physicians will not recommend therapies using any of our future products
until such time as clinical data or other factors demonstrate the safety and
efficacy of such products as compared to conventional drug and other treatments.
Even if the clinical safety and efficacy of therapies using our products is
established, physicians may elect not to recommend the therapies for any number
of other reasons, including whether the mode of administration of our products
is effective for certain indications. Our product candidates, if successfully
developed, will compete with a number of drugs and therapies manufactured and
marketed by major pharmaceutical and other biotechnology companies. Our products
may also compete with new products currently under development by others.
Physicians, patients, third-party payors and the medical community may not
accept and utilize any product candidates that we, or our collaborative
partners, develop. If our products do not achieve significant market acceptance,
we will not be able to recover the significant investment we have made in
developing such products and our business would be severely harmed.
 
WE MAY BE UNABLE TO COMPETE SUCCESSFULLY.
 
    The markets in which we compete are well-established and intensely
competitive. We may be unable to compete successfully against our current and
future competitors. Our failure to compete successfully may result in pricing
reductions, reduced gross margins and failure to achieve market acceptance for
our potential products. Our competitors include pharmaceutical companies,
biotechnology companies, chemical companies, academic and research institutions
and government agencies. Many of these organizations have substantially more
experience and more capital, research
 
                                       18
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and development, regulatory, manufacturing, sales, marketing, human and other
resources than we do. As a result, they may:
 
    - develop products that are safer or more effective than our product
      candidates;
 
    - obtain FDA and other regulatory approvals or reach the market with their
      products more rapidly than we can, reducing the potential sales of our
      product candidates;
 
    - devote greater resources to market or sell their products;
 
    - adapt more quickly to new technologies and scientific advances;
 
    - initiate or withstand substantial price competition more successfully than
      we can;
 
    - have greater success in recruiting skilled scientific workers from the
      limited pool of available talent;
 
    - more effectively negotiate third-party licensing and collaboration
      arrangements; and
 
    - take advantage of acquisition or other opportunities more readily than we
      can.
 
    A number of pharmaceutical and biotechnology companies are currently
developing products targeting the same types of cancer that we target, and some
of our competitors' products have entered clinical trials or already are
commercially available. In addition, our product candidates, if approved and
commercialized, will compete against well-established, existing, therapeutic
products that are currently reimbursed by government health administration
authorities, private health insurers and health maintenance organizations. We
face and will continue to face intense competition from other companies for
collaborative arrangements with pharmaceutical and biotechnology companies, for
relationships with academic and research institutions, and for licenses to
proprietary technology. In addition, we anticipate that we will face increased
competition in the future as new companies enter our markets and as scientific
developments surrounding prodrug and antibody-based therapeutics for cancer
continue to accelerate. While we will seek to expand our technological
capabilities to remain competitive, research and development by others may
render our technology or product candidates obsolete or noncompetitive or result
in treatments or cures superior to any therapy developed by us.
 
IF WE ARE UNABLE TO PROTECT OUR INTELLECTUAL PROPERTY RIGHTS ADEQUATELY, THE
VALUE OF OUR TAP TECHNOLOGY AND OUR PRODUCT CANDIDATES COULD BE DIMINISHED.
 
    Our success is dependent in part on obtaining, maintaining and enforcing our
patents and other proprietary rights and our ability to avoid infringing the
proprietary rights of others. Patent law relating to the scope of claims in the
biotechnology field in which we operate is still evolving and surrounded by a
great deal of uncertainty and involves complex legal, scientific and factual
questions. To date, no consistent policy has emerged regarding the breadth of
claims allowed in biotechnology patents. Accordingly, our pending patent
applications may not result in issued patents. Although we own several patents,
the issuance of a patent is not conclusive as to its validity or enforceability.
Through litigation, a third party may challenge the validity or enforceability
of a patent after its issuance. Also, patents and applications owned or licensed
by us may become the subject of interference proceedings in the United States
Patent and Trademark Office to determine priority of invention which could
result in substantial cost to us. An adverse decision in an interference
proceeding may result in our loss of rights under a patent or patent application
subject to such a proceeding. We cannot assure you how much protection, if any,
will be given to our patents if we attempt to enforce them and they are
challenged in court or in other proceedings. It is possible that a competitor
may successfully challenge our patents or that a challenge will result in
limitations of their coverage. In addition, the cost of litigation or
interference proceedings to uphold the validity of patents can be substantial.
If we are unsuccessful in such proceedings, third parties may be able to use our
patented technology without paying us licensing fees or royalties. Moreover,
competitors may infringe our patents or successfully avoid them through design
 
                                       19
<Page>
innovation. To prevent infringement or unauthorized use, we may need to file
infringement claims, which are expensive and time-consuming. In an infringement
proceeding a court may decide that a patent of ours is not valid. Even if the
validity of our patents were upheld, a court may refuse to stop the other party
from using the technology at issue on the ground that its activities are not
covered by our patents. Policing unauthorized use of our intellectual property
is difficult, and we may not be able to prevent misappropriation of our
proprietary rights, particularly in countries where the laws may not protect
such rights as fully as in the United States. In addition to our patent rights,
we also rely on unpatented technology, trade secrets and confidential
information. Others may independently develop substantially equivalent
information and techniques or otherwise gain access to or disclose our
technology. We may not be able to effectively protect our rights in unpatented
technology, trade secrets and confidential information. We require each of our
employees, consultants and corporate partners to execute a confidentiality
agreement at the commencement of an employment, consulting or collaborative
relationship with us. However, these agreements may not provide effective
protection of our information or, in the event of unauthorized use or
disclosure, they may not provide adequate remedies.
 
WE MAY BE SUBJECT TO SUBSTANTIAL COSTS AND LIABILITY OR BE PROHIBITED FROM
COMMERCIALIZING OUR POTENTIAL PRODUCTS AS A RESULT OF LITIGATION AND OTHER
PROCEEDINGS RELATING TO PATENT RIGHTS.
 
    Patent litigation is very common in the biotechnology and pharmaceutical
industries. Third parties may assert patent or other intellectual property
infringement claims against us with respect to our technologies, products or
other matters. Any claims that might be brought against us relating to
infringement of patents may cause us to incur significant expenses and, if
successfully asserted against us, may cause us to pay substantial damages and
limit our ability to use the intellectual property subject to these claims. Even
if we were to prevail, any litigation could be costly and time-consuming and
could divert the attention of our management and key personnel from our business
operations. Furthermore, as a result of a patent infringement suit, we may be
forced to stop or delay developing, manufacturing or selling potential products
that incorporate the challenged intellectual property unless we enter into
royalty or license agreements. Furthermore, because patent applications in the
United States are maintained in secrecy until a patent issues, others may have
filed patent applications for technology covered by our pending applications.
There may be third-party patents, patent applications and other intellectual
property relevant to our potential products that may block or compete with our
products or processes. In addition, we sometimes undertake research and
development with respect to potential products even when we are aware of
third-party patents that may be relevant to our potential products, on the basis
that such patents may be challenged or licensed by us. If our subsequent
challenge to such patents were not to prevail, we may not be able to
commercialize our potential products after having already incurred significant
expenditures unless we are able to license the intellectual property on
commercially reasonable terms. We may not be able to obtain royalty or license
agreements on terms acceptable to us, if at all. Even if we were able to obtain
licenses to such technology, some licenses may be non-exclusive, thereby giving
our competitors access to the same technologies licensed to us. Ultimately, we
may be unable to commercialize some of our potential products or may have to
cease some of our business operations, which could severely harm our business.
 
WE FACE UNCERTAINTIES OVER REIMBURSEMENT AND HEALTHCARE REFORM.
 
    In both domestic and foreign markets, future sales of our potential
products, if any, will depend in part on the availability of reimbursement from
third-party payors such as government health administration authorities, private
health insurers and other organizations. Third-party payors are increasingly
challenging the price and cost-effectiveness of medical products and services.
Significant uncertainty exists as to the reimbursement status of newly-approved
health care products. Even if they were to obtain regulatory approval, our
product candidates may not be considered cost-effective and adequate third-party
reimbursement may not be available to enable us to maintain price levels
 
                                       20
<Page>
sufficient to realize an appropriate return on our investments in product
development. Legislation and regulations affecting the pricing of
pharmaceuticals may change before any of our product candidates is approved for
marketing. Adoption of such legislation and regulations could further limit
reimbursement for medical products and services. If the government and
third-party payors fail to provide adequate coverage and reimbursement rates for
our potential products, the market acceptance of our products may be adversely
affected.
 
WE USE HAZARDOUS MATERIALS IN OUR BUSINESS, AND ANY CLAIMS RELATING TO IMPROPER
HANDLING, STORAGE OR DISPOSAL OF THESE MATERIALS COULD HARM OUR BUSINESS.
 
    Our research and development activities involve the controlled use of
hazardous materials, chemicals, biological materials and radioactive compounds.
We are subject to federal, state and local laws and regulations governing the
use, manufacture, storage, handling and disposal of such materials and certain
waste products. Although we believe that our safety procedures for handling and
disposing of such materials comply with the standards prescribed by such laws
and regulations, the risk of accidental contamination or injury from these
materials cannot be completely eliminated. In the event of such an accident, we
could be held liable for any resulting damages, and any such liability could
exceed our resources. We may be required to incur significant costs to comply
with these laws in the future. Failure to comply with these laws could result in
fines and the revocation of permits, which could prevent us from conducting our
business.
 
WE FACE PRODUCT LIABILITY RISKS AND MAY NOT BE ABLE TO OBTAIN ADEQUATE
INSURANCE.
 
    The use of our product candidates during testing or after approval entails
an inherent risk of adverse effects which could expose us to product liability
claims. Regardless of their merit or eventual outcome, product liability claims
may result in:
 
    - decreased demand for our product;
 
    - injury to our reputation and significant media attention;
 
    - withdrawal of clinical trial volunteers;
 
    - costs of litigation;
 
    - distraction of management; and
 
    - substantial monetary awards to plaintiffs.
 
    We may not have sufficient resources to satisfy any liability resulting from
these claims. We currently have $5.0 million of product liability insurance for
products which are in clinical testing. This coverage may not be adequate in
scope to protect us in the event of a successful product liability claim.
Further, we may not be able to maintain such insurance or obtain general product
liability insurance on reasonable terms and at an acceptable cost if we or our
collaborative partners begin commercial production of our proposed product
candidates or that such insurance will be in sufficient amounts to provide us
with adequate coverage against potential liabilities.
 
WE DEPEND ON OUR KEY PERSONNEL AND WE MUST CONTINUE TO ATTRACT AND RETAIN KEY
EMPLOYEES AND CONSULTANTS.
 
    We depend on the principal members of our scientific and management
personnel. Our ability to pursue the development of our current and future
product candidates depends largely on retaining the services of our existing
personnel and hiring additional qualified scientific personnel to perform
research and development. We will also need to hire personnel with expertise in
clinical testing, government regulation, manufacturing, marketing and finance.
Attracting and retaining qualified personnel will be critical to our success. We
may not be able to attract and retain personnel on
 
                                       21
<Page>
acceptable terms given the competition for such personnel among biotechnology,
pharmaceutical and healthcare companies, universities and non-profit research
institutions. Failure to retain our existing key management and scientific
personnel or to attract additional highly qualified personnel could delay the
development of our product candidates and harm our business.
 
IF WE ARE UNABLE TO OBTAIN ADDITIONAL FUNDING WHEN NEEDED, WE MAY HAVE TO DELAY
OR SCALE BACK SOME OF OUR PROGRAMS OR GRANT RIGHTS TO THIRD PARTIES TO DEVELOP
AND MARKET OUR PRODUCTS.
 
    We will continue to expend substantial resources developing new and existing
product candidates, including costs associated with research and development,
acquiring new technologies, conducting pre-clinical and clinical trials,
obtaining regulatory approvals and manufacturing products. We believe that our
current working capital and future payments, if any, from our collaboration
arrangements will be sufficient to meet our operating and capital requirements
for at least the next three years. However, we may need additional financing
sooner due to a number of factors including:
 
    - higher costs and slower progress than expected in developing product
      candidates and obtaining regulatory approvals;
 
    - acquisition of technologies and other business opportunities that require
      financial commitments; or
 
    - lower revenues than expected under our collaboration agreements.
 
    Additional funding may not be available to us on favorable terms, or at all.
We may raise additional funds through public or private financings,
collaborative arrangements or other arrangements. Debt financing, if available,
may involve covenants which could restrict our business activities. If we are
unable to raise additional funds through equity or debt financing when needed,
we may be required to delay, scale back or eliminate expenditures for some of
our development programs or grant rights to develop and market product
candidates that we would otherwise prefer to internally develop and market. If
we are required to grant such rights, the ultimate value of these product
candidates to us may be reduced.
 
FLUCTUATIONS IN OUR QUARTERLY REVENUE AND OPERATING RESULTS MAY CAUSE OUR STOCK
PRICE TO DECLINE.
 
    Our operating results have fluctuated in the past and are likely to continue
to do so in the future. Our revenue is unpredictable and may fluctuate due to
the timing of non-recurring licensing fees, reimbursement for manufacturing
services, the achievement of milestones and our receipt of the related milestone
payments under new and existing licensing and collaboration agreements. Revenue
historically recognized under our prior collaboration agreements may not be an
indicator of revenue from any future collaborations. In addition, our expenses
are unpredictable and may fluctuate from quarter-to-quarter due to the timing of
expenses, which may include obligations to manufacture or supply product or
payments owed by us under licensing or collaboration agreements. It is possible
that in the future, our quarterly operating results will not meet the
expectations of securities analysts or investors, causing the market price of
our common stock to decline. We believe that quarter-to-quarter comparisons of
our operating results are not a good indicator of our future performance and
should not be relied upon to predict the future performance of our stock price.
 
WE DO NOT INTEND TO PAY CASH DIVIDENDS ON OUR COMMON STOCK.
 
    We have not paid cash dividends since our inception and do not intend to pay
cash dividends in the foreseeable future. Therefore, you will have to rely on
appreciation in our stock price in order to achieve a gain on your investment.
 
                                       22
<Page>

I
TEM 2.  PROPERTIES
 
    We lease approximately 37,700 square feet of laboratory and office space in
Cambridge, Massachusetts. The Cambridge lease expires on March 31, 2003. We also
lease approximately 30,750 square feet of space in Norwood, Massachusetts, which
serves as the Company's pilot manufacturing facility as well as other office
space. The Norwood lease expires on June 30, 2008. We believe that the
manufacturing portion of the Norwood facility complies with all applicable FDA
Current Good Manufacturing Practice Regulations.
 

ITEM 3.  LEGAL PROCEEDINGS
 
    The Company is not a party to any material legal proceedings.
 

ITEM 4.  SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS
 
    No matters were submitted to a vote of our security holders through
soliciation of proxies or otherwise during the last quarter of the fiscal year
ended June 30, 2001.
 
                                       23
<Page>

                                    PART II
 

ITEM 5.  MARKET FOR REGISTRANT'S COMMON EQUITY AND RELATED STOCKHOLDER MATTERS
 
    ImmunoGen's Common Stock is quoted on The Nasdaq National Market under the
symbol IMGN. The table below sets forth the high and low sale prices on the
Nasdaq National Market for our Common Stock for each of the quarters indicated.
 

<Table>
<Caption>
                                            FISCAL YEAR 2001      FISCAL YEAR 2000
                                           -------------------   -------------------
                                             HIGH       LOW        HIGH       LOW
                                           --------   --------   --------   --------
<S>                                        <C>        <C>        <C>        <C>
First Quarter............................  $36.375    $ 9.500    $ 3.063     $1.750
Second Quarter...........................   45.500     17.000      6.000      2.000
Third Quarter............................   24.250      8.875      20.50      4.500
Fourth Quarter...........................   20.820     10.750     14.250      6.625
</Table>

 
    As of September 14, 2001, there were approximately 592 holders of record of
the Company's Common Stock and, according to the Company's estimates,
approximately 24,700 beneficial owners of the Company's Common Stock.
 
    The Company has not paid any cash dividends on its Common Stock since its
inception and does not intend to pay any cash dividends in the foreseeable
future.
 
    On September 7, 2000, in connection with a collaboration agreement entered
into between Abgenix and the Company, Abgenix purchased 789,473 shares of the
Company's common stock at a purchase price of $19.00 per share from the Company
in a private placement exempt from registration under Section 4(2) of the
Securities Act of 1933, as amended. No underwriter or placement agent was used
in connection with this sale, and no commissions were paid to any party in
connection with the sale.
 

ITEM 6.  SELECTED FINANCIAL DATA
 
    The following table sets forth consolidated financial data with respect to
the Company for each of the five years in the period ended June 30, 2001. The
information set forth below should be read in conjunction with "Management's
Discussion and Analysis of Financial Condition and Results of
 
                                       24
<Page>
Operations" and the consolidated financial statements and related notes included
elsewhere in this report on Form 10-K.
 

<Table>
<Caption>
                                                                           YEAR ENDED JUNE 30,
                                                   -------------------------------------------------------------------
                                                      1997          1998          1999          2000          2001
                                                   -----------   -----------   -----------   -----------   -----------
                                                       IN THOUSANDS, EXCEPT PER SHARE DATA AND SHARES OUTSTANDING
<S>                                                <C>           <C>           <C>           <C>           <C>
STATEMENT OF OPERATIONS DATA:
 
Total revenues...................................  $       421   $       307   $     3,401   $    11,181   $     4,479
Total expense excluding in-process research and
  development expense............................        9,632         7,477         7,874        11,924        20,291
In-process research and development expense......           --           872            --            --            --
Non-operating income.............................          130           271           297           430         6,339
Non-cash dividends and other expenses............        3,512           605           918            --            83
Minority interest................................           --           160           101            76            --
Net loss to common stockholders before cumulative
  effect of a change in accounting principle.....      (12,595)       (8,216)       (4,993)         (238)       (9,556)
Cumulative effect of a change in accounting
  principle......................................           --            --            --            --        (5,734)
Net loss to common stockholders..................      (12,595)       (8,216)       (4,993)         (238)      (15,291)
Basic and diluted loss per common share..........        (0.70)        (0.34)        (0.20)        (0.01)        (0.42)
Weighted average common shares outstanding.......   17,930,164    24,210,340    25,525,061    29,520,576    36,675,324
 
PRO FORMA AMOUNTS ASSUMING SAB 101 FOLLOWED SINCE
  INCEPTION:
 
Total revenues...................................          421           307         2,471         6,320         4,479
Net loss to common stockholders..................      (12,595)       (8,216)       (5,923)       (5,098)       (9,556)
Basic and diluted loss per common share..........        (0.70)        (0.34)        (0.23)        (0.17)        (0.26)
 
CONSOLIDATED BALANCE SHEET DATA:
 
Total assets.....................................        6,350         5,877         7,171        19,344       159,161
Long-term debt and capital lease obligations,
  less current portion...........................           59            35            68             8            --
Stockholders' equity.............................        4,462         4,311         5,329        10,508       142,447
</Table>

 

ITEM 7.  MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
  OF OPERATIONS
 
OVERVIEW
 
    Since our inception, we have been principally engaged in the development of
antibody-based cancer therapeutics. Our product candidates, TAPs, consist of an
antibody chemically linked, or conjugated, to a highly potent cell-killing, or
cytotoxic agent which is delivered directly to the tumor cell where it bonds to
and is internalized by the tumor cell. Once internalized, the cytotoxic agent
kills the tumor cell. The cytotoxic agent we currently use in all of our TAPs is
maytansinoid, a chemical derivation of a naturally occurring substance called
maytansine.
 
    We have entered into collaborative agreements that allow companies to use
our TAP technology to develop commercial products with antibodies. We also have
licensed certain rights to our first two internally-developed TAP product
candidates to companies that have product development and commercialization
capabilities we wish to access in exchange for fees, milestone payments and
royalties on product sales. Our collaborative partners include GlaxoSmithKline,
Genentech, Abgenix, British Biotech, Millennium, MorphoSys, Genzyme Transgenics,
Avalon and Raven. We expect that substantially all of our revenue for the
foreseeable future will result from payments under our collaborative
arrangements. The terms of the collaborative agreements vary, reflecting the
value we add to the development of any particular product candidate.
 
    To date, we have not generated revenues from commercial product sales and we
expect to incur significant operating losses over the foreseeable future. As of
June 30, 2001, we had approximately $151.0 million in cash and short-term and
long-term investments. We do not anticipate having a commercially-approved
product within the foreseeable future. Research and development expenses are
expected to increase significantly in the near term as we continue our
development efforts. Moreover,
 
                                       25
<Page>
in the next twelve to eighteen months we expect to spend approximately
$4.4 million to further expand our development and pilot manufacturing facility
in Norwood, Massachusetts. We anticipate that the increase in total cash
expenditures will be partially offset by collaboration-derived proceeds.
Accordingly, period-to-period operational results may fluctuate dramatically. We
believe that our established collaborative agreements, while subject to
specified milestone achievements, will provide funding sufficient to allow us to
meet our obligations under all collaborative agreements while also allowing the
aggressive development of internal product candidates and technologies. However,
we can give no assurances that such collaborative agreement funding will, in
fact, be realized. Should we not meet some or all of the terms and conditions of
our various collaboration agreements, we may be required to pursue additional
strategic partners, secure alternative financing arrangements, and/or defer or
limit some or all of our research, development and/or clinical projects.
 
RESULTS OF OPERATIONS
 
    REVENUES
 
    Prior to June 30, 2000, we recognized collaboration revenue on up-front,
non-refundable license payments upon receipt and milestone payments upon
achievement of the milestone and when collection was probable. Revenues
recognized were based on the collaboration agreement milestone value and the
relationship of costs incurred to our estimates of total cost expected to
complete that milestone.
 
    Effective July 1, 2000, we changed our method of accounting for revenue
recognition in accordance with Staff Accounting Bulletin No. 101, "Revenue
Recognition in Financial Statements" (SAB 101). Under the new accounting method,
adopted retroactively to July 1, 2000, we recognize revenue from non-refundable,
up-front license payments, not specifically tied to a separate earnings process,
ratably over the term of the research contract. The cumulative effect of the
change in accounting on prior years resulted in a non-cash charge to income of
$5.7 million, which is included in our net loss to common stockholders for the
year ended June 30, 2001.
 
    For all periods presented, when milestone payments are specifically tied to
a separate earnings process, revenue is recognized when the milestone is
achieved. In addition, when appropriate, we recognize revenue from certain
research payments based upon the level of research services performed during the
period of the research contract. Deferred revenue represents amounts received
under collaborative agreements and not yet earned pursuant to these policies.
Where we have no continuing involvement, we will record non-refundable license
fees as revenue upon receipt and will record milestone revenue upon achievement
of the milestone by the collaborative partner.
 
    The following discussions relating to revenue for the years ended June 30,
2001 (2001), June 30, 2000 (2000) and June 30, 1999 (1999) reflect pro forma
results as if we had followed SAB 101 from our inception.
 
    Our total revenues for the year ended June 30, 2001 were $4.5 million,
compared with $6.3 million for the year ended June 30, 2000 and $2.5 million for
the year ended June 30, 1999. The 29% decrease in revenues from 2000 to 2001 is
primarily attributable to milestone payments and access fees we recognized under
the GlaxoSmithKline agreement in 2000. During 2001 we recognized collaboration
revenue of $2.6 million from GlaxoSmithKline, $700,000 from Genentech, $300,000
from Abgenix, and $100,000 from Millennium. During 2000 and 1999, we recognized
$6.2 million and $2.1 million in collaboration revenue, respectively, under the
GlaxoSmithKline agreement. Deferred revenue of $12.9 million as of June 30, 2001
represents progress payments received from collaborators pursuant to contract
revenues not yet earned.
 
    In the years ended June 30, 2000 and 1999, we derived revenues of $4,800 and
$400,000, respectively, from development fees received under the Small Business
Innovation Research (SBIR)
 
                                       26
<Page>
program of the National Cancer Institute. We recognize SBIR revenue when
reimbursable expenses are incurred. We did not derive revenues under this
program during the year ended June 30, 2001.
 
    Revenues for the year ended June 30, 2001 include $597,000 of reimbursements
related to our manufacture of clinical material under certain collaboration
agreements. Under the terms of these agreements, our collaborators reimburse our
fully-burdened cost to manufacture clinical product.
 
    RESEARCH AND DEVELOPMENT EXPENSES
 
    Research and development expenses, which constitute the principal component
of our total operational expenditures (75%, 74% and 77% in 2001, 2000 and 1999,
respectively), were $15.2 million in 2001 as compared to $8.9 million in 2000
and $6.1 million in 1999. The $6.3 million, or 71%, increase from 2000 to 2001
primarily reflects increased costs associated with supporting our ongoing
huC242-DM1/SB-408075 human clinical trials, pre-clinical and development costs
of huN901-DM1/BB-10901 and other TAPs, up-front payments related to the Genzyme
Transgenics, Avalon and Raven collaborations, as well as increased salaries
associated with headcount increases, a calendar year 2000 bonus awarded by the
Board of Directors and estimated fiscal 2001/2002 bonuses that were accrued. The
$2.8 million, or 46%, increase in research and development expenses between 1999
and 2000 was primarily related to increased costs associated with supporting the
huC242-DM1/SB-408075 human clinical trials, as well as the development efforts
related to huN901-DM1/BB-10901 and other TAP product candidates in advance of
their respective human clinical studies. We expect future research and
development expenses to significantly increase in connection with the further
development of our other TAP product candidates, effector molecules and other
technologies.
 
    GENERAL AND ADMINISTRATIVE EXPENSES
 
    General and administrative expenses were $4.5 million in 2001 compared to
$3.1 million in 2000 and $1.8 million in 1999. The approximate $1.4 million, or
46%, increase from 2000 to 2001 was primarily due to increased administrative
and business development personnel costs, increased expenditures associated with
investor relations and business development as well as the calendar year 2000
bonus awarded by the Board of Directors and estimated fiscal 2001/2002 bonuses
that were accrued. We expect future general and administrative expenses to
increase in support of the continued development of our product candidates and
technologies.
 
    INTEREST INCOME
 
    Net interest income was $5.9 million in 2001 compared to $361,000 in 2000
and $242,000 in 1999. The increase in interest income from 2000 to 2001 is
primarily attributable to higher cash and investment balances resulting from our
November 2000 public stock offering, a collaborator investment of $15.0 million
in September 2000, and the receipt of $9.0 million in collaborator milestone
payments during the year ended June 30, 2001.
 
    NON-OPERATING INCOME
 
    Net non-operating income was $464,000 in 2001 compared to $69,000 in 2000
and $55,000 in 1999. In 2001, non-operating income included a settlement in a
securities litigation case filed on our behalf and realized gains on
investments. Non-operating income in 2000 and 1999 was primarily comprised of
prior-period, retroactive favorable insurance rate adjustments as well as gains
on sales of idle assets.
 
    MINORITY INTEREST
 
    Apoptosis Technology, Inc., or ATI, operating losses of $76,000 and $101,000
for fiscal 2000 and 1999, respectively, were allocated to ATI's minority
stockholders within our consolidated financial
 
                                       27
<Page>
statements. ATI ceased operations in July 2000 and generated no losses that were
allocated to the minority stockholders in 2001.
 
    NON-CASH DIVIDENDS
 
    Non-cash dividends of $918,000 recorded in 1999 represented the
Black-Scholes derived fair value of warrants to purchase shares of ImmunoGen
Common Stock issued in connection with the sale of the Company's Series E
Convertible Preferred Stock (Series E Stock).
 
LIQUIDITY AND CAPITAL RESOURCES
 

<Table>
<Caption>
                                                              JUNE 30,
                                                   ------------------------------
                                                     2001       2000       1999
                                                   --------   --------   --------
<S>                                                <C>        <C>        <C>
Cash and short-term investments..................  $ 94,496   $17,329     $4,226
Working capital..................................    94,215    15,324      3,770
Stockholders' equity.............................   142,447    15,368      5,329
</Table>

 
    As of June 30, 2001, we had approximately $94.5 million in cash and
short-term investments. In November 2000, we completed a public offering of
4.0 million shares of our common stock. Net proceeds of the offering were
$124.8 million. We intend to use the net proceeds from the offering for working
capital and general corporate purposes, including research and development.
Since July 1, 2000, we have financed the net cash used to support operating
activities primarily from various collaborative and financing sources. These
sources include milestone revenues earned under our collaboration agreements
with GlaxoSmithKline, Genentech, Abgenix, and Millennium, the sale of equity
securities to Abgenix, the exercise of stock options and warrants to purchase
common stock and income earned on invested assets.
 
    Net cash used in operations during the year ended June 30, 2001 was
$6.4 million compared to net cash provided by operations of $2.4 million in the
year ended June 30, 2000. This increase in operational cash use is largely due
to the increase in operating expenses discussed previously as well as the
increase in clinical materials inventory produced on behalf of our
collaborators. During 2001, we received $9.0 million in up-front and milestone
payments, compared to $11.5 million received in 2000.
 
    Net cash used in investing activities was $122.4 million for the year ended
June 30, 2001, and primarily represents our investment of excess cash in
marketable securities. Capital purchases were $2.4 million for the fiscal year
ended June 30, 2001, and consisted primarily of costs associated with the
build-out of our existing Norwood, Massachusetts, development and pilot
manufacturing facility.
 
    Net cash provided by financing activities increased to $142.2 million for
the year ended June 30, 2001 versus $10.5 million provided by financing
activities for the year ended June 30, 2000. The increase is largely due to our
November 2000 public offering of 4.0 million shares of common stock, the
exercise of 381,342 warrants and 313,928 stock options during the year ended
June 30, 2001 and the September 7, 2000 issuance of 789,473 shares of our common
stock to Abgenix. Our total net proceeds from all common stock issued for the
year ended June 30, 2001 were $142.3 million.
 
    We anticipate that our capital resources will enable us to meet our
operational expenses and capital expenditures for the foreseeable future. We
believe that the proceeds from our November 2000 public stock offering in
addition to our established collaborative agreements will provide funding
sufficient to allow us to meet our obligations under all collaborative
agreements while also allowing us to develop product candidates and technologies
not covered by collaborative agreements. However, we cannot assure you that such
collaborative agreement funding will, in fact, be realized. Should we not meet
some or all of the terms and conditions of our various collaboration agreements,
we may be required to pursue additional strategic partners, secure alternative
financing arrangements, and/or defer or limit some or all of our research,
development and/or clinical projects.
 
                                       28
<Page>
CERTAIN FACTORS THAT MAY AFFECT FUTURE RESULTS OF OPERATIONS
 
    This report contains certain forward-looking statements as that term is
defined in the Private Securities Litigation Reform Act of 1995. Such statements
are based on management's current expectations and are subject to a number of
factors and uncertainties which could cause actual results to differ materially
from those described in the forward-looking statements. We caution investors
that there can be no assurance that actual results or business conditions will
not differ materially from those projected or suggested in such forward-looking
statements as a result of various factors, including, but not limited to, the
following: the uncertainties associated with pre-clinical studies and clinical
trials; the early stage of our initial product development and lack of product
revenues; our history of operating losses and accumulated deficit; our lack of
commercial manufacturing experience and commercial sales, distribution and
marketing capabilities; reliance on suppliers of key materials necessary for
production of our products and technologies; the potential development by
competitors of competing products and technologies; our dependence on existing
and potential collaborative partners, and the lack of assurance that we will
receive any funding under such relationships to develop and maintain strategic
alliances; the lack of assurance regarding patent and other protection for our
proprietary technology; governmental regulation of our activities, facilities,
products and personnel; the dependence on key personnel; uncertainties as to the
extent of reimbursement for the costs of our potential products and related
treatments by government and private health insurers and other organizations;
the potential adverse impact of government-directed health care reform; the risk
of product liability claims; and economic conditions, both generally and those
specifically related to the biotechnology industry. As a result, our future
development efforts involve a high degree of risk. For further information,
refer to the more specific risks and uncertainties discussed throughout this
Annual Report on Form 10-K.
 
    RECENT ACCOUNTING PRONOUNCEMENTS
 
    In June 2001, the Financial Accounting Standards Board, or FASB, issued
Statement of Financial Accounting Standard (SFAS) No. 141, "Business
Combinations" (SFAS No. 141). SFAS No. 141 requires that all business
combinations be accounted for under the purchase method only and that certain
acquired intangible assets in a business combination be recognized as assets
apart from goodwill. SFAS No. 141 is effective for all business combinations
initiated after June 30, 2001 and for all business combinations accounted for by
the purchase method for which the date of acquisition is after June 30, 2001. We
do not believe the adoption of SFAS No. 141 will have a material effect on the
Company's financial position or results of operations.
 
    In June 2001, the FASB issued SFAS No. 142, "Goodwill and Other Intangible
Assets" (SFAS No. 142), which requires that ratable amortization of goodwill be
replaced with periodic tests of the goodwill's impairment and that intangible
assets other than goodwill be amortized over their useful lives. The provisions
of SFAS No. 142 will be effective for fiscal years beginning after December 15,
2001, and will thus be adopted by the Company, as required, on July 1, 2002. We
do not believe the adoption of SFAS No. 142 will have a material effect on the
Company's financial position or results of operations.
 

I
TEM 7A.  QUANTITATIVE AND QUALITATIVE DISCLOSURE ABOUT MARKET RISK
 
    We maintain an investment portfolio in accordance with our Investment
Policy. The primary objectives of our Investment Policy are to preserve
principal, maintain proper liquidity to meet operating needs and maximize
yields. Although our investments are subject to credit risk, our Investment
Policy specifies credit quality standards for our investments and limits the
amount of credit exposure from any single issue, issuer or type of investment.
Our investments are also subject to interest rate risk and will decrease in
value if market interest rates increase. However, due to the conservative nature
of our investments and relatively short duration, interest rate risk is
mitigated. We do not own derivative financial instruments in our investment
portfolio.
 
    Accordingly, we do not believe that there is any material market risk
exposure with respect to derivative or other financial instruments which would
require disclosure under this item.
 
                                       29
<Page>

ITEM 8.   FINANCIAL STATEMENTS AND SUPPLEMENTARY DATA
 
                   INDEX TO CONSOLIDATED FINANCIAL STATEMENTS
 

<Table>
<Caption>
                                                              PAGE(S)
                                                              --------
<S>                                                           <C>
Report of Independent Accountants...........................     31
Consolidated Financial Statements:
  Consolidated Balance Sheets as of June 30, 2001 and
    2000....................................................     32
  Consolidated Statements of Operations for the Years Ended
    June 30, 2001, 2000, and 1999...........................     33
  Consolidated Statements of Stockholders' Equity for the
    Years Ended June 30, 2001, 2000, and 1999...............     34
  Consolidated Statements of Cash Flows for the Years Ended
    June 30, 2001, 2000, and 1999...........................     35
  Notes to Consolidated Financial Statements................     36

</Table>

 
                                       30
<Page>

                       REPORT OF INDEPENDENT ACCOUNTANTS
 
To the Board of Directors and Stockholders of ImmunoGen, Inc.:
 
    In our opinion, the accompanying consolidated balance sheets and the related
consolidated statements of operations, stockholders' equity and cash flows
present fairly, in all material respects, the financial position of
ImmunoGen, Inc. (the Company) at June 30, 2001 and 2000, and the results of its
operations and its cash flows for each of the three years in the period ended
June 30, 2001, in conformity with accounting principles generally accepted in
the United States of America. These financial statements are the responsibility
of the Company's management; our responsibility is to express an opinion on
these financial statements based on our audits. We conducted our audits of these
statements in accordance with auditing standards generally accepted in the
United States of America, which require that we plan and perform the audit to
obtain reasonable assurance about whether the financial statements are free of
material misstatement. An audit includes examining, on a test basis, evidence
supporting the amounts and disclosures in the financial statements, assessing
the accounting principles used and significant estimates made by management, and
evaluating the overall financial statement presentation. We believe that our
audits provide a reasonable basis for the opinion expressed above.
 
    As discussed in Note B to the consolidated financial statements, during the
year ended June 30, 2001, the Company changed its method of accounting for
revenue recognition.
 
                                          /s/ PRICEWATERHOUSECOOPERS LLP
 
Boston, Massachusetts
August 14, 2001

 
                                       31
<Page>
                                IMMUNOGEN, INC.
 
                          CONSOLIDATED BALANCE SHEETS
 

<Table>
<Caption>
                                                                        JUNE 30,
                                                              -----------------------------
                                                                  2001            2000
                                                              -------------   -------------
<S>                                                           <C>             <C>
                                          ASSETS
 
Cash and cash equivalents...................................  $  14,822,519   $   1,408,908
Marketable securities.......................................     79,673,934      15,920,484
Due from related parties....................................             --          47,352
Earned and unbilled revenue.................................        693,835              --
Inventory...................................................      2,942,267              --
Prepaid and other current assets............................      1,443,116         415,441
                                                              -------------   -------------
Total current assets........................................     99,575,671      17,792,185
 
Long term marketable securities.............................     56,303,267              --
Property and equipment, net of accumulated depreciation.....      3,238,082       1,508,396
Other assets................................................         43,700          43,700
                                                              -------------   -------------
    Total assets............................................  $ 159,160,720   $  19,344,281
                                                              =============   =============
 
                           LIABILITIES AND STOCKHOLDERS' EQUITY
 
Accounts payable............................................  $     842,927   $     891,419
Accrued compensation........................................        703,036         204,210
Other current accrued liabilities...........................      2,245,874         987,475
Current portion of capital lease obligations................          8,137          60,083
Current portion of deferred revenue.........................      1,560,865         325,000
                                                              -------------   -------------
Total current liabilities...................................      5,360,839       2,468,187
 
Capital lease obligations...................................             --           8,137
Deferred revenue............................................     11,353,115       1,500,000
                                                              -------------   -------------
    Total liabilities.......................................     16,713,954       3,976,324
 
Commitments and contingencies (Note K)
 
Stockholders' equity:
Common stock, $.01 par value; authorized 50,000,000 shares
  as of June 30, 2001 and 2000; issued and outstanding
  38,535,402 shares and 33,050,659 shares as of June 30,
  2001 and 2000, respectively...............................        385,354         330,507
Additional paid-in capital..................................    310,971,161     168,682,991
Accumulated deficit.........................................   (169,246,607)   (153,955,925)
Accumulated other comprehensive income......................        336,858         310,384
                                                              -------------   -------------
    Total stockholders' equity..............................    142,446,766      15,367,957
                                                              -------------   -------------
      Total liabilities and stockholders' equity............  $ 159,160,720   $  19,344,281
                                                              =============   =============
</Table>

 
   The accompanying notes are an integral part of the consolidated financial
                                  statements.
 
                                       32
<Page>
                                IMMUNOGEN, INC.
 
                     CONSOLIDATED STATEMENTS OF OPERATIONS
 

<Table>
<Caption>
                                                                  YEAR ENDED JUNE 30,
                                                        ----------------------------------------
                                                            2001          2000          1999
                                                        ------------   -----------   -----------
<S>                                                     <C>            <C>           <C>
Revenues:
  Revenue earned under collaboration agreements.......  $  3,645,498   $11,175,000   $ 3,000,000
  Clinical materials reimbursement....................       597,050            --            --
  Development fees....................................       236,815         4,800       400,105
  Licensing...........................................            --           705         1,158
                                                        ------------   -----------   -----------
    Total revenues....................................     4,479,363    11,180,505     3,401,263
 
Expenses:
  Cost of clinical materials reimbursed...............       597,050            --            --
  Research and development............................    15,213,164     8,878,105     6,097,869
  General and administrative..........................     4,481,802     3,046,054     1,776,413
                                                        ------------   -----------   -----------
    Total expenses....................................    20,292,016    11,924,159     7,874,282
 
Net loss from operations..............................   (15,812,653)     (743,654)   (4,473,019)
 
  Gain/(loss) on the sale of assets...................        (1,900)       19,538         4,200
  Interest income, net................................     5,874,975       361,173       241,657
  Realized gains on investments.......................       132,766            --            --
  Other income........................................       333,208        49,513        51,042
                                                        ------------   -----------   -----------
Net loss before income tax expense, minority interest
  and cumulative effect of change in accounting
  principle...........................................    (9,473,604)     (313,430)   (4,176,120)
  Income tax expense..................................        82,600            --            --
                                                        ------------   -----------   -----------
Net loss before minority interest and cumulative
  effect of change in accounting principle............    (9,556,204)     (313,430)   (4,176,120)
  Minority interest in net loss of consolidated
    subsidiary........................................            --        75,870       101,160
                                                        ------------   -----------   -----------
Net loss before cumulative effect of change in
  accounting principle................................    (9,556,204)     (237,560)   (4,074,960)
  Cumulative effect of change in accounting
    principle.........................................    (5,734,478)           --            --
                                                        ------------   -----------   -----------
Net loss..............................................   (15,290,682)     (237,560)   (4,074,960)
  Non-cash dividends on convertible preferred stock...            --            --      (917,583)
                                                        ------------   -----------   -----------
Net loss to common stockholders.......................  $(15,290,682)  $  (237,560)  $(4,992,543)
                                                        ============   ===========   ===========
Basic and diluted net loss per common share before
  cumulative effect of change in accounting
  principle...........................................  $      (0.26)  $     (0.01)  $     (0.20)
Cumulative effect of change in accounting
  principle--basic and diluted........................         (0.16)  $        --   $        --
                                                        ------------   -----------   -----------
 
Basic and diluted net loss per common share...........  $      (0.42)  $     (0.01)  $     (0.20)
                                                        ============   ===========   ===========
Basic and diluted average common shares outstanding...    36,675,324    29,520,576    25,525,061
                                                        ============   ===========   ===========
</Table>

 
   The accompanying notes are an integral part of the consolidated financial
                                  statements.
 
                                       33
<Page>
                                IMMUNOGEN, INC.
 
                CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY

<Table>
<Caption>
                                                                                                                   ACCUMULATED
                                                                                                                      OTHER
                                         COMMON STOCK          PREFERRED STOCK      ADDITIONAL                    COMPREHENSIVE
                                     ---------------------   -------------------     PAID-IN       ACCUMULATED        INCOME
                                       SHARES      AMOUNT     SHARES     AMOUNT      CAPITAL         DEFICIT          (LOSS)
                                     ----------   --------   --------   --------   ------------   -------------   --------------
<S>                                  <C>          <C>        <C>        <C>        <C>            <C>             <C>
Balance at June 30, 1998...........  25,419,552   $254,195     1,200      $ 12     $152,782,585   $(148,725,822)   $         --
                                                                                                                             --
Stock options exercised............     174,245      1,742        --        --          313,545              --              --
Issuance of Series E Convertible
  Preferred Stock, net of financing
  costs............................          --         --     1,200        12        1,495,193              --              --
Issuance of common stock in
  exchange for Series E Preferred
  Stock placement services.........      75,000        750        --        --             (750)             --              --
Value of common stock purchase
  warrants issued..................          --         --        --        --          917,583              --              --
Compensation for stock option
  vesting acceleration for retired
  director.........................          --         --        --        --           13,275              --              --
Value ascribed to ImmunoGen
  warrants issued to BioChem
  Pharma, Inc., net of financing
  costs............................          --         --        --        --        3,269,390              --              --
Non-cash dividends on convertible
  preferred stock..................          --         --        --        --               --        (917,583)             --
Net loss for the year ended
  June 30, 1999....................          --         --        --        --               --      (4,074,960)             --
                                     ----------   --------    ------      ----     ------------   -------------    ------------
Balance at June 30, 1999...........  25,668,797   $256,687     2,400      $ 24     $158,790,821   $(153,718,365)   $         --
                                     ----------   --------    ------      ----     ------------   -------------    ------------
Unrealized gains on marketable
  securities, net..................          --         --        --        --               --              --         310,384
Net loss for the year ended
  June 30, 2000....................          --         --        --        --               --        (237,560)             --
Comprehensive income...............          --         --        --        --               --              --              --
Stock options exercised............     131,567      1,316        --        --          219,192              --              --
Exercise of put option.............   1,023,039     10,231        --        --        2,489,769              --              --
Warrants exercised.................   3,403,728     34,037        --        --        4,408,575              --              --
Conversion of Series E Convertible
  Preferred Stock into common
  stock............................   2,823,528     28,236    (2,400)      (24)         (28,212)             --              --
Compensation for stock option
  vesting acceleration for
  terminated officer...............          --         --        --        --          349,716              --              --
Value ascribed to ImmunoGen
  warrants issued to BioChem
  Pharma, Inc., net of financing
  costs............................          --         --        --        --        2,453,130              --              --
                                     ----------   --------    ------      ----     ------------   -------------    ------------
Balance at June 30, 2000...........  33,050,659   $330,507        --      $ --     $168,682,991   $(153,955,925)   $    310,384
                                     ----------   --------    ------      ----     ------------   -------------    ------------
Unrealized gain on marketable
  securities, net..................          --         --        --        --               --              --          26,474
Net loss for the year ended
  June 30, 2001....................          --         --        --        --               --     (15,290,682)
Comprehensive loss.................          --         --        --        --               --                              --
Stock options exercised............     313,928      3,139        --        --          772,741              --              --
Warrants exercised.................     381,342      3,813        --        --        1,706,735              --              --
Issuance of common stock to
  Abgenix, Inc.....................     789,473      7,895        --        --       14,992,105              --              --
Issuance of common stock to public,
  net of financing costs...........   4,000,000     40,000        --        --      124,736,202              --              --
Compensation for stock options.....          --         --        --        --           80,387              --              --
                                     ----------   --------    ------      ----     ------------   -------------    ------------
Balance at June 30, 2001...........  38,535,402   $385,354        --      $ --     $310,971,161   $(169,246,607)   $    336,858
                                     ==========   ========    ======      ====     ============   =============    ============
 
<Caption>
 
                                                          TOTAL
                                     COMPREHENSIVE    STOCKHOLDERS'
                                         INCOME          EQUITY
                                     --------------   -------------
<S>                                  <C>              <C>
Balance at June 30, 1998...........   $         --    $  4,310,970
                                                --
Stock options exercised............             --         315,287
Issuance of Series E Convertible
  Preferred Stock, net of financing
  costs............................             --       1,495,205
Issuance of common stock in
  exchange for Series E Preferred
  Stock placement services.........             --              --
Value of common stock purchase
  warrants issued..................             --         917,583
Compensation for stock option
  vesting acceleration for retired
  director.........................             --          13,275
Value ascribed to ImmunoGen
  warrants issued to BioChem
  Pharma, Inc., net of financing
  costs............................             --       3,269,390
Non-cash dividends on convertible
  preferred stock..................             --        (917,583)
Net loss for the year ended
  June 30, 1999....................     (4,074,960)     (4,074,960)
                                      ------------    ------------
Balance at June 30, 1999...........   $         --    $  5,329,167
                                      ------------    ------------
Unrealized gains on marketable
  securities, net..................        310,384         310,384
Net loss for the year ended
  June 30, 2000....................       (237,560)       (237,560)
                                      ------------
Comprehensive income...............   $     72,824              --
                                      ============
Stock options exercised............             --         220,508
Exercise of put option.............             --       2,500,000
Warrants exercised.................             --       4,442,612
Conversion of Series E Convertible
  Preferred Stock into common
  stock............................             --              --
Compensation for stock option
  vesting acceleration for
  terminated officer...............             --         349,716
Value ascribed to ImmunoGen
  warrants issued to BioChem
  Pharma, Inc., net of financing
  costs............................             --       2,453,130
                                      ------------    ------------
Balance at June 30, 2000...........   $         --    $ 15,367,957
                                      ------------    ------------
Unrealized gain on marketable
  securities, net..................         26,474          26,474
Net loss for the year ended
  June 30, 2001....................    (15,290,682)    (15,290,682)
                                      ------------
Comprehensive loss.................   $(15,264,208)             --
                                      ============
Stock options exercised............             --         775,880
Warrants exercised.................             --       1,710,548
Issuance of common stock to
  Abgenix, Inc.....................             --      15,000,000
Issuance of common stock to public,
  net of financing costs...........             --     124,776,202
Compensation for stock options.....             --          80,387
                                      ------------    ------------
Balance at June 30, 2001...........   $         --    $142,446,766
                                      ============    ============
</Table>

 
   The accompanying notes are an integral part of the consolidated financial
                                  statements.
 
                                       34
<Page>
                                IMMUNOGEN, INC.
 
                     CONSOLIDATED STATEMENTS OF CASH FLOWS
 

<Table>
<Caption>
                                                                          YEAR ENDED JUNE 30,
                                                              --------------------------------------------
                                                                   2001             2000          1999
                                                              ---------------   ------------   -----------
<S>                                                           <C>               <C>            <C>
Cash flows from operating activities:
  Net loss to common stockholders...........................  $   (15,290,682)  $   (237,560)  $(4,992,543)
  Adjustments to reconcile net loss to net cash used for
    operating activities:
    Cumulative effect on change in accounting principle.....        5,734,478             --            --
    Depreciation and amortization...........................          612,824        498,619       555,357
    (Gain)/loss on sale of property and equipment...........            1,900        (19,539)       (4,200)
    Interest earned on note receivable......................               --             --       (77,362)
    Compensation for stock options..........................           80,387        349,716        13,275
    Non-cash dividend on convertible preferred stock........               --             --       917,583
    Minority interest in net loss of consolidated
      subsidiary............................................               --        (75,870)     (101,160)
    Amortization of deferred lease..........................               --        (35,172)      (52,760)
    Changes in operating assets and liabilities:
      Due from related parties..............................           47,352         19,756         5,365
      Earned and unbilled revenue...........................         (693,835)            --            --
      Inventory.............................................       (2,942,267)            --            --
      Prepaid and other current assets......................       (1,027,675)      (357,526)       (6,555)
      Accounts payable......................................          (48,492)        21,423       170,578
      Accrued compensation..................................          498,826        (78,180)       57,264
      Other current accrued liabilities.....................        1,258,399        458,506            --
      Deferred revenue......................................        5,354,502      1,825,000       (24,277)
                                                              ---------------   ------------   -----------
        Net cash (used for) provided by operating
          activities........................................       (6,414,283)     2,369,173    (3,539,435)
                                                              ---------------   ------------   -----------
 
Cash flows from investing activities:
  Payments received on note receivable......................               --        350,000       960,000
  Purchase of marketable securities.........................   (1,269,265,213)   (20,560,447)           --
  Proceeds from maturities or sales of marketable
    securities..............................................    1,149,234,970      4,950,347            --
  Proceeds from sale of property and equipment..............            7,500         19,795         4,200
  Capital expenditures......................................       (2,351,910)      (423,921)     (120,223)
                                                              ---------------   ------------   -----------
        Net cash (used for) provided by investing
          activities........................................     (122,374,653)   (15,664,226)      843,977
                                                              ---------------   ------------   -----------
 
Cash flows from financing activities:
  Proceeds from common stock issuance, net..................      139,776,202             --            --
  Proceeds from convertible preferred stock issuance, net...               --             --     1,495,205
  Proceeds from issuance of subsidiary convertible preferred
    stock, net..............................................               --      3,372,000     3,370,550
  Proceeds from exercise of put option......................               --      2,500,000            --
  Proceeds from stock options exercised, net................          775,880        220,508       315,287
  Proceeds from warrants exercised, net.....................        1,710,548      4,442,612            --
  Principal payments on capital lease obligations...........          (60,083)       (56,739)       (1,829)
                                                              ---------------   ------------   -----------
        Net cash provided by financing activities...........      142,202,547     10,478,381     5,179,213
                                                              ---------------   ------------   -----------
 
Net change in cash and cash equivalents.....................       13,413,611     (2,816,672)    2,483,755
 
Cash and cash equivalents, beginning balance................        1,408,908      4,225,580     1,741,825
                                                              ---------------   ------------   -----------
 
Cash and cash equivalents, ending balance...................  $    14,822,519   $  1,408,908   $ 4,225,580
                                                              ===============   ============   ===========
 
Supplemental disclosure of non-cash financing activities:
 
  Capital lease obligations assumed on acquired equipment...  $            --   $         --   $   126,788
                                                              ===============   ============   ===========
 
  Due from related party for quarterly investment payment...  $            --   $         --   $   843,000
                                                              ===============   ============   ===========
 
  Cash paid for income taxes................................  $        77,500   $         --   $        --
                                                              ===============   ============   ===========
</Table>

 
   The accompanying notes are an integral part of the consolidated financial
                                  statements.
 
                                       35
<Page>
                                IMMUNOGEN, INC.
 
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
 
A.  NATURE OF BUSINESS AND PLAN OF OPERATIONS
 
    ImmunoGen, Inc. was incorporated in Massachusetts in 1981 to develop,
produce and market commercial anti-cancer and other pharmaceuticals based on
molecular immunology. The Company continues to research and develop its various
products and technologies and does not expect to derive revenue from
commercially-approved product sales within the foreseeable future. It is
anticipated that the Company's existing capital resources, enhanced by
collaborative agreement funding, will enable current and planned operations to
be maintained for the foreseeable future. However, if the Company is unable to
achieve subsequent milestones under its collaborative agreements (see Note C),
the Company may be required to defer or limit some or all of its research,
development and/or clinical projects.
 
    The Company is subject to risks common to companies in the biotechnology
industry including, but not limited to, the development by the Company or its
competitors of new technological innovations, dependence on key personnel,
protection of proprietary technology, manufacturing and marketing limitations,
collaboration arrangements, third-party reimbursements and compliance with
governmental regulations.
 
B.  BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
 
    PRINCIPLES OF CONSOLIDATION
 
    The consolidated financial statements include the accounts of the Company
and its wholly-owned subsidiary, ImmunoGen Securities Corp. (established in
December 1989), and its 97% owned subsidiary Apoptosis Technology, Inc., or ATI
(established in January 1993). All intercompany transactions and balances have
been eliminated.
 
    REVENUE RECOGNITION--CHANGE IN ACCOUNTING PRINCIPLE
 
    The Company enters into licensing and development agreements with
collaborative partners for the development of monoclonal antibody-based cancer
therapeutics. The terms of the agreements typically include nonrefundable
license fees, payments based upon the achievement of certain milestones and
royalties on product sales.
 
    Prior to June 30, 2000, the Company recognized collaboration revenue on
up-front, non-refundable license payments upon receipt and milestone payments
upon achievement of the milestone and when collection was probable. Revenues
recognized were based on the collaboration agreement milestone value and the
relationship of costs incurred to the Company's estimates of total cost expected
to complete that milestone.
 
    Effective July 1, 2000, ImmunoGen changed its method of accounting for
revenue recognition in accordance with Staff Accounting Bulletin No. 101,
"Revenue Recognition in Financial Statements," (SAB 101). Under the new
accounting method, adopted retroactively to July 1, 2000, the Company recognizes
revenue from non-refundable, up-front license payments, not specifically tied to
a separate earnings process, ratably over the term of the research contract. The
cumulative effect of the change in accounting on prior years resulted in a
non-cash charge to income of $5.7 million, which is included in the net loss to
common stockholders for the year ended June 30, 2001.
 
    For all periods presented, when milestone payments are specifically tied to
a separate earnings process, revenue is recognized when the milestone is
achieved. In addition, when appropriate, the Company recognizes revenue from
certain research payments based upon the level of research services performed
during the period of the research contract. Deferred revenue represents amounts
received under collaborative agreements not yet earned pursuant to these
policies. Where the Company has no
 
                                       36
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
B.  BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
(CONTINUED)
continuing involvement, it will record non-refundable license fees as revenue
upon receipt and will record milestone revenue upon achievement of the milestone
by the collaborative partner.
 
    Royalty revenue is recognized based upon actual net sales of licensed
products in licensed territories as provided by the collaborative partner and is
generally recognized in the period the sales occur.
 
    Revenue from clinical material reimbursement is recognized upon shipment,
when title to product and associated risk of loss has passed to the customer.
Development revenues of approximately $4,800 and $400,000 in fiscal years 2000
and 1999, respectively, represent income earned, on a cost reimbursement basis,
under the Small Business Innovation Research Program, or SBIR, of the National
Institute of Health and amounts received pursuant to licensing agreements of the
Company and ATI. The Company did not earn any SBIR income during 2001.
 
    INVENTORY
 
    Inventory costs primarily relate to clinical trial materials being
manufactured for our collaborators. Inventory is stated at the lower of cost or
market. At June 30, 2001, approximately $93,500 of general and administrative
costs were allocated to and remained in inventory.
 
    Inventory at June 30, 2001 is summarized below:
 

<Table>
<S>                                                           <C>
Raw materials...............................................  $1,737,620
Work in process.............................................   1,021,902
Finished goods..............................................     182,745
                                                              ----------
Total.......................................................  $2,942,267
                                                              ==========
</Table>

 
    EARNED AND UNBILLED REVENUE
 
    The Company performs certain research and development activities on behalf
of certain collaborative partners. The Company is generally reimbursed at cost,
including allocated overhead. The Company recognizes revenue as the activities
are performed, but bills the customer at the completion of the effort. Amounts
earned, but not billed to the customer at period-end are classified as earned
and unbilled revenue in the accompanying balance sheet.
 
    USE OF ESTIMATES
 
    The preparation of financial statements in conformity with accounting
principles generally accepted in the United States of America requires
management to make estimates and assumptions that affect the reported amounts of
assets and liabilities and disclosure of contingent assets and liabilities at
the date of the financial statements and the reported amounts of revenues and
expenses during the reporting period. Actual results could differ from those
estimates.
 
    COST OF CLINICAL MATERIALS REIMBURSED
 
    The Company manufactures pre-clinical and clinical materials for certain of
its collaborative partners. The Company is generally reimbursed at cost,
including allocated overhead, for this manufacturing of pre-clinical and
clinical materials. When the pre-clinical and clinical materials are shipped and
title to the product and risk of loss has transferred to the Company's
collaborative partner,
 
                                       37
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
B.  BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
(CONTINUED)
the Company records revenue and the associated cost of manufacturing the
pre-clinical and clinical materials. In the accompanying Statement of
Operations, the cost of manufacturing pre-clinical and clinical materials
shipped to collaborative partners is reported as Cost of Clinical Materials
Reimbursed.
 
    RESEARCH AND DEVELOPMENT COSTS
 
    Research and development costs are expensed as incurred.
 
    INCOME TAXES
 
    The company uses the liability method to account for income taxes. Deferred
tax assets and liabilities are determined based on differences between the
financial reporting and income tax basis of assets and liabilities, as well as
net operating loss carryforwards and tax credits and are measured using the
enacted tax rates and laws that will be in effect when the differences reverse.
A valuation allowance against net deferred tax assets is recorded if, based on
the available evidence, it is more likely than not that some or all of the
deferred tax assets will not be realized.
 
    FINANCIAL INSTRUMENTS AND CONCENTRATION OF CREDIT RISK
 
    The Company has no significant off-balance sheet concentration of credit
risk such as foreign exchange contracts, option contracts or other foreign
hedging arrangements. Cash and cash equivalents are primarily maintained with
two financial institutions in the United States. Deposits with banks may exceed
the amount of insurance provided on such deposits. Generally, these deposits may
be redeemed upon demand and, therefore, bear minimal risk. Financial instruments
that potentially subject the Company to concentrations of credit risk consist
principally of marketable securities. Marketable securities consist of United
States Treasury bonds, high-grade corporate bonds, asset-backed and United
States government agency securities. The Company's investment policy, approved
by the Board of Directors, limits the amount it may invest in any one type of
investment, thereby reducing credit risk concentrations.
 
    CASH AND CASH EQUIVALENTS
 
    The Company considers all investments purchased to be marketable securities.
Cash and cash equivalents include money market funds and cash at June 30, 2001
and 2000.
 
    MARKETABLE SECURITIES
 
    In accordance with our investment policy, surplus cash is invested in
investment-grade corporate and United States Government debt and asset-backed
securities typically with maturity dates of less than one year. We determine the
appropriate classification of marketable securities at each balance sheet date.
Marketable securities are classified as available for sale and are carried at
fair value based on quoted market prices. Unrealized gains and losses are
reported net, as comprehensive income, within stockholders' equity. The cost of
debt securities is adjusted for amortization of premiums and accretion of
discounts to maturity with all amortization/accretion included in interest
income.
 
                                       38
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
B.  BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
(CONTINUED)
    PROPERTY AND EQUIPMENT
 
    Property and equipment are stated at cost. The Company provides for
depreciation based upon expected useful lives using the straight-line method
over the following estimated useful lives:
 

<Table>
<S>                                                <C>
Machinery and equipment..........................  3-5 years
Computer hardware and software...................  3-5 years
Furniture and fixtures...........................  5 years
Leasehold improvements...........................  Shorter of lease term or
                                                     estimated useful life
</Table>

 
    Maintenance and repairs are charged to expense as incurred. Upon retirement
or sale, the cost of disposed assets and the related accumulated depreciation
are removed from the accounts and any resulting gain or loss is credited or
charged to non-operating income. Gains recorded under sale/ leaseback
arrangements are deferred and amortized to operations over the life of the
lease.
 
    IMPAIRMENT OF LONG-LIVED ASSETS
 
    The Company periodically evaluates the potential impairment of its
long-lived assets whenever events or changes in circumstances indicate that the
carrying amount of an asset may not be recoverable. At the occurrence of a
certain event or change in circumstances, the Company evaluates the potential
impairment of an asset based on estimated future undiscounted cash flows. In the
event impairment exists, the Company will measure the amount of such impairment
based on the present value of estimated future cash flows using a discount rate
commensurate with the risks involved. Based on management's assessment as of
June 30, 2001, the Company determined that no impairment of long-lived assets
exists.
 
    DEBT AND EQUITY INSTRUMENTS ISSUED WITH PROVISIONS FOR CONVERSION INTO
     COMMON STOCK AT A DISCOUNT TO THE MARKET PRICE OF COMMON STOCK
 
    The value of discounts inherent in convertible instruments issued with
provisions for conversion into common stock at a discount to the market price of
common stock or the value of any warrants issued in connection with those
instruments, is calculated as of the date of issuance of the convertible
securities as either dividends to preferred shareholders or as interest to
debtholders. The calculated value of the discount is amortized over the period
in which the discount is earned. In certain instances, the number and/or
exercise prices of warrants to be issued are tied to the market price of the
common stock at a future date (the future price). Therefore, the number of
warrants to be issued and/or the exercise price of those warrants is not readily
determinable at the date of issuance, when the value is required to be
calculated. In those instances, for warrant valuation purposes, the Company
assumes that the future price is equal to the quoted market price of the common
stock on the date of issuance. Accordingly, upon conversion, actual numbers
and/or prices may differ from original estimates.
 
    STOCK-BASED COMPENSATION
 
    In accounting for its stock-based compensation plans, the Company applies
Accounting Principles Board Opinion No. 25, "Accounting for Stock Issued to
Employees" (APB 25), and related interpretations for all awards granted to
employees. Under APB 25, when the exercise price of options granted to employees
under these plans equals the market price of the common stock on the date of
 
                                       39
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
B.  BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
(CONTINUED)
grant, no compensation expense is recorded. When the exercise price of options
granted to employees under these plans is less than the market price of the
common stock on the date of grant, compensation expense is recognized over the
vesting period. For stock options granted to non-employees, the Company
recognizes compensation expense in accordance with the requirements of Statement
of Financial Accounting Standards (SFAS) No. 123, "Accounting for Stock Based
Compensation" (SFAS 123). SFAS 123 requires that companies recognize
compensation expense for grants of stock, stock options and other equity
instruments based on fair value.
 
    SEGMENT INFORMATION
 
    The Company is in one business segment under the management approach, the
business of discovery of monoclonal antibody-based cancer therapeutics.
 
    RECENT ACCOUNTING PRONOUNCEMENTS
 
    In June 2001, the Financial Accounting Standards Board (FASB) issued
SFAS No. 141, "Business Combinations" (SFAS No. 141). SFAS No. 141 requires that
all business combinations be accounted for under the purchase method only and
that certain acquired intangible assets in a business combination be recognized
as assets apart from goodwill. SFAS No. 141 is effective for all business
combinations initiated after June 30, 2001 and for all business combinations
accounted for by the purchase method for which the date of acquisition is after
June 30, 2001. Management does not believe the adoption of SFAS No. 141 will
have a material effect on the Company's financial position or results of
operations.
 
    In June 2001, the FASB issued SFAS No. 142, "Goodwill and Other Intangible
Assets" (SFAS No. 142), which requires that ratable amortization of goodwill be
replaced with periodic tests of the goodwill's impairment and that intangible
assets other than goodwill be amortized over their useful lives. The provisions
of SFAS No. 142 will be effective for fiscal years beginning after December 15,
2001, and will thus be adopted by the Company, as required, on July 1, 2002.
Management does not believe the adoption of SFAS No. 142 will have a material
effect on the Company's financial position or results of operations.
 
    RECLASSIFICATIONS
 
    Certain prior year balances have been reclassified to conform with current
year presentation.
 
C.  AGREEMENTS
 
    As discussed further in Note B, effective June 30, 2001, the Company adopted
SAB 101. The following descriptions relating to revenue recognized under the
Company's collaborative agreements reflect the effects of the adoption of
SAB 101.
 
    In February 1999, the Company entered into an exclusive license agreement
with SmithKline Beecham plc, London, England and SmithKline Beecham,
Philadelphia, Pennsylvania, wholly-owned subsidiaries of GlaxoSmithKline plc, to
develop and commercialize the Company's lead TAP, huC242-DM1/SB-408075 for the
treatment of colorectal, pancreatic and certain non-small lung cancers. Under
the terms of the agreement, the Company could receive up to $41.5 million,
subject to the achievement by the Company and/or GlaxoSmithKline of certain
development milestones. The Company is also entitled to receive royalty payments
on future product sales, if and when they commence. Finally, at the Company's
option, and subject to certain conditions, GlaxoSmithKline will
 
                                       40
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
C.  AGREEMENTS (CONTINUED)
purchase up to $5.0 million of its Common Stock. Between the signing of the
agreement and June 30, 2001, GlaxoSmithKline had purchased, pursuant to
ImmunoGen's put option, $2.5 million of the Company's common stock.
 
    The GlaxoSmithKline agreement is expected to provide the Company with
sufficient cash funding to carry out its responsibilities in developing
huC242-DM1/SB-408075. All costs subsequent to the Phase I clinical studies will
be the responsibility of GlaxoSmithKline.
 
    As of June 30, 2001, the Company had received an up-front fee of
$1.0 million and four milestones totaling $10.5 million under the
GlaxoSmithKline agreement. The up-front fee was deferred and is being recognized
ratably over the Company's period of involvement during development. All of the
milestones have been recorded as collaboration revenue, with the exception of
$10,000 of the fourth milestone, which has been recorded as deferred revenue
until such time as the remaining ongoing commitments associated with this
milestone have been satisfied.
 
    In May 2000, the Company executed two separate licensing agreements with
Genentech. The first agreement grants an exclusive license to Genentech for
ImmunoGen's TAP technology for use with antibodies such as
Herceptin-Registered Trademark-. Under the terms of the agreement, Genentech
will receive exclusive worldwide rights to commercialize anti-HER2 targeting
products using ImmunoGen's TAP platform. Genentech will be responsible for
manufacturing, product development and marketing of any products resulting from
the agreement; ImmunoGen will be reimbursed for any pre-clinical and clinical
materials that it manufactures under the agreement. ImmunoGen received a
$2.0 million non-refundable payment for execution of the agreement. The up-front
fee was deferred and is being recognized ratably over the Company's period of
involvement during development. In addition to royalties on net sales, the terms
of the agreement include certain other payments based upon Genentech's
achievement of milestones. Assuming all benchmarks are met, ImmunoGen will
receive approximately $40.0 million.
 
    In addition to the Herceptin-Registered Trademark- agreement described
above, the Company announced in May 2000 that it entered into an additional
agreement with Genentech. This second collaboration provides Genentech with
broad access to ImmunoGen's TAP technology for use with Genentech's other
proprietary antibodies. This multi-year agreement provides Genentech with a
license to utilize ImmunoGen's TAP platform in its antibody product research
efforts and an option to obtain product licenses for a limited number of antigen
targets over the agreement's five-year term. Under this agreement, the Company
received a non-refundable technology access fee of $3.0 million in May 2000. The
up-front fee was deferred and is being recognized ratably over the Company's
period of involvement during development. This agreement also provides for other
payments based on Genentech's achievement of milestones per antigen target, and
royalties on net sales of any resulting products. Assuming all benchmarks are
met, the Company would receive approximately $40.0 million in payments per
antigen target under this agreement. Genentech will be responsible for
manufacturing, product development and marketing of any products developed
through this collaboration; ImmunoGen will be reimbursed for any pre-clinical
and clinical materials that it manufactures under the agreement. The agreement
can be renewed for one subsequent three-year period, for an additional
technology access fee.
 
    Also in May 2000, the Company entered into a development, commercialization
and license agreement with British Biotech to develop and commercialize its
huN901-DM1/BB-10901 TAP for the treatment of small-cell lung cancer. The
agreement grants British Biotech exclusive rights to develop and commercialize
huN901-DM1/BB-10901 in the European Union and Japan. The Company retains
 
                                       41
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
C.  AGREEMENTS (CONTINUED)
the rights to develop and commercialize huN901-DM1/BB-10901 in the United States
and the rest of the world, as well as the right to manufacture the product
worldwide. Under the terms of the agreement, British Biotech will be responsible
for conducting the clinical trials necessary to achieve marketing approval in
the United States, European Union and Japan. ImmunoGen is responsible for the
remaining pre-clinical development, and will be reimbursed for manufacturing the
product for clinical trials. British Biotech paid a fee of $1.5 million for its
territorial rights to huN901-DM1/BB-10901, which has been deferred. Upon
approval of the product for marketing in the United States, the Company will pay
to British Biotech a one-time milestone payment of $3.0 million. ImmunoGen will
receive royalties on sales of huN901-DM1/BB-10901 in the European Union and
Japan, if and when they commence.
 
    In September 2000, the Company entered into a collaboration agreement with
Abgenix. The agreement provides Abgenix with access to the Company's TAP
technology for use with Abgenix's antibodies along with options to obtain
product licenses for antigen targets. The Company received a total of
$5.0 million in technology access fee payments and is entitled to potential
milestone payments and royalties on net sales of any resulting products. At
June 30, 2001, $4.7 million of the technology access fees remained as deferred
revenue to be recognized over the period of the Company's involvement during
development. In addition, on September 7, 2000, Abgenix purchased $15.0 million
of the Company's common stock in accordance with the agreement. Abgenix has the
right to extend its options to obtain product licenses for a specified period of
time for an extension fee. The Company's agreement with Abgenix will terminate
upon expiration of a specified time period during which ImmunoGen has given
Abgenix access to the Company's technology. Either party can terminate the
agreement for any material breach by the other party that remains uncured for a
certain period of time. For the year ended June 30, 2001, the Company recognized
$333,000 of the technology access fees as collaboration revenue.
 
    In September 2000, the Company entered into a collaboration agreement with
MorphoSys. Pursuant to this agreement, MorphoSys will identify fully human
antibodies against a specific cell surface marker that the Company identified
through its apoptosis research and is associated with a number of forms of
cancer. The Company intends to develop products using antibodies generated by
MorphoSys against this marker. The Company expensed and paid MorphoSys an
$825,000 technology access payment, recorded as a research and development
charge, and will pay development-related milestone payments and royalties on net
sales of any resulting products. The Company reimburses MorphoSys for its
research and development efforts related to identifying these antibodies. During
the year ended June 30, 2001, the Company reimbursed Morphosys approximately
$562,000 recorded as a research and development charge. The Company can
terminate this agreement unilaterally at any time and either party can terminate
the agreement for any material breach by the other party that remains uncured
for a certain period of time.
 
    In June 2001, the Company and MorphoSys expanded their existing
collaboration by entering into an agreement that provides ImmunoGen access to
the MorphoSys HuCAL-Registered Trademark- technology. The Company will utilize
this technology for the generation of internal research antibodies. The Company
paid MorphoSys a technology access fee of $300,000 and a license fee of
$300,000, both of which were recorded as research and development charges, and
will pay annual subscription fees during the four-year term of the agreement.
 
    In November 2000, the Company entered into a collaboration agreement with
Genzyme Transgenics. Pursuant to this agreement, Genzyme Transgenics will
produce ImmunoGen's humanized
 
                                       42
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
C.  AGREEMENTS (CONTINUED)
monoclonal antibody, huN901. huN901 is the antibody component of the Company's
TAP huN901-DM1/BB-10901, being developed in collaboration with British Biotech
for treatment of small-cell lung cancer. The Company paid Genzyme Transgenics a
$500,000 project start-up fee, recorded as a research and development charge,
and will pay development-related milestone payments and royalties on net sales
of any resulting products. The Company can terminate this agreement unilaterally
at any time and either party can terminate the agreement for any material breach
by the other party that remains uncured for a certain period of time.
 
    In January 2001, the Company entered into a collaboration agreement with
Avalon. Pursuant to the agreement, Avalon will provide gene targets to
ImmunoGen. The Company will be responsible for the development, manufacture and
commercialization of any resulting products. The Company paid Avalon an up-front
fee that was recorded as a research and development charge. Either party can
terminate the agreement for any material breach by the other party that remains
uncured for a certain period of time.
 
    In March 2001, the Company entered into a five-year collaboration agreement
with Millennium. The agreement provides Millennium access to the Company's TAP
technology for use with Millennium's proprietary antibodies. Millennium acquired
a license to utilize the TAP technology in its antibody product research efforts
and an option to obtain product licenses for a restricted number of antigen
targets during the collaboration. The Company received an up-front fee of
$2.0 million in the third quarter of 2001. The full amount of the up-front fee
was deferred and will be recorded as revenue as it is earned over the
development period. This agreement also provides for certain other payments
based on Millennium's achievement of milestones. Assuming all benchmarks are
met, the Company could receive more than $40.0 million per antigen target. The
Company will also receive royalties on net sales of any resulting products.
Millennium will be responsible for manufacturing, product development and
marketing of any products developed through this collaboration; the Company will
be reimbursed for any pre-clinical and clinical materials that it makes under
the agreement. The agreement can be renewed for one subsequent three-year
period, for an additional technology access fee.
 
    Also in March 2001, the Company and Raven entered into a collaboration aimed
at identifying targets and therapeutic antibodies with the potential to treat
ovarian cancer. Raven will discover and provide to the Company's cell surface
targets and monoclonal antibodies. We will use these targets and antibodies to
develop therapeutic products. We will have the development, manufacturing and
commercialization rights to these products in North America and Europe in
exchange for an up-front licensing fee, research support, milestones and
royalties. Research and development expense for the year ended June 30, 2001,
include the up-front licensing fee and quarterly research support paid to Raven.
 
    In June 2001, ImmunoGen and BioInvent International AB entered into a
monoclonal antibody supply agreement. Under the terms of the agreement,
BioInvent will perform process qualification and Current Good Manufacturing
Practices manufacturing of one of the Company's monoclonal antibodies. Under the
terms of the agreement, the Company pays a stated price per gram of antibody,
adjustable based upon production volumes. The Company prepaid $265,000 and
$517,000 upon signing of the letter of intent and the signing of the agreement,
respectively. These payments have been recorded as inventory in the accompanying
balance sheet. The Company anticipates that the antibody will be used in
producing clinical product on behalf of a collaborator, and will be included in
the Cost of Clinical Materials Reimbursed when it is shipped and invoiced.
 
                                       43
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
D.  COMPUTATION OF LOSS PER COMMON SHARE
 
    Basic and diluted loss per share is calculated based upon the weighted
average number of common shares outstanding during the period. Diluted earnings
per share incorporates the dilutive effect of stock options, warrants and other
convertible securities. As of June 30, 2001, 2000 and 1999, the total number of
options, warrants and other securities convertible into ImmunoGen Common Stock
equaled 7,334,101, 6,964,225 and 12,610,917, respectively. ImmunoGen Common
Stock equivalents as calculated in accordance with the treasury-stock accounting
method, totaled 5,042,380, 4,698,751 and 3,666,523 as of June 30, 2001, 2000 and
1999, respectively. ImmunoGen Common Stock equivalents have not been included in
the loss per share calculation because their effect is antidilutive.
 
E.  MARKETABLE SECURITIES
 
    As of June 30, 2001, $14.8 million in cash and money market funds was
classified as cash and cash equivalents. The Company's cash, cash equivalents
and marketable securities as of June 30, 2001 are as follows:
 

<Table>
<Caption>
                                                                GROSS        GROSS
                                                AMORTIZED     UNREALIZED   UNREALIZED    ESTIMATED
                                                   COST         GAINS        LOSSES      FAIR VALUE
                                               ------------   ----------   ----------   ------------
<S>                                            <C>            <C>          <C>          <C>
Cash and money market funds..................  $ 14,822,519    $     --     $     --    $ 14,822,519
Commercial paper.............................     2,272,020         148           --       2,272,168
Government treasury notes....................    44,297,109      86,485         (626)     44,382,968
Federal agencies.............................    13,255,817       3,564         (206)     13,259,175
Asset-back securities........................    20,761,640     119,356       (5,495)     20,875,501
Corporate notes..............................    51,991,833     123,592      (12,675)     52,102,750
Bank notes...................................     3,061,924      22,715           --       3,084,639
                                               ------------    --------     --------    ------------
    Total....................................   150,462,862     355,860      (19,002)    150,799,720
                                               ------------    --------     --------    ------------
Less amounts classified as cash and cash
  equivalents................................    14,822,519          --           --      14,822,519
                                               ------------    --------     --------    ------------
    Total marketable securities..............  $135,640,343    $355,860     $(19,002)   $135,977,201
                                               ============    ========     ========    ============
</Table>

 
    During the twelve-month period ended June 30, 2001, $26,474 of unrealized
gains on available-for-sale securities were recognized as comprehensive income.
 
    In determining realized gains or losses on the sale of marketable
securities, the cost of securities sold is based on the specific identification
method.
 
    Short term marketable securities mature within one year of the balance sheet
date and long term marketable securities mature within two to three years of the
balance sheet date.
 
                                       44
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
F.  PROPERTY AND EQUIPMENT
 
    Property and equipment consisted of the following at June 30, 2001 and 2000:
 

<Table>
<Caption>
                                                                      JUNE 30,
                                                              -------------------------
                                                                 2001          2000
                                                              -----------   -----------
<S>                                                           <C>           <C>
Machinery and equipment.....................................  $ 3,093,619   $ 2,085,037
Computer hardware and software..............................      805,830       761,497
Assets under construction...................................      143,815       104,400
Furniture and fixtures......................................       68,173        67,229
Leasehold improvements......................................    9,613,746     8,378,609
                                                              -----------   -----------
                                                               13,725,183    11,396,772
Less accumulated depreciation...............................   10,487,101     9,888,376
                                                              -----------   -----------
Property and equipment, net of accumulated depreciation.....  $ 3,238,082   $ 1,508,396
                                                              ===========   ===========
</Table>

 
    Depreciation expense was approximately $613,000, $499,000 and $555,000 for
the years ended June 30, 2001, 2000 and 1999, respectively.
 
    As of June 30, 2001 and June 30, 2000 capital lease amortization totaled
approximately $49,000 and $59,000, respectively. As of June 30, 2001 and
June 30, 2000 the cost of capitalized equipment equaled $142,000. All of this
capitalized equipment is classified under Computer hardware & software.
 
G.  COMPREHENSIVE INCOME (LOSS)
 
    The Company presents comprehensive income (loss) in accordance with
SFAS 130, "Reporting Comprehensive Income." For the years ended June 30, 2001,
2000 and 1999, total comprehensive income (loss) equaled $(15.3) million,
$73,000 and $(4.1) million, respectively. Other comprehensive income was
comprised entirely of unrealized gains recognized on available-for-sale debt
securities.
 
H.  MINORITY INTEREST
 
    In July 1997, ATI entered into a collaboration agreement with BioChem
Pharma, Inc. This agreement granted BioChem an exclusive worldwide license to
ATI's proprietary screens based on two families of proteins involved in
apoptosis, for use in identifying leads for anti-cancer drug development. As of
April 2000, BioChem had fulfilled all of its funding obligations under the
agreement by purchasing a total of $11.1 million in non-voting,
non-dividend-bearing convertible preferred stock of ATI.
 
    In April 2000, BioChem informed ATI of its decision not to extend the
agreement beyond its scheduled July 31, 2000 termination date. Consequently,
under the terms of the agreement, rights to all screens delivered to BioChem
reverted to ATI effective August 1, 2000. However, certain provisions pertaining
to the license of any products resulting from the collaboration will remain in
force. As of August 1, 2000, no compound leads had been identified.
 
    The preferred stock issued to BioChem is convertible into ATI common stock
at any time after three years from the date of first issuance, at a conversion
price equal to the then current market price of the ATI common stock, but in any
event at a price that will result in BioChem acquiring at least 15% of the then
outstanding ATI common stock. Through June 30, 2001, 11,125 shares of ATI
preferred stock had been issued to BioChem, representing a 15% minority interest
(on an if-converted
 
                                       45
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
H.  MINORITY INTEREST (CONTINUED)
and fully diluted basis) in the net equity of ATI. Minority interest in ATI's
loss reduced ImmunoGen's net loss for the years ended June 30, 2000 and 1999 by
$76,000 and $101,000, respectively. Based upon an independent appraisal,
approximately 3% of the $11.1 million invested, or approximately $337,000, was
allocated to the minority interest in ATI, with the remainder, or approximately
$10.8 million, allocated to the Company's equity.
 
    As part of the BioChem agreement, BioChem also received warrants to purchase
shares of ImmunoGen Common Stock equal to the amount invested in ATI during the
three-year research term. Beginning July 31, 2000, these warrants became
exercisable for a number of shares of ImmunoGen Common Stock determined by
dividing $11.1 million, the amount of BioChem's investment in ATI, by the market
price of ImmunoGen Common Stock on the exercise date, subject to certain
limitations imposed by the Nasdaq Stock Market rules, which limit the sale or
issuance by an issuer of certain securities at a price less than the greater of
book or market value of such securities. Consequently, BioChem's ability to
convert all of its ImmunoGen warrants into ImmunoGen Common Stock is limited to
a total of 20% of the number of shares of ImmunoGen's Common Stock outstanding
on the date of the initial transaction to the extent that the conversion price
would be less than the market price of ImmunoGen Common Stock on that date,
unless stockholder approval for such conversion is obtained, if required, or
unless the Company has obtained a waiver of that requirement. The exercise price
is payable in cash or shares of ATI's preferred stock, at BioChem's option. The
warrants are expected to be exercised only in the event that the shares of ATI
common stock do not become publicly traded. ImmunoGen expects that BioChem will
use its shares of ATI preferred stock, in lieu of cash, to exercise the
warrants.
 
I.  INCOME TAXES
 
    The difference between the Company's "expected" tax benefit, as computed by
applying the U.S. federal corporate tax rate of 34% to income (loss) before
minority interest, the cumulative effect of accounting change and provision for
income taxes, and actual tax is reconciled in the following chart (in
thousands):
 

<Table>
<Caption>
                                                                   YEAR ENDED JUNE 30,
                                                              ------------------------------
                                                                2001       2000       1999
                                                              --------   --------   --------
<S>                                                           <C>        <C>        <C>
Loss before income tax expense, minority interest and
  cumulative effect of accounting change....................  $(9,473)    $(313)    $(4,176)
                                                              =======     =====     =======
 
Expected tax benefit at 34%.................................  $(3,221)    $(107)    $(1,420)
State tax benefit net of federal benefit....................     (429)      (16)       (220)
Change in valuation allowance for deferred tax assets
  allocated to tax expense..................................    3,697       104       1,618
Other.......................................................       36        19          22
                                                              -------     -----     -------
Income tax provision........................................  $    83     $  --     $    --
                                                              =======     =====     =======
</Table>

 
    At June 30, 2001, the Company has net operating loss carryforwards of
approximately $134.7 million available to reduce federal taxable income expiring
in 2002 through 2021 and $45.4 million available to reduce state taxable income
expiring in 2002 through 2006. Of the total $134.7 million federal net operating
loss carryforwards and $45.4 million state net operating loss
 
                                       46
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
I.  INCOME TAXES (CONTINUED)
carryforwards, $4.1 million of net operating loss carryforwards relates to the
exercise of stock options. The tax benefit of this amount will result in an
increase in additional paid-capital if and when realized. The Company also has
federal and state research tax credits of approximately $7.6 million available
to offset federal and state income taxes, which expire beginning in 2002. Due to
the degree of uncertainty related to the ultimate use of the loss carryforwards
and tax credits, the Company has fully reserved these tax benefits.
 
    Deferred income taxes reflect the net tax effects of temporary differences
between the carrying amounts of assets and liabilities for financial reporting
purposes and the amounts used for income tax purposes. Significant components of
the Company's deferred tax assets as of June 30 are as follows (in thousands):
 

<Table>
<Caption>
                                                                  AT JUNE 30,
                                                              -------------------
                                                                2001       2000
                                                              --------   --------
<S>                                                           <C>        <C>
Net operating loss carryforwards............................  $ 48,210   $ 44,987
Research and development tax credit carryforwards...........     6,708      7,137
Capitalized research costs..................................     1,630      1,953
Property and other intangible assets........................     2,445      2,369
Deferred revenue............................................     5,073         --
Other liabilities...........................................     1,235         --
                                                              --------   --------
Total deferred tax assets...................................    65,301     56,446
Valuation allowance.........................................   (65,301)   (56,446)
                                                              --------   --------
Net deferred tax assets.....................................  $     --   $     --
                                                              ========   ========
</Table>

 
    The valuation allowance increased by $8.9 million during 2001 due primarily
to the increase in net operating loss carryforwards related to the cumulative
effect of accounting change and the Company's net loss before the cumulative
effect of accounting change offset by write-offs of expiring federal and state
net operating loss carryforwards and research and development credits.
 
J.  CAPITAL STOCK
 
    COMMON AND PREFERRED STOCK
 
    In October 1996, the Company's $2.5 million debenture issued in June 1996
was converted into 2,500 shares of the Company's Series A Convertible Preferred
Stock (Series A Stock), with a stated value of $1,000 per share. Holders of the
Series A Stock were entitled to receive, when and as declared by the Board of
Directors, cumulative dividends in cash, or at the Company's option, shares of
the Company's common stock, in arrears on the conversion date. The 2,500 shares
of Series A Stock were convertible into the same number of shares of common
stock as the $2.5 million debenture. Each share of Series A Stock was
convertible into a number of shares of common stock determined by dividing
$1,000 by the lower of (i) $2.50 (subject to certain restrictions) and (ii) 85%
of the average of the closing bid price of the common stock for the five days
prior to conversion. In addition, holders of Series A Stock were entitled to
receive, on conversion of the Series A Stock, a number of warrants equal to 50%
of the number of shares of common stock issued on conversion. On January 5,
1998, the remaining 1,100 unconverted shares of the Series A Stock plus accrued
dividends thereon were converted into 1,347,491 shares of the Company's common
stock. In connection with the Series A Stock
 
                                       47
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
J.  CAPITAL STOCK (CONTINUED)
conversions, warrants to purchase 1,338,117 shares of common stock were issued.
The warrants were issued with an exercise price of $4.00 per share and expired
at various dates during 2002 and 2003. The warrants were valued at $623,000 and
were accounted for as non-cash dividends on convertible preferred stock at the
time of issuance of the Series A Stock. The warrant agreements contain
anti-dilution provisions. In connection with ImmunoGen's November 2000 public
offering of stock, the Company and the warrant holder negotiated a revision to
the warrants based upon the anti-dilution provisions. Under the revised warrant
terms, the holder may purchase 1,347,811 shares of common stock at exercise
prices ranging from $3.95 to $4.00 per share. The warrants expire at various
dates in 2002 and 2003.
 
    Also in October 1996, the Company sold 3,000 shares of its Series B
Convertible Preferred Stock (Series B Stock). As of February 4, 1997, all 3,000
shares of Series B Stock plus accrued dividends thereon had been converted into
1,384,823 shares of the Company's common stock. In connection with the issuance
of the Series B Stock, warrants to purchase 500,000 shares of the Company's
common stock were also issued. Of these, 250,000 warrants are exercisable at
$5.49 per share and expire in October 2001. The remaining 250,000 warrants are
exercisable at $3.68 per share and expire in January 2002. These warrants were
valued at $618,900, and were accounted for as non-cash dividends on convertible
preferred stock at the time of issuance of the Series B Stock.
 
    In January 1997, the Company sold $3.0 million of its Series C Convertible
Preferred Stock (Series C Stock) in connection with the October 1996 Private
Placement (the October 1996 Private Placement) to an institutional investor.
Each share of Series C Stock was convertible into a number of shares of common
stock determined by dividing $1,000 by the lower of (i) $2.61 and (ii) 85% of
the market price of the Company's common stock at the time of conversion. On
August 1, 1997, the remaining 700 unconverted shares of the Series C Stock plus
accrued dividends thereon were converted into 701,180 shares of the Company's
common stock. In connection with all Series C Stock, warrants to purchase
1,147,754 shares of common stock were issued to the investor. These warrants are
exercisable at $2.31 per share and expire in April 2002. The $1.2 million value
of these warrants was accounted for as non-cash dividends on convertible
preferred stock at the time of issuance of the Series C Stock.
 
    In June 1997, the Company sold $1.0 million of its Series D Convertible
Preferred Stock (Series D Stock) in connection with a financing agreement that
was entered into in October 1996. The Series D Stock was convertible at any time
into a number of shares of common stock determined by dividing $1,000 by the
lower of (i) $1.44 and (ii) 85% of the market price of the Company's common
stock at the time of conversion. As of December 31, 1997, all 1,000 shares of
Series D Stock and accumulated dividends thereon had been converted into
1,001,387 shares of common stock. In addition, the investor received warrants to
purchase 454,545 shares of the Company's common stock. These warrants have an
exercise price of $1.94 per share and expire in 2002. The value of these
warrants, $278,000, was determined at the time of issuance of the convertible
securities and was accounted for as non-cash dividends on convertible preferred
stock at that time.
 
    Also in June 1997, the Company and ATI satisfied an obligation of ATI to one
of its scientific advisors, totaling $120,000, by paying the advisor a
combination of cash and 41,481 shares of the Company's common stock.
 
                                       48
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
J.  CAPITAL STOCK (CONTINUED)
 
    In December 1997, the Company entered into an agreement, which was amended
in March 1998, to sell $3.0 million of its non-dividend-bearing Series E
Convertible Preferred Stock (Series E Stock) to an institutional investor. The
investment was completed in three installments: $1.0 million in December 1997;
$500,000 in March 1998; and $1.5 million in July 1998. The issued Series E Stock
became convertible into common stock at the end of a two-year holding period at
$1.0625 per share. In addition, through June 30, 1999, warrants to purchase
2,823,528 shares of common stock had been issued. These warrants became
exercisable at the end of a two-year holding period, subject to certain
provisions. The value of the warrants was determined at the time of their
issuance and accounted for as non-cash dividends on convertible preferred stock.
Approximately $580,500 and $918,000 in non-cash dividends were recorded in the
each of fiscal 1998 and 1999, respectively. These warrants had an exercise price
of $2.125 per share, and vested over a period of two years subject to certain
provision. Of the total 2,823,528 warrants issued, 941,176 would have expired in
2004 and 1,882,352 would have expired in 2005. Also in relation to this
agreement, 75,000 shares of common stock were issued to a third party as a
finder's fee. The value of these issued shares equaled $107,000 based on closing
prices on the date of grant and was charged to operations.
 
    In July 2000, one warrant holder exercised its right to acquire 50,000 of
shares of common stock at an exercise price of $3.11 per share. In
September 2000, one holder of warrants exercised its right to acquire 50,000
shares of common stock at an exercise price of $3.11 per share. In
October 2000, two holders of warrants exercised their rights to acquire 77,500
shares of common stock at a price range of $3.11 to $5.49 per share. During the
twelve-month period ended June 30, 2001, holders of options issued through the
Company's Restated Incentive Stock Option Plan exercised their rights to acquire
an aggregate of 313,928 shares at prices ranging from $0.84 per share to $14.75
per share. The total proceeds from these option and warrant exercises,
$2.5 million, will be used to fund current operations.
 
    In February 1999, as part of the exclusive license agreement with
GlaxoSmithKline, at ImmunoGen's option, GlaxoSmithKline agreed to purchase up to
$5.0 million of ImmunoGen Common Stock over the next two years, subject to
certain conditions. As of June 30, 2001, GlaxoSmithKline had purchased, pursuant
to the Company's put option, 1,023,039 shares of ImmunoGen Common Stock for
$2.5 million.
 
    In July 1997, the Company's majority-owned subsidiary, ATI, entered into a
collaboration with BioChem. As part of the agreement, BioChem received warrants
to purchase shares of ImmunoGen Common Stock equal to $11.1 million, the amount
invested in ATI by BioChem during the three-year research term. These warrants
are exercisable at any time on or after July 31, 2000, until and including
July 31, 2002, into a number of shares of ImmunoGen common stock determined by
dividing $11.1 million by the market price of the ImmunoGen common stock on the
exercise date, subject to certain limitations. In April 2000, the last quarterly
investment of $843,000 was received and warrants corresponding to that amount
were issued. Until July 31, 2000, proceeds from this investment were restricted
to fund the ongoing ATI research collaboration. After that date, all residual
proceeds represented unrestricted assets of ATI.
 
    On September 5, 2000, the Company entered into a collaboration agreement
with Abgenix, Inc. of Fremont, California. The agreement provides Abgenix with
access to ImmunoGen's maytansinoid TAP technology for use with Abgenix's fully
human antibodies generated with XenoMouse technology. Immunogen will receive
$5.0 million in technology access fee payments, as well as potential milestone
 
                                       49
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
J.  CAPITAL STOCK (CONTINUED)
payments, and royalties on net sales of any resulting products. In addition, on
September 7, 2000 Abgenix purchased $15.0 million of ImmunoGen Common Stock at
$19.00 per share.
 
    In November 2000, the Company completed a public offering of 4.0 million
shares of Common Stock at $33.00 per share. Proceeds to the Company were
$124.8 million, net of offering costs of $7.2 million. Proceeds from the public
offering will be used for working capital and general corporate purposes,
including research and development.
 
    WARRANTS
 
    In addition to the warrants discussed in this footnote, subheading Common
and Preferred Stock, the Company issued warrants to purchase 509,000 and 500,000
shares of Common Stock at exercise prices of $4.00 and $6.00 per share,
respectively, in connection with a private placement of the Company's
convertible debentures in March 1996. The warrant agreements contain
anti-dilution provisions. In connection with ImmunoGen's November 2000 public
offering of stock, the Company and the warrant holder negotiated a revision to
the warrants based upon the anti-dilution provisions. Under the revised warrant
terms, the holder may purchase 568,715 and 558,659 share of common stock at
exercise prices of $3.58 and $5.37 per share, respectively. The warrants expire
in September 2001. Additionally, the Company issued the holder a warrant,
expiring in November 2005, to acquire 340,000 shares of common stock at an
exercise price of $38.00 per share.
 
    In connection with ImmunoGen's March 1996 convertible debt offering, the
Company issued warrants to purchase 250,000 shares of the Company's Common Stock
to a third party as a finder's fee. The 250,000 warrants have an exercise price
of $3.11 and expire in 2003.
 
    STOCK OPTIONS
 
    Under the Company's Restated Stock Option Plan (the Plan), originally
adopted by the Board of Directors on February 13, 1986, and subsequently amended
and restated, employees, consultants and directors may be granted options to
purchase shares of common stock of the Company. In July 1999, the Board of
Directors authorized, and the shareholders subsequently approved, amendments to
the Plan to increase the total number of shares reserved for the grant of
options to 4.85 million shares of common stock. In addition to options granted
under the Plan, the Board previously approved the
 
                                       50
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
J.  CAPITAL STOCK (CONTINUED)
granting of other, non-qualified options. Information related to stock option
activity under the Plan and outside of the Plan during fiscal years 1999, 2000
and 2001 is as follows:
 

<Table>
<Caption>
                                                      OPTIONS ISSUED             NON-QUALIFIED OPTIONS
                                                      UNDER THE PLAN           ISSUED OUTSIDE OF THE PLAN
                                                ---------------------------   ----------------------------
                                                                AVERAGE                        AVERAGE
                                                 SHARES     PRICE PER SHARE     SHARES     PRICE PER SHARE
                                                ---------   ---------------   ----------   ---------------
<S>                                             <C>         <C>               <C>          <C>
Outstanding at June 30, 1998..................  2,492,353       $ 2.92          20,000         $ 7.69
                                                ---------       ------          ------         ------
Granted.......................................    642,700         2.06              --             --
Exercised.....................................    174,245         1.81              --             --
Canceled......................................    151,659         5.58              --             --
                                                ---------       ------          ------         ------
Outstanding at June 30, 1999..................  2,809,149       $ 2.65          20,000         $ 7.69
                                                ---------       ------          ------         ------
Granted.......................................    596,200         7.27              --             --
Exercised.....................................    131,567         1.67              --             --
Canceled......................................     61,774         4.92              --             --
                                                ---------       ------          ------         ------
Outstanding at June 30, 2000..................  3,212,008       $ 3.50          20,000         $ 7.69
                                                ---------       ------          ------         ------
Granted.......................................  1,051,300        19.89          12,500          14.49
Exercised.....................................    303,928         2.47          10,000           3.38
Canceled......................................    100,999        10.86              --             --
                                                ---------       ------          ------         ------
Outstanding at June 30, 2001..................  3,858,381       $ 7.85          22,500         $13.38
                                                =========       ======          ======         ======
</Table>

 
    The following table summarizes aggregate information about total stock
options under the Plan and outside the Plan, outstanding at June 30, 2001:
 

<Table>
<Caption>
                                           OPTIONS OUTSTANDING                       OPTIONS EXERCISABLE
                            -------------------------------------------------   ------------------------------
                                          WEIGHTED-AVERAGE
                                             REMAINING
                              NUMBER        CONTRACTUAL      WEIGHTED-AVERAGE     NUMBER      WEIGHTED-AVERAGE
RANGE OF EXERCISE PRICES    OUTSTANDING     LIFE (YEARS)      EXERCISE PRICE    EXERCISABLE    EXERCISE PRICE
------------------------    -----------   ----------------   ----------------   -----------   ----------------
<S>                         <C>           <C>                <C>                <C>           <C>
$  .84 -  1.31............     964,128          6.39              $  .98           964,003         $  .98
  1.38 -  2.50............   1,026,450          6.06                2.15           829,704           2.18
  2.50 - 11.50............     811,841          6.43                7.50           446,232           7.70
 11.88 - 19.97............     220,500          7.28               14.61            77,250          14.38
 20.75 - 39.13............     857,962          9.58               21.05                --             --
                             ---------                                           ---------
                             3,880,881                                           2,317,189
                             =========                                           =========
</Table>

 
                                       51
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
J.  CAPITAL STOCK (CONTINUED)
    The Company has granted options at the fair market value of the common stock
on the date of such grant. The following options and their respective average
prices per share were outstanding and exercisable at June 30, 2001, 2000 and
1999:
 

<Table>
<Caption>
                                               AVERAGE                         AVERAGE
                             OUTSTANDING   PRICE PER SHARE   EXERCISABLE   PRICE PER SHARE
                             -----------   ---------------   -----------   ---------------
<S>                          <C>           <C>               <C>           <C>
June 30, 2001..............   3,880,881         $7.85         2,317,189         $3.15
June 30, 2000..............   3,232,008          3.50         1,863,312          3.12
June 30, 1999..............   2,829,149          2.65         1,343,651          3.94
</Table>

 
    Options vest at various rates over periods of up to four years and may be
exercised within ten years from the date of grant.
 
    The Company applies APB 25 and related interpretations in accounting for its
Plan. Accordingly, no compensation expense is generally recognized for its
stock-based compensation plans. However, in April 2000, 52,916 options
previously granted to a terminating officer were granted accelerated vesting
and, accordingly, the Company charged $350,000 to compensation expense
representing the difference between the exercise price and the fair value of the
stock at the acceleration date. In 2001, the Company also recorded $43,000 of
compensation expense related to a terminating employee and $37,000 in connection
with variable stock option grants.
 
    Had compensation costs for the Company's stock-based compensation been
determined based on the fair value at the grant dates as calculated in
accordance with SFAS 123, the Company's net basic and diluted loss per common
share for the years ended June 30, 2001, 2000 and 1999 would have been adjusted
to the pro forma amounts indicated below:
 

<Table>
<Caption>
                                                        JUNE 30,
                                        ----------------------------------------
                                            2001          2000          1999
                                        ------------   -----------   -----------
<S>                                     <C>            <C>           <C>
Pro forma net loss....................  $(18,817,066)  $(1,378,740)  $(5,648,419)
Pro forma basic and diluted loss per
  share...............................  $      (0.51)  $     (0.05)  $     (0.22)
</Table>

 
    The fair value of each stock option is estimated on the date of grant using
the Black-Scholes option-pricing model with the following weighted-average
assumptions:
 

<Table>
<Caption>
                                                                  JUNE 30,
                                                    ------------------------------------
                                                      2001          2000          1999
                                                    --------      --------      --------
<S>                                                 <C>           <C>           <C>
Dividend..........................................     None          None          None
Volatility........................................    97.00%       107.00%        85.00%
Risk-free interest rate...........................     5.00%         6.72%         4.96%
Expected life (years).............................      5.5           5.5           5.5
</Table>

 
    Using the Black-Scholes option-pricing model, the fair value of options
granted during fiscal 2001, 2000 and 1999 was $16.12, $6.00 and $1.47,
respectively.
 
    The Black-Scholes option-pricing model was developed for use in estimating
the fair value of traded options that have no vesting restrictions and are fully
transferable. In addition, option-pricing models require the use of highly
subjective assumptions, including the expected stock price volatility. Because
the Company's employee stock options have characteristics significantly
different from those of
 
                                       52
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
J.  CAPITAL STOCK (CONTINUED)
traded options, and because changes in the subjective assumptions can materially
affect the fair value estimates, in management's opinion, the existing models do
not necessarily provide a reliable single measure of the fair value of its
employee stock-based compensation.
 
    COMMON STOCK RESERVED
 
    Shares of authorized common stock have been reserved for the exercise of all
options and warrants outstanding.
 
K.  COMMITMENTS
 
    OPERATING LEASES
 
    At June 30, 2001, the Company leased facilities in Norwood and Cambridge,
Massachusetts. In fiscal year 2001, the Company amended its lease on the Norwood
facility, extending the lease term to June 30, 2008. The Cambridge facility was
subject to a sublease agreement, which expired in April 2000. The lease term for
the Cambridge facility expires in 2003. Total net receipts under the sublease
agreement, which were credited to rent expense, were approximately $3.4 million
through April 2000, of which approximately $707,000 and $796,000 was received by
the Company in fiscal 2000 and 1999, respectively. The Company is required to
pay all operating expenses for the leased premises subject to escalation charges
for certain expense increases over a base amount. Facilities rent expense, net
of the above mentioned subleased income, was approximately $635,000, $318,000
and $146,000 during fiscal years 2001, 2000 and 1999.
 
    The minimum rental commitments, including real estate taxes and other
expenses, for the next four years under the non-cancelable capital and operating
lease agreements are as follows:
 

<Table>
<Caption>
                                                          OPERATING    CAPITAL
                                                            LEASES      LEASES
                                                          ----------   --------
<S>                                                       <C>          <C>
2002....................................................  $  932,805    $8,683
2003....................................................     942,719        --
2004....................................................     576,064        --
2005....................................................     576,064        --
2006....................................................     576,064        --
Thereafter..............................................   1,152,128        --
                                                          ----------    ------
Total minimum lease payment.............................   4,755,844     8,683
Total lease commitments.................................   4,755,844     8,683
                                                          ----------    ------
Less amount representing interest.......................                   546
                                                                        ------
Present value of net minimum capital lease payments.....                $8,137
                                                                        ======
</Table>

 
L.  EMPLOYEE BENEFIT PLANS
 
    Effective September 1, 1990, the Company implemented a deferred compensation
plan under Section 401(k) of the Internal Revenue Code (the "401(k) Plan").
Under the 401(k) Plan, eligible employees are permitted to contribute, subject
to certain limitations, up to 15% of their gross salary. The Company makes a
matching contribution that currently totals 20% of the employee's contribution,
 
                                       53
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
L.  EMPLOYEE BENEFIT PLANS (CONTINUED)
up to a maximum amount equal to 1% of the employee's gross salary. In fiscal,
2001, 2000 and 1999, the Company's contributions to the 401(k) Plan amounted to
approximately $47,500, $41,075, and $26,000, respectively.
 
M.  QUARTERLY FINANCIAL INFORMATION (UNAUDITED)
 
    The quarterly information for the four quarters of 2001 reflects the
quarters as previously reported prior to the adoption of SAB 101 and as adjusted
for the retroactive adoption of SAB 101 to July 1, 2000, as noted in the column
headings. The 2000 quarterly information has been presented as originally
reported and pro forma for the adoption of SAB 101.
 

<Table>
<Caption>
                                                                        FISCAL YEAR 2001
                            --------------------------------------------------------------------------------------------------------
                               FIRST QUARTER ENDED        SECOND QUARTER ENDED         THIRD QUARTER ENDED      FOURTH QUARTER ENDED
                               SEPTEMBER 30, 2000           DECEMBER 31, 2000            MARCH 31, 2001            JUNE 30, 2001
                            -------------------------   -------------------------   -------------------------   --------------------
                                AS            AS            AS            AS            AS            AS                 AS
                             REPORTED      ADJUSTED      REPORTED      ADJUSTED      REPORTED      ADJUSTED           REPORTED
                            -----------   -----------   -----------   -----------   -----------   -----------   --------------------
<S>                         <C>           <C>           <C>           <C>           <C>           <C>           <C>
Revenues:
  Revenue earned under
    collaboration
    agreement.............  $ 1,759,000   $ 2,213,162   $   526,000   $   614,750   $   155,411   $   429,870       $   387,716
  Clinical materials
    reimbursement.........           --            --            --            --       561,615       561,615            35,435
  Development fees........           --            --       100,069       100,069        35,164        35,164           101,582
                            -----------   -----------   -----------   -----------   -----------   -----------       -----------
    Total revenues........    1,759,000     2,213,162       626,069       714,819       752,190     1,026,649           524,733
Expenses:
  Cost of clinical
    materials
    reimbursed............           --            --            --            --       561,615       561,615            35,435
  Research and
    development...........    3,568,933     3,568,933     3,619,171     3,619,171     3,739,396     3,739,396         4,285,664
  General and
    administrative........      853,909       853,909     1,047,265     1,047,265     1,179,697     1,179,697         1,400,931
                            -----------   -----------   -----------   -----------   -----------   -----------       -----------
    Total Expenses........    4,422,842     4,422,842     4,666,436     4,666,436     5,480,708     5,480,708         5,722,030
Net loss from
  operations..............   (2,663,842)   (2,209,680)   (4,040,367)   (3,951,617)   (4,728,518)   (4,454,059)       (5,197,297)
  Loss on the sale of
    assets................       (1,900)       (1,900)           --            --            --            --                --
  Interest income, net....      213,601       213,601     1,242,923     1,242,923     2,583,606     2,583,606         1,834,845
  Realized gains on
    investments...........           --            --            --            --        92,582        92,582            40,184
  Other income............       19,349        19,349       248,706       248,706        20,226        20,226            44,927
                            -----------   -----------   -----------   -----------   -----------   -----------       -----------
Net loss before income tax
  expense and cumulative
  effect of change in
  accounting principle....   (2,432,792)   (1,978,630)   (2,548,738)   (2,459,988)   (2,032,104)   (1,757,645)       (3,277,341)
  Income tax expense......           --            --        55,000        55,000        27,600        27,600                --
                            -----------   -----------   -----------   -----------   -----------   -----------       -----------
Net loss before cumulative
  effect of change in
  accounting principle....   (2,432,792)   (1,978,630)   (2,603,738)   (2,514,988)   (2,059,704)   (1,785,245)       (3,277,341)
                            -----------   -----------   -----------   -----------   -----------   -----------       -----------
Cumulative effect of
  change in accounting
  principle...............           --    (5,734,478)           --            --            --            --                --
                            -----------   -----------   -----------   -----------   -----------   -----------       -----------
Net loss to common
  stockholders............  $(2,432,792)  $(7,713,108)  $(2,603,738)  $(2,514,988)  $(2,059,704)  $(1,785,245)      $(3,277,341)
                            ===========   ===========   ===========   ===========   ===========   ===========       ===========
Basic and diluted net loss
  per share...............  $     (0.07)  $     (0.23)  $     (0.07)  $     (0.07)  $     (0.05)  $     (0.05)      $     (0.09)
                            ===========   ===========   ===========   ===========   ===========   ===========       ===========
</Table>

 
                                       54
<Page>
                                IMMUNOGEN, INC.
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
M.  QUARTERLY FINANCIAL INFORMATION (UNAUDITED) (CONTINUED)

<Table>
<Caption>
                                                                  FISCAL YEAR 2000
                                    -----------------------------------------------------------------------------
                                      FIRST QUARTER ENDED      SECOND QUARTER ENDED        THIRD QUARTER ENDED
                                      SEPTEMBER 30, 1999         DECEMBER 31, 1999           MARCH 31, 2000
                                    -----------------------   -----------------------   -------------------------
                                        AS                        AS                        AS
                                     REPORTED    PRO FORMA     REPORTED    PRO FORMA     REPORTED      PRO FORMA
                                    ----------   ----------   ----------   ----------   -----------   -----------
<S>                                 <C>          <C>          <C>          <C>          <C>           <C>
Revenues:
  Revenue earned under
    collaboration agreement.......  $4,000,000   $4,041,667   $2,500,000   $1,795,667   $        --   $   231,667
  Licensing.......................         290         290           195         195             --            --
  Development fees................       4,800       4,800            --          --             --            --
                                    ----------   ----------   ----------   ----------   -----------   -----------
    Total revenues................   4,005,090   4,046,757     2,500,195   1,795,862             --       231,667
Expenses:
  Research and development........   1,831,023   1,831,023     1,890,695   1,890,695      2,262,513     2,262,513
  General and administrative......     503,335     503,335       643,212     643,212        689,167       689,167
                                    ----------   ----------   ----------   ----------   -----------   -----------
    Total expenses................   2,334,358   2,334,358     2,533,907   2,533,907      2,951,680     2,951,680
Net earnings/(loss) from
  operations......................   1,670,732   1,712,399       (33,712)   (738,045)    (2,951,680)   (2,720,013)
  Gain/(loss) on the sale of
    assets........................        (157)       (157)        1,645       1,645             50            50
  Interest income, net............      54,296      54,296        69,931      69,931        115,961       115,961
  Other income....................          --          --        42,030      42,030          6,000         6,000
                                    ----------   ----------   ----------   ----------   -----------   -----------
Net earnings/(loss) before
  minority interest...............   1,724,871   1,766,538        79,894    (624,439)    (2,829,669)   (2,598,002)
  Minority interest in net loss of
    consolidated subsidiary.......      25,290      25,290        25,290      25,290         25,290        25,290
                                    ----------   ----------   ----------   ----------   -----------   -----------
Net earnings/(loss) to common
  stockholders....................  $1,750,161   $1,791,828   $  105,184   $(599,149)   $(2,804,379)  $(2,572,712)
                                    ==========   ==========   ==========   ==========   ===========   ===========
Basic net earnings/(loss) per
  share...........................  $     0.07   $    0.07    $     0.00   $   (0.02)   $     (0.09)  $     (0.08)
                                    ==========   ==========   ==========   ==========   ===========   ===========
Diluted net earnings/(loss) per
  share...........................  $     0.05   $    0.05    $     0.00   $   (0.02)   $     (0.09)  $     (0.08)
                                    ==========   ==========   ==========   ==========   ===========   ===========
 
<Caption>
                                        FISCAL YEAR 2000
                                    ------------------------
                                      FOURTH QUARTER ENDED
                                         JUNE 30, 2000
                                    ------------------------
                                        AS
                                     REPORTED     PRO FORMA
                                    ----------   -----------
<S>                                 <C>          <C>
Revenues:
  Revenue earned under
    collaboration agreement.......  $4,675,000   $   245,162
  Licensing.......................         220           220
  Development fees................          --            --
                                    ----------   -----------
    Total revenues................   4,675,220       245,382
Expenses:
  Research and development........   2,893,874     2,893,874
  General and administrative......   1,210,340     1,210,340
                                    ----------   -----------
    Total expenses................   4,104,214     4,104,214
Net earnings/(loss) from
  operations......................     571,006    (3,858,832)
  Gain/(loss) on the sale of
    assets........................      18,000        18,000
  Interest income, net............     120,985       120,985
  Other income....................       1,483         1,483
                                    ----------   -----------
Net earnings/(loss) before
  minority interest...............     711,474    (3,718,364)
  Minority interest in net loss of
    consolidated subsidiary.......          --            --
                                    ----------   -----------
Net earnings/(loss) to common
  stockholders....................  $  711,474   $(3,718,364)
                                    ==========   ===========
Basic net earnings/(loss) per
  share...........................  $     0.02   $     (0.11)
                                    ==========   ===========
Diluted net earnings/(loss) per
  share...........................  $     0.02   $     (0.11)
                                    ==========   ===========
</Table>

 
                                       55
<Page>

I
TEM 9.  CHANGES IN AND DISAGREEMENTS WITH ACCOUNTANTS ON ACCOUNTING AND
  FINANCIAL DISCLOSURE
 
    The information reported in the Registrant's Current Report on Form 8-K
filed with the Securities and Exchange Commission on September 7, 2001 is hereby
incorporated by reference.
 

                                    PART III
 

ITEM 10.  DIRECTORS AND EXECUTIVE OFFICERS OF THE REGISTRANT
 
DIRECTORS
 
    The section entitled "Election of Directors" in the Company's definitive
proxy statement for its 2001 Annual Meeting of Shareholders, which the Company
intends to file with the Securities and Exchange Commission on or before
October 12, 2001, is hereby incorporated by reference.
 
EXECUTIVE OFFICERS
 
    The following is a list of the executive officers of the Company and their
positions with the Company. Each individual executive officer serves at the
pleasure of the Board of Directors.
 

<Table>
<Caption>
NAME                                     AGE                     POSITIONS WITH THE COMPANY
----                                   --------   --------------------------------------------------------
<S>                                    <C>        <C>
Mitchel Sayare, Ph.D.................     53      Chairman of the Board of Directors, Chief Executive
                                                  Officer and President
 
Walter A. Blattler, Ph.D.............     52      Executive Vice President, Science and Technology and
                                                    Treasurer
 
Gregg D. Beloff......................     33      Vice President and Chief Financial Officer
 
John M. Lambert, Ph.D................     50      Senior Vice President, Research and Development
 
Pauline Jen Ryan.....................     34      Vice President, Business Development
 
Virginia A. Lavery...................     37      Senior Corporate Controller and Treasurer
</Table>

 
    The background of each executive officer is as follows:
 
    Mitchel Sayare, Chief Executive Officer since 1986, a Director since 1986
and Chairman of the Board of Directors since 1989, joined the Company in 1986.
From 1986 to July 1992 and currently since 1994, Mr. Sayare has served as
President of the Company. From 1982 to 1985, Mr. Sayare was Vice President for
Development at Xenogen, Inc., a biotechnology company specializing in monoclonal
antibody-based diagnostic systems for cancer. From 1977 to 1982, Mr. Sayare was
Assistant Professor of Biophysics and Biochemistry at the University of
Connecticut. He holds a Ph.D. in Biochemistry from Temple University School of
Medicine.
 
    Walter A. Blattler, Ph.D., elected a Director in September 1995, served as
Vice President, Research and Development from 1987 to October 1994 and as Senior
Vice President, Research and Development from October 1994 to October 1996.
Since October 1996 Dr. Blattler has served as Executive Vice President, Science
and Technology. Dr. Blattler joined the Company in October 1987. From 1981 to
1987 Dr. Blattler was chief scientist for the ImmunoGen-supported research
program at Dana-Farber Cancer Institute. Dr. Blattler received his Ph.D. from
the Swiss Federal Institute of Technology in Zurich in 1978.
 
    Gregg D. Beloff, Vice President and Chief Financial Officer, joined the
Company in March 2001. From 1998 to 2001 he was employed at Adams, Harkness &
Hill, Inc., most recently as a Vice President in Investment Banking. From 1993
to 1996, Mr. Beloff was employed as an attorney at the law firm of Gaffin &
Krattenmaker, P.C. Mr. Beloff holds a Juris Doctorate from the University of
Pittsburgh and a Masters of Business Administration from Carnegie Mellon
University.
 
                                       56
<Page>
    John M. Lambert, Ph.D., Senior Vice President, Research and Development
since November 1996, joined the Company in 1987. Dr. Lambert served as Senior
Director of Research from November 1992 to October 1994 and served as Vice
President of Research from October 1994 to November 1996. Prior to joining
ImmunoGen, Dr. Lambert was Assistant Professor of Pathology at the Dana-Farber
Cancer Institute, where he worked on the research program supported by
ImmunoGen. Dr. Lambert received his Ph.D. in Biochemistry from Cambridge
University in England.
 
    Pauline Jen Ryan, Vice President, Business Development, joined the Company
in May of 1999, with more than ten years of experience in the pharmaceutical and
biotechnology industries. Most recently, she was Vice President at Capital
Management Consulting, Inc., where she provided strategic counsel. Before that,
she managed business development at Organogenesis, Inc. Ms. Ryan holds a Masters
degree in Management from Northwestern University's Kellogg Graduate School of
Management.
 
    Virginia A. Lavery, Senior Corporate Controller and Treasurer, joined the
Company in December 2000. During 2000, Ms. Lavery was self-employed as a
financial consultant. From August 1999 to February 2000, Ms. Lavery was interim
Chief Financial Officer of Dynamics Research Corporation, a publicly-traded
government contractor, after having served as Corporate Controller since
July 1998. From 1989 to 1998, Ms. Lavery was a Certified Public Accountant with
Arthur Andersen, LLP.
 
    The section entitled "Section 16(a) Beneficial Ownership Reporting
Compliance" in the Company's definitive proxy statement for its 2001 Annual
Meeting of Shareholders is hereby incorporated by reference.
 

ITEM 11.  EXECUTIVE COMPENSATION
 
    The sections entitled "Executive Compensation" and "Employment Contracts,
Termination of Employment and Change in Control Agreements" in the Company's
definitive proxy statement for its 2001 Annual Meeting of Shareholders are
hereby incorporated by reference.
 

ITEM 12.  SECURITIES OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT
 
    The section entitled "Security Ownership of Certain Beneficial Owners and
Management" in the Company's definitive proxy statement for its 2001 Annual
Meeting of Shareholders is hereby incorporated by reference.
 

ITEM 13.  CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS
 
    The section entitled "Certain Transactions" in the Company's definitive
proxy statement for its 2001 Annual Meeting of Shareholders is hereby
incorporated by reference.
 
                                       57
<Page>

                                    PART IV
 

ITEM 14.  EXHIBITS, FINANCIAL STATEMENT SCHEDULES AND REPORTS ON FORM 8-K
 
    (a) Financial Statements
 
        (1) and (2) See "Index to Consolidated Financial Statements and
    Supplemental Schedules" at Item 8 of this Annual Report on Form 10-K.
    Schedules not included herein are omitted because they are not applicable or
    the required information appears in the Consolidated Financial Statements or
    Notes thereto.
 
        (3) Exhibits
 

<Table>
<Caption>
EXHIBIT NO.                                     DESCRIPTION
-----------             ------------------------------------------------------------
<S>                     <C>
 (3.1)                  Restated Articles of Organization(1)
 
 (3.2)                  By-Laws, as amended(2)
 
 (4.1)                  Article 4 of the Restated Articles of Organization as
                        amended (See Exhibits 3.1 and 3.2)(1)
 
 (4.2)                  Form of Common Stock Certificate(7)
 
(10.1)                  Research and License Agreement dated as of May 22, 1981 by
                        and between the Registrant and Sidney Farber Cancer
                        Institute, Inc. (now Dana-Farber Cancer Institute, Inc.)
                        with addenda dated as of August 13, 1987 and August 22,
                        1989(5)
 
(10.2)                  Amended and Restated Registration Rights Agreement dated as
                        of December 23, 1988 by and among the Registrant and various
                        beneficial owners of the Registrant's securities(5)
 
(10.3)x                 Restated Stock Option Plan(6)
 
(10.4)x                 Letter Agreement Regarding Employment dated as of
                        October 1, 1987 between the Registrant and Dr. Walter A.
                        Blattler(5)
 
(10.5)                  Lease dated May 15, 1997 by and between Harry F. Stimpson,
                        III, as trustees, lessor, and the Registrant, lessee(3)
 
(10.6)                  Leases dated as of December 1, 1986 and June 21, 1988 by and
                        between James H. Mitchell, Trustee of New Providence Realty
                        Trust, lessor, and Charles River Biotechnical
                        Services, Inc. ("Lessee") together with Assignment of Leases
                        dated June 29, 1989 between Lessee and the Registrant(7)
 
(10.7)                  First Amendment, dated as of May 9, 1991, to Lease dated as
                        of June 21, 1988 by and between James A. Mitchell, Trustee
                        of New Providence Realty Trust, lessor, and the
                        Registrant(8)
 
(10.8)                  Confirmatory Second Amendment to Lease dated June 21, 1988
                        by and between James A. Mitchell, Trustee of New Providence
                        Realty Trust, lessor, and the Registrant, Lessee(3)
 
(10.9)x                 Letter Agreement Regarding Compensation of Mitchel Sayare,
                        dated April 29, 1994(9)
 
(10.10)                 Lease dated as of December 23, 1992 by and between
                        Massachusetts Institute of Technology, lessor, and the
                        Registrant, lessee(6)
 
(10.11)                 Option Agreement dated April 5, 1990 by and between the
                        Registrant and Takeda Chemical Industries, Ltd.(10)
 
(10.12)                 Capital Lease Agreement dated March 31, 1994 by and between
                        the Registrant and Aberlyn Capital Management Limited
                        Partnership(9)
</Table>

 
                                       58
<Page>
 

<Table>
<Caption>
EXHIBIT NO.                                     DESCRIPTION
-----------             ------------------------------------------------------------
<S>                     <C>
(10.13)                 Sublease dated as of August 31, 1995 by and between the
                        Registrant, as landlord, and Astra Research Center Boston,
                        Inc., as tenant(11)
 
(10.14)                 Equipment Use and Services Agreement dated as of August 31,
                        1995 by and between the Registrant, as landlord, and Astra
                        Research Center Boston, Inc., as tenant(11)
 
(10.15)                 Consent to Sublease and Agreement dated as of August 31,
                        1995 by and between Massachusetts Institute of Technology,
                        as lessor, the Registrant, as sublessor, and Astra Research
                        Center Boston, Inc., as sublessee(11)
 
(10.16)                 Amendment to Lease dated August 31, 1995 between
                        Massachusetts Institute of Technology, as lessor, and the
                        Registrant, as lessee(12)
 
(10.17)                 Securities Purchase Agreement, including the Form of
                        Convertible Debenture and The Form of Stock Purchase
                        Warrant, dated as of March 15, 1996 by and among the
                        Registrant and Capital Ventures International(12)
 
(10.18)                 Registration Rights Agreement dated as of March 15, 1996 by
                        and among the Registrant and Capital Ventures
                        International(12)
 
(10.19)                 Letter Agreement dated as of March 21, 1996 by and among the
                        Registrant and Capital Ventures International regarding the
                        Securities Purchase Agreement dated as of March 15,
                        1996(12)
 
(10.20)                 Letter Agreement dated as of June 6, 1996 by and among the
                        Registrant and Capital Ventures International regarding an
                        amendment to their agreement dated March 15, 1996(13)
 
(10.21)                 First Amendment to Sublease dated August 31, 1995 by and
                        between the Registrant, as landlord, and Astra Research
                        Center Boston, Inc., as tenant(14)
 
(10.22)                 Convertible Debenture, dated as of June 28, 1996, by and
                        among the Registrant and The Dana-Farber Cancer Institute,
                        Inc.(15)
 
(10.23)                 Form of Warrant issued by the Registrant to LBC Capital
                        Resources, Inc.(15)
 
(10.24)                 Research Collaboration Agreement dated July 31, 1997 between
                        Apoptosis Technology, Inc. and BioChem
                        Therapeutic Inc.*(3)
 
(10.25)                 License Agreement dated July 31, 1997 between Apoptosis
                        Technology, Inc., BioChem Pharma Inc., Tanaud Holdings
                        (Barbados) Ltd. and Tanaud L.L.C.*(3)
 
(10.26)                 Stock Purchase Agreement dated July 31, 1997 by and among
                        Apoptosis Technology, Inc., BioChem Pharma (International)
                        Inc., and the Registrant*(3)
 
(10.27)                 Registration Agreement dated July 31, 1997 between the
                        Registrant and BioChem Pharma (International) Inc.(3)
 
(10.28)                 Registration Agreement dated July 31, 1997 between Apoptosis
                        Technology, Inc. and the Registrant(3)
 
(10.29)                 Form of Warrant issued by the Registrant to BioChem Pharma
                        (International) Inc.(3)
 
(10.30)                 Warrant Certificate dated September 16, 1997 issued to
                        Southbrook International Investments, Ltd.(16)
 
(10.31)                 Warrant Certificate dated July 31, 1997 issued to Capital
                        Ventures International(16)
 
(10.32)                 Warrant Certificate dated August 1, 1997 issued to Capital
                        Ventures International(16)
</Table>

 
                                       59
<Page>
 

<Table>
<Caption>
EXHIBIT NO.                                     DESCRIPTION
-----------             ------------------------------------------------------------
<S>                     <C>
(10.33)                 Warrant Certificate dated August 21, 1997 issued to Capital
                        Ventures International(16)
 
(10.34)                 Warrant Certificate dated October 6, 1997 issued to BioChem
                        Pharma (International)(16)
 
(10.35)                 Series E Convertible Preferred Stock Purchase Agreement by
                        and among ImmunoGen, Inc., Biotechnology Venture Partners,
                        L.P., Biotechnology Value Fund, L.P., Biotechnology Value
                        Fund, Ltd. and Investment 10, L.L.C. dated December 10,
                        1997*(4)
 
(10.36)                 Registration Agreement among ImmunoGen, Inc., Biotechnology
                        Venture Partners, L.P., Biotechnology Value Fund, L.P.,
                        Biotechnology Value Fund, Ltd. and Investment 10, L.L.C.
                        dated December 10, 1997(4)
 
(10.37)                 Form of Warrant Certificate issued by the Registrant to
                        Biotechnology Venture Partners, L.P., Biotechnology Value
                        Fund, L.P., Biotechnology Value Fund, Ltd. and Investment
                        10, L.L.C.(4)
 
(10.38)                 Warrant Certificate dated December 1,1997 issued to Capital
                        Ventures International(4)
 
(10.39)                 Warrant Certificate dated December 5,1997 issued to Capital
                        Ventures International(4)
 
(10.40)                 Warrant Certificate dated January 5,1998 issued to Capital
                        Ventures International(4)
 
(10.41)                 Warrant Certificate dated January 5, 1998 issued to BioChem
                        Pharma Inc.(4)
 
(10.42)                 First Amendment to Stock Purchase Agreement dated as of
                        March 18, 1998 by and among ImmunoGen, Inc., Biotechnology
                        Venture Partners, L.P., Biotechnology Value Fund, Ltd. and
                        Investment 10, L.L.C.*(17)
 
(10.43)                 License Agreement dated effective June 1, 1998 by and
                        between the Registrant and Pharmacia & Upjohn AB*(19)
 
(10.44)                 License Agreement dated February 1, 1999 between the
                        Registrant and SmithKline Beecham Corporation*(18)
 
(10.45)                 Stock Purchase Agreement dated February 1, 1999 between the
                        Registrant and SmithKline Beecham plc*(18)
 
(10.46)                 License Agreement dated effective May 2, 2000 by and between
                        the Registrant and Genentech, Inc.*(20)
 
(10.47)                 Heads of Agreement dated effective May 2, 2000 by and
                        between the Registrant and Genentech, Inc.*(20)
 
(10.48)                 Development,Commercialization and License Agreement dated
                        effective May 4, 2000 by and between the Registrant and
                        British Biotech Pharmaceuticals Limited*(20)
 
(10.49)                 Collaboration and License Agreement dated as of
                        September 29, 2000 by and between the Company and
                        MorphoSys AG.*(21)
 
(10.50)                 Option and License Agreement dated September 5, 2000 by and
                        between Abgenix, Inc. and the Company.*(22)
 
(10.51)                 Letter Agreement for Stock Purchase dated September 6, 2000
                        by and between Abgenix, Inc. and the Company.*(22)
 
(10.52)                 Agreement between ImmunoGen, Inc. and Millennium
                        Pharmaceuticals, Inc., dated March 30, 2001.*(23)
 
(10.53)                 Agreement between ImmunoGen, Inc. and Raven
                        Biotechnologies, Inc., dated March 28, 2001.*(23)
</Table>

 
                                       60
<Page>
 

<Table>
<Caption>
EXHIBIT NO.                                     DESCRIPTION
-----------             ------------------------------------------------------------
<S>                     <C>
(21)                    Subsidiaries of the Registrant, filed herewith
 
(23)                    Consent of PricewaterhouseCoopers LLP, filed herewith
</Table>

 
------------------------
 
 (1) Previously filed with the Commission as Exhibits to, and incorporated
     herein by reference from, the Registrant's Registration Statement on
     Form S-1, File No. 33-38883.
 
 (2) Previously filed with the Commission as Exhibits to, and incorporated
     herein by reference from, the Registrant's annual report on Form 10-K for
     the fiscal year ended June 30, 1990.
 
 (3) Previously filed with the Commission as an exhibit to, and incorporated
     herein by reference from, the Registrant's annual report on Form 10-K for
     the year ended June 30, 1997.
 
 (4) Previously filed as an exhibit to, and incorporated herein by reference
     from, the Registrant's quarterly report on Form 10-Q for the quarter ended
     December 31, 1997.
 
 (5) Previously filed with the Commission as Exhibits to, and incorporated
     herein by reference from, the Registrant's Registration Statement on
     Form S-1, File No. 33-31219.
 
 (6) Previously filed with the Commission as Exhibits to, and incorporated
     herein by reference from, the Registrant's quarterly report on Form 10-Q
     for the quarter ended December 31, 1992.
 
 (7) Previously filed with the Commission as Exhibit No. 10.10 to, and
     incorporated herein by reference from, the Registrant's Registration
     Statement on Form S-1, File No. 33-31219.
 
 (8) Previously filed with the Commission as Exhibit No. 10.10a to, and
     incorporated herein by reference from, the Registrant's Registration
     Statement on Form S-1, File No. 33-43725, as amended.
 
 (9) Previously filed with the Commission as Exhibits to, and incorporated
     herein by reference from the registrant's annual report on Form 10-K in the
     fiscal year ended June 30, 1994.
 
 (10) Previously filed with the Commission as Exhibit No. 10.15 to, and
      incorporated herein by reference from, the Registrant's Registration
      Statement on Form S-1, File No. 33-38883.
 
 (11) Previously filed with the Commission as Exhibits to, and incorporated
      herein by reference from, the Registrant's annual report on Form 10-K for
      the fiscal year ended June 30, 1995.
 
 (12) Previously filed as exhibits to the Registrant's Current Report on
      Form 8-K for the March 25, 1996 event, and incorporated herein by
      reference.
 
 (13) Previously filed as Exhibit 10.29 to the Registrant's Current Report on
      Form 8-K for the June 6, 1996 event, and incorporated herein by reference.
 
 (14) Previously filed as an exhibit to, and incorporated herein by reference
      from, the Registrant's annual report on Form 10-K for the fiscal year
      ended June 30, 1996.
 
 (15) Previously filed as an exhibit to, and incorporated herein by reference
      from, the Registrant's Registration Statement on Form S-3, File
      No. 333-07661.
 
 (16) Previously filed as an exhibit to, and incorporated herein by reference
      from, the Registrant's quarterly report on Form 10-Q, as amended by
      Form 10-Q/A, for the quarter ended September 30, 1997.
 
 (17) Previously filed as an exhibit to, and incorporated herein by reference
      from, the Registrant's quarterly report on Form 10-Q for the quarter ended
      March 31, 1998.
 
 (18) Previously filed as an exhibit to, and incorporated herein by reference
      from, the Registrant's quarterly report on Form 10-Q for the quarter ended
      December 31, 1998.
 
                                       61
<Page>
 (19) Previously filed as an exhibit to, and incorporated herein by reference
      from, the Registrant's annual report on Form 10-K for the fiscal year
      ended June 30, 1998.
 
 (20) Previously filed as an exhibit to, and incorporated herein by reference
      from, the Registrant's annual report on Form 10-K for the fiscal year
      ended June 30, 2000.
 
 (21) Previously filed as an exhibit to, and incorporated herein by reference
      from, the Registrant's current report on Form 8-K filed October 10, 2000.
 
 (22) Previously filed as an exhibit to, and incorporated herein by reference
      from, the Registrant's current report on Form 8-K/A filed October 10,
      2000.
 
 (23) Previously filed as an exhibit to, and incorporated herein by reference
      from, the Registrant's quarterly report on Form 10-Q for the fiscal
      quarter ended March 31, 2001.
 
 (x) Exhibit is a management contract or compensatory plan, contract or
     arrangement required to be filed as an exhibit to Form 10-K.
 
 (*) The Registrant has filed a confidential treatment request with the
     Commission with respect to this document.
 
 (b) Form 8-K dated September 6, 2000--Item 5: Other Events.
 
    Form 8-K dated September 29, 2000--Item 5: Other Events.
 
    Form 8-K dated November 8, 2000--Item 5: Other Events.
 
    Form 8-K dated March 5, 2001--Item 5: Other Events.
 
    Form 8-K dated March 7, 2001--Item 5: Other Events.
 
    Form 8-K dated March 29, 2001--Item 5: Other Events.
 
                                       62
<Page>

                                   SIGNATURES
 
    Pursuant to the requirements of Section 13 or 15(d) of the Securities and
Exchange Act of 1934, the Registrant has duly caused this report to be signed on
its behalf by the undersigned, thereunto duly authorized.
 

<Table>
<S>                                                    <C>  <C>
                                                       IMMUNOGEN, INC.
 
                                                       By:              /s/ MITCHEL SAYARE
                                                            -----------------------------------------
                                                                          MITCHEL SAYARE
                                                                    CHAIRMAN OF THE BOARD AND
                                                                     CHIEF EXECUTIVE OFFICER
Dated: September 28, 2001
</Table>

 
    Pursuant to the requirements of the Securities and Exchange Act of 1934,
this report has been signed below by the following persons on behalf of the
Registrant in the capacities and on the dates indicated.
 

<Table>
<Caption>
                  SIGNATURE                                  TITLE                        DATE
                  ---------                                  -----                        ----
<C>                                            <S>                                 <C>
                                               Chairman of the Board of
             /s/ MITCHEL SAYARE                  Directors, Chief Executive
    ------------------------------------         Officer and President (principal  September 28, 2001
               Mitchel Sayare                    executive)
 
           /s/ WALTER A. BLATTLER
    ------------------------------------       Executive Vice President, Science   September 28, 2001
             Walter A. Blattler                  and Technology, and Director
 
             /s/ GREGG D. BELOFF
    ------------------------------------       Vice President and Chief Financial  September 28, 2001
               Gregg D. Beloff                   Officer
 
             /s/ DAVID W. CARTER
    ------------------------------------       Director                            September 28, 2001
               David W. Carter
 
           /s/ MICHAEL R. EISENSON
    ------------------------------------       Director                            September 28, 2001
             Michael R. Eisenson
 
            /s/ STUART F. FEINER
    ------------------------------------       Director                            September 28, 2001
              Stuart F. Feiner
 
            /s/ MARK B. SKALETSKY
    ------------------------------------       Director                            September 28, 2001
              Mark B. Skaletsky
</Table>

 
                                       63
<Page>

                               INDEX TO EXHIBITS
 

<Table>
<Caption>
     EXHIBIT NO.                                DESCRIPTION
---------------------                           -----------
<S>                     <C>
 Ex. 21                 Subsidiaries
 Ex. 23                 Consent of PricewaterhouseCoopers LLP
</Table>





<Page>
                                                                      EXHIBIT 21
 
                                  SUBSIDIARIES
 
ImmunoGen Securities Corp.
 
Apoptosis Technology, Inc.




<Page>
                                                                      EXHIBIT 23
 
                       CONSENT OF INDEPENDENT ACCOUNTANTS
 
    We hereby consent to the incorporation by reference in the Registration
Statements on Forms S-3 (File Nos. 333-2441, 333-15819, 333-22153, 333-31795,
333-07661, 333-48385 and 333-57234) and Form S-8 (File No. 333-53292) of
ImmunoGen, Inc. of our report dated August 14, 2001 relating to the financial
statements and financial statement schedules, which appears in this Form 10-K.
 
Boston, Massachusetts
September 28, 2001