1
   
    AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON OCTOBER 16, 1998
    

                           REGISTRATION NO. 333-48385

                       SECURITIES AND EXCHANGE COMMISSION
   

                               AMENDMENT NO. 3 TO
    


                                    FORM S-3
                             REGISTRATION STATEMENT
                                      UNDER

                           THE SECURITIES ACT OF 1933

                                 ---------------

                                 IMMUNOGEN, INC.

             (Exact name of registrant as specified in its charter)

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

       333 PROVIDENCE HIGHWAY, NORWOOD, MASSACHUSETTS 02062 (781) 769-4242
      (Address, including zip code, and telephone, including area code, of
                   registrant's principal executive offices)

                                 ---------------

                                 MITCHEL SAYARE
                            CHAIRMAN OF THE BOARD AND
                             CHIEF EXECUTIVE OFFICER
                                 IMMUNOGEN, INC.
                             333 PROVIDENCE HIGHWAY
                                NORWOOD, MA 02062
                                 (781) 769-4242
 (Name, address, including zip code, and telephone number, including area code, 
                             of agent for service)

                                    COPY TO:

                          JONATHAN L. KRAVETZ, ESQUIRE
               MINTZ, LEVIN, COHN, FERRIS, GLOVSKY AND POPEO, P.C.
                              ONE FINANCIAL CENTER
                                BOSTON, MA 02111
                                 (617) 542-6000

                                 ---------------


   2


    Approximate date of commencement of proposed sale to public: As soon as
practicable after the effective date of this Registration Statement.

    If the only securities being registered on this Form are being offered
pursuant to dividend or interest reinvestment plans, please check the following
box.

    If any of the securities being registered on this Form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, as amended (the "Securities Act"), other than securities offered only in
connection with dividend or interest reinvestment plans, check the following
box.    [X]

    If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following box
and list the Securities Act registration statement number of the earliest
effective registration statement for the same offering.

    If this Form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering.

    If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box.


    THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNITL THE REGISTRANT SHALL
FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(a) OF
THE SECURITIES ACT OF 1933, AS AMENDED, OR UNTIL THE REGISTRATION STATEMENT
SHALL BECOME EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID
SECTION 8(a), MAY DETERMINE.


   3

                                   PROSPECTUS

                                 IMMUNOGEN, INC.
                         475,425 SHARES OF COMMON STOCK
                          (PAR VALUE OF $.01 PER SHARE)


The 475,425 shares of Common Stock, par value $.01 per share, ("Common Stock")
of ImmunoGen, Inc., a Massachusetts corporation ("ImmunoGen" or the "Company"),
offered hereby are being sold by the selling stockholders identified herein (the
"Selling Stockholders"). Such offers and sales may be made on one or more
exchanges, in the over-the-counter market, or otherwise, at prices and on terms
then prevailing, or at prices related to the then-current market price, or in
negotiated transactions, or by underwriters pursuant to underwriting agreements
in customary form, or in a combination of any such methods of sale. The Selling
Stockholders may also sell such shares in accordance with Rule 144 under the
Securities Act of 1933, as amended (the "Securities Act"). The Selling
Stockholders are identified and certain information with respect to them is
provided under the caption "Selling Stockholders" herein, to which reference is
made. The expenses of the registration of the securities offered hereby,
including fees of counsel for the Company, will be paid by the Company. The
following expenses will be borne by the Selling Stockholders: underwriting
discounts and selling commissions, if any, and the fees of legal counsel, if
any, for the Selling Stockholders.
   

The Selling Stockholders have advised the Company that they have not engaged any
person as an underwriter or selling agent for any of such shares, but may in the
future elect to do so, and they will be responsible for paying such a person or
persons customary compensation for so acting. The Selling Stockholders and any
broker executing sell orders on behalf of any Selling Stockholder may be deemed
to be "underwriters" within the meaning of the Securities Act, in which event
commissions received by any such broker may be deemed to be underwriting
commissions under the Securities Act. The Company will not receive any of the
proceeds from the sale of the securities offered hereby. The Common Stock is
listed on the Nasdaq Stock Market ("Nasdaq") under the symbol IMGN. On 
October 7, 1998, the closing sale price of the Common Stock, as reported by 
Nasdaq, was $1.25 per share.
    

                                 ---------------

         THE COMMON STOCK OFFERED HEREBY INVOLVES A HIGH DEGREE OF RISK.
                SEE "RISK FACTORS" ON PAGE 4 OF THIS PROSPECTUS.

                                 ---------------

    THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES
       AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION NOR HAS
         THE SECURITIES AND EXCHANGE COMMISSION OR ANY STATE SECURITIES
             COMMISSION PASSED UPON THE ACCURACY OR ADEQUACY OF THIS
                 PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY
                             IS A CRIMINAL OFFENSE.

                                 ---------------

    No person is authorized in connection with any offering made hereby to give
any information or to make any representations other than as contained in this
Prospectus, and, if given or made, such information or representations must not
be relied upon as having been authorized by the Company. This Prospectus is not
an offer to sell, or a solicitation of an offer to buy, by any person in any
jurisdiction in which it is unlawful for such person to make such an offer or
solicitation. Neither the delivery of this Prospectus nor any sales made
hereunder shall under any circumstances create any implication that the
information contained herein is correct as of any time subsequent to the date
hereof.

                                 ---------------
   

                    THE DATE OF THIS PROSPECTUS IS OCTOBER 16, 1998.
    
   4

                              AVAILABLE INFORMATION

The Company is subject to the reporting requirements of the Securities Exchange
Act of 1934, as amended (the "1934 Act"), and in accordance therewith files
reports and other information with the Securities and Exchange Commission (the
"Commission"). These reports, proxy statements and other information can be
inspected and copied, at prescribed rates, at the public reference facilities
maintained by the Commission at Room 1024 of the Commission's office at 450
Fifth Street, N.W., Judiciary Plaza, Washington, D.C. 20549, and at its regional
offices located at 7 World Trade Center, Suite 1300, New York, NY 10048 and
Citicorp Center, 500 West Madison Street, Suite 1400, Chicago, IL 60661. The
Commission maintains a Web site that contains reports, proxy and information
statements and other information regarding registrants that file electronically
with the Commission. The address of the Commission's Web site is
http://www.sec.gov. Additional updating information with respect to the
securities covered herein may be provided in the future to purchasers by means
of appendices to this Prospectus.

The Company has filed with the Commission in Washington, D.C. a registration
statement (herein, together with all amendments and exhibits, referred to as the
"Registration Statement") under the Securities Act with respect to the
securities offered or to be offered hereby. This Prospectus does not contain all
of the information included in the Registration Statement, certain items of
which are omitted in accordance with the rules and regulations of the
Commission. For further information about the Company and the securities offered
hereby, reference is made to the Registration Statement and the exhibits
thereto.

The Company will provide, without charge to each person to whom this Prospectus
is delivered, on the written or oral request of such person, a copy of any
document incorporated herein by reference, excluding exhibits. Requests should
be made to ImmunoGen, Inc., 333 Providence Highway, Norwood, MA 02062, telephone
(781) 769-4242 and directed to the attention of the Treasurer.

                                      - 2 -


   5




                                TABLE OF CONTENTS

                                                                  PAGE
                                                                  ----
    RISK FACTORS                                                 -  4 -
    THE COMPANY                                                  - 10 -
    SELLING STOCKHOLDERS                                         - 10 -
    PLAN OF DISTRIBUTION                                         - 11 -
    LEGALITY OF COMMON STOCK                                     - 11 -
    EXPERTS                                                      - 11 -
    INCORPORATION OF CERTAIN INFORMATION BY REFERENCE            - 12 -
    PART II. INFORMATION NOT REQUIRED IN PROSPECTUS               II-1
    SIGNATURES                                                    II-4






                                           -3-


   6



                                  RISK FACTORS

   
An investment in the shares being offered by this Prospectus involves a high
degree of risk. The following factors, in addition to those discussed elsewhere
in the Prospectus or incorporated herein by reference, should be carefully
considered in evaluating the Company and its business prospects before
purchasing shares offered by this Prospectus. This Prospectus contains and
incorporates by reference forward-looking statements within the "safe harbor"
provisions of the Private Securities Litigation Reform Act of 1995. Reference is
made in particular to the discussion set forth under "Management's Discussion
and Analysis of Financial Condition and Results of Operations" in the Company's
Annual Report on Form 10-K for the fiscal year ended June 30, 1998 (the "Form
10-K"), and under "Business" in the Form 10-K, incorporated in this Prospectus
by reference. Such statements are based on current expectations that involve a
number of uncertainties including those set forth in the risk factors below.
Actual results could differ materially from those projected in the forward
looking statements.
    

   
NEED FOR ADDITIONAL FUNDS AND ACCESS TO CAPITAL FUNDING. Since June 30, 1998,
the Company has raised capital from the following sources: $1,686,000
received to date by Apoptosis Technology, Inc. ("ATI"), the Company's
96.5%-owned subsidiary, from its collaborator, BioChem Pharma, Inc. ("BioChem"),
pursuant to a Stock Purchase Agreement dated July 31, 1997; $125,000 received
under a $750,000 grant from the Small Business Innovation Research ("SBIR")
Program of the National Cancer Institute to advance development over a two-year
period of the Company's lead product candidate, huC242-DM1; $1.5 million
received from the sale of the Company's Series E Convertible Preferred Stock to
an institutional investor; and $90,000 received from the assignee of one of the
Company's facilities. At September 30, 1998, the Company's remaining cash
resources totalled approximately $2.4 million. Based on the Company's current
operations, the Company believes that its present funds should be sufficient to
fund the Company's operations through at least January, 1999. In addition, the
Company is actively engaged in discussions with third parties regarding
potential financing and/or strategic partnering arrangements involving an equity
investment or other funding in the Company by such third parties. However, there
can be no assurance that these discussions will result in a completed
transaction. Any such additional funding may result in significant dilution to
the Company's current stockholders. If the Company is unable to obtain financing
on acceptable terms in the future or obtain funds through arrangements with
collaborative partners, which may require the Company to relinquish certain
material rights to its technology, it could be forced to further scale back or
discontinue its operations.
    

   
HISTORY OF OPERATING LOSSES AND ACCUMULATED DEFICIT. As of June 30, 1998, the
Company's accumulated deficit totalled approximately $148.7 million. The Company
expects to incur additional net losses over the next several years, assuming it
is able to raise sufficient working capital to continue operations. Based on the
amount of available cash at June 30, 1998, the report by the Company's auditors
for the year ended June 30, 1998 included an explanatory paragraph concerning
the uncertainties surrounding the Company's ability to continue as a going
concern. The amount of net losses and the time required by the Company to reach
sustained profitability are highly uncertain. To achieve profitability, the
Company must, among other things, successfully complete development of its
products, obtain regulatory approvals and establish manufacturing and marketing
capabilities. There can be no assurance that the Company will be able to achieve
profitability at all or on a sustained basis.
    

UNCERTAINTIES ASSOCIATED WITH CLINICAL TRIALS. The Company has conducted and
plans to continue to undertake extensive and costly clinical testing to assess
the safety and efficacy of its potential products. The rate of completion of the
Company's clinical trials is dependent upon, among other factors, the rate of
patient enrollment. Patient enrollment is a function of many factors, including
the nature of the Company's clinical trial protocols, existence of competing
protocols, size of the patient population, proximity of patients to clinical
sites and eligibility criteria for the study. Delays in patient enrollment will
result in increased costs and delays, which could have a material adverse effect
on the Company. The Company cannot assure that patients enrolled in the
Company's clinical trials will continue in the trials or will respond to the
Company's product candidates. Setbacks are to be expected in conducting human
clinical trials. Failure to comply with the United States Food and Drug
Administration (the"FDA") regulations applicable to such testing can result in
delay, suspension or cancellation of such testing, and/or refusal by the FDA to
accept the results of such testing. In addition, the FDA may suspend clinical
trials at any time if it concludes that the subjects or patients participating
in such trials are being exposed to unacceptable health risks. Alternatively,
the FDA may require additional trials if it finds that the results of the
ongoing trials are inconclusive. Further, there can be no assurance that human
clinical testing will show any current or future product candidate to be safe
and effective or that data derived therefrom will be suitable for submission to
the FDA. See "Business--Regulatory Issues," in the Company's Form 10-K.

                                       -4-
   7
EARLY STAGE OF INITIAL PRODUCT DEVELOPMENT. The Company has not begun to market
or generate revenues from the sale of products. In March 1997, the Company
discontinued development of its then lead product candidate, Oncolysin B, based
on preliminary data from the Phase III clinical trial. The Company intends to
focus its resources on potential products under development for the treatment of
colorectal cancer and small-cell lung cancer, now in research and preclinical
development, and on drug development based on proprietary apoptosis screens. The
Company's products will require significant additional development, laboratory
and clinical testing and investment prior to commercialization. There can be no
assurance that such products will be successfully developed, prove to be safe
and efficacious in clinical trials, meet applicable regulatory standards, obtain
required regulatory approvals, be capable of being produced in commercial
quantities at reasonable costs or be successfully marketed.

MANUFACTURING RISKS. The Company has not yet commercially introduced any
products. To be successful, the Company's products must be manufactured in
commercial quantities, in compliance with regulatory requirements and at
acceptable costs. Although the Company has produced its products in the
laboratory and scaled its production process to pilot levels, production in
commercial quantities will create substantial technical as well as financial
challenges for the Company. The Company's current facilities are not yet
approved by the FDA for commercial production of its proposed products, and
there can be no assurance that such approval will be obtained. In order to
manufacture its products in commercial quantities, the Company will have to
enhance its existing manufacturing facilities, which will require additional
funds, or rely on the manufacturing expertise of a strategic partner or contract
manufacturer. The Company has no experience in large-scale manufacturing, and no
assurance can be given that the Company will be able to make the transition to
commercial production successfully.

LACK OF MARKETING AND SALES EXPERIENCE. Although the Company may market certain
of its products through a direct sales force if and when regulatory approval is
obtained, it currently has no marketing or sales staff. To the extent that the
Company determines not to, or is unable to, arrange third-party distribution for
its products, significant additional expenditures, management resources and time
will be required to develop a sales force. There can be no assurance that the
Company will be able to establish such a sales force or be successful in gaining
market acceptance for its products.

RAPID AND SIGNIFICANT TECHNOLOGICAL CHANGE; COMPETITION. The biotechnology
industry is subject to rapid and significant technological change. Competitors
of the Company engaged in all areas of biotechnology in the United States and
abroad are numerous and include major pharmaceutical and chemical companies,
specialized biotechnology firms, universities and other research institutions.
There can be no assurance that the Company's competitors will not succeed in
developing technologies and products that are more effective than any which have
been or are being developed by the Company or which would render the Company's
technology and products obsolete and noncompetitive. The Company's competitors
may also succeed in obtaining patent protection in other intellectual property
rights that would block the Company's ability to develop its proposed products.
Many of these competitors have substantially greater financial and technical
resources and production and marketing capabilities than the Company. In
addition, many of the Company's competitors have significantly greater
experience than the Company in preclinical testing and human clinical trials of
new or improved pharmaceutical products and in obtaining the FDA and other
regulatory approvals of products for use in health care. The Company has limited
experience in conducting and managing preclinical and clinical testing necessary
to obtain government approvals. Accordingly, the Company's competitors may
succeed in obtaining FDA approval for products more rapidly than the Company. If
the Company commences significant commercial sales of its products, it will also
be competing with respect to manufacturing efficiency and marketing
capabilities, areas in which it currently has no experience.

RISK OF THIRD-PARTY CLAIMS OF INFRINGEMENT; DEPENDENCE ON PATENTS AND
PROPRIETARY RIGHTS. The patent situation in the field of biotechnology generally
is highly uncertain 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, there can be no assurance
that patent applications relating to the Company's products or technology will
result in patents being issued or that, if issued, the patents will afford
protection against competitors with similar technology.

There has been significant litigation in the biotechnology industry regarding
patent and other intellectual property rights. This litigation is likely to
continue in the future. If the Company becomes involved in such litigation, it
could consume a substantial portion of the Company's resources. Also, patents
and applications owned or licensed by the Company may become the subject of
infringement proceedings in the United States Patent and Trademark Office to
determine priority of invention, which could result in substantial cost to the
Company, as well as a possible adverse decision as to priority of invention of
the patent or patent application involved. An adverse decision in an
infringement proceeding may result in the Company's loss of rights under a
patent or patent application subject to such a proceeding. In addition,
companies may obtain patents claiming products or processes that are necessary
for or useful to the development of the Company's products and bring legal
actions against the Company claiming infringement and

                                       -5-
   8

may seek to recover damages and to enjoin the Company from manufacturing and
marketing the affected product or process. If any such actions are successful,
in addition to any potential liability for damages, the Company may be required
to obtain licenses from others to continue to develop, manufacture or market its
products. There can be no assurance that the Company will prevail in any such
action or that it will be able to obtain such licenses on commercially
reasonable terms.
   

The Company and its subsidiary, ATI, seek patent protection for their
proprietary technology and products both in the United States and abroad. The
Company has received two United States patents and one European patent claiming
the use of maytansinoids in conjugated form as an invention, two United States
patents and one Notice of Allowance of a United States patent claiming use of
DC1 and its analogs in immunoconjugates and one United States patent claiming
methods and use of its resurfacing technology. ATI has received two United
States patents. One patent claims the GD (BH3) domain as a molecular target for
the development of drugs which regulate apoptosis and one patent claims specific
antibodies to the apoptosis-related protein, Bcl-Y (also referred to as Bak).
ATI has also received four Notices of Allowance of United States patents. One
Notice of Allowance claims composition of matter of the GD (BH3) domain, one
claims the anti-proliferation domain of Bcl-2, one claims composition of matter
of the apoptosis-related protein, Bbk, and one claims the apoptosis gene, EI24.
    

In addition, four patents have been issued to Dana-Farber Cancer Institute
("Dana-Farber") in the United States covering immunoconjugate technology
exclusively licensed by ImmunoGen from Dana-Farber. Two of these patents claim a
monoclonal antibody specific to small-cell lung carcinoma cells as an invention
and two of these patents claim the use of blocked ricin in immunoconjugates. A
patent assigned to Dana-Farber and exclusively licensed to ATI, claiming
composition of TIA 1 binding proteins, was allowed. Nine additional Dana-Farber
patents had been exclusively licensed to ImmunoGen, or ATI, and, at the
Company's option, have reverted back to Dana-Farber.

In order to practice its antibody humanization technology using either
Complementarity Determining Region grafting or resurfacing, the Company may need
to obtain one or more licenses under patents issued to third parties. The
Company understands that such licenses may be available on what it believes to
be commercially acceptable terms. However, there can be no assurance that any
such licenses will in fact be, or continue to be, available on commercially
acceptable terms, if at all.

The Company also relies upon unpatented proprietary technology. No assurance can
be given that others will not duplicate or independently develop substantially
equivalent technology, or otherwise gain access to the Company's proprietary
technology or disclose such technology, or that the Company can meaningfully
protect its rights in such unpatented proprietary technology.

DEPENDENCE ON OTHERS. The Company plans to conduct certain aspects of its future
operations with third-party collaborators who may fund collaborative work with
the Company. While the Company believes its potential collaborators will have an
economic motivation to succeed in performing their obligations under such
arrangements, the amount and timing of funds and other resources to be devoted
under such arrangements will be controlled by such other parties and would be
subject to financial or other difficulties that may befall such other parties.
Thus, no assurance can be given that the Company will generate any revenues from
such arrangements. In addition, although the Company is currently exploring
entry into such arrangements, no such arrangements have been concluded nor is
there any assurance that any such arrangements will ever come into effect.

DEPENDENCE ON KEY PERSONNEL. The Company's success is dependent on certain key
management and scientific personnel. Competition for qualified employees among
biotechnology companies is intense, and the loss of key personnel, or the
inability to attract and retain the additional, highly skilled employees
required for the expansion of the Company's activities, could adversely affect
its business, financial condition and results of operation. The Company has
entered into employment contracts with certain of its key employees. "Key
Person" life insurance is not maintained with respect to any employee of the
Company.

GOVERNMENT REGULATION. The production and marketing of the Company's products
and its ongoing research and development activities are subject to regulation by
numerous governmental authorities in the United States and other countries. The
rigorous preclinical and clinical testing requirements and regulatory approval
processes typically take a number of years and require the expenditure of
substantial resources. Delays in obtaining regulatory approvals would adversely
affect the marketing of products developed by the Company and the Company's
ability to receive product revenues or royalties. In light of the limited
regulatory history of monoclonal antibody-based therapeutics, there can be no
assurance that regulatory approvals for the Company's products will be obtained
without lengthy delays, if at all. Moreover, the Company is, 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 the Company's research work. In
addition, the Company cannot predict the extent to which existing or proposed
governmental regulations might have an adverse effect on the production and
marketing of the Company's products.

                                       -6-
   9
DEPENDENCE ON THIRD-PARTY REIMBURSEMENT. In both domestic and foreign markets,
sales of the Company's proposed products 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. There can be no
assurance that the Company's proposed products will be considered cost effective
or that adequate third-party reimbursement will be available to enable ImmunoGen
to maintain price levels sufficient to realize an appropriate return on its
investments in product development. Legislation and regulations affecting the
pricing of pharmaceuticals may change before any of the Company's proposed
products are approved for marketing. Adoption of such legislation and
regulations could further limit reimbursement for medical products and services.

HAZARDOUS MATERIALS; ENVIRONMENTAL MATTERS. The Company's research and
development activities involve the controlled use of hazardous materials,
chemicals, biological materials and radioactive compounds. The Company is
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 the Company believes that its 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, the
Company could be held liable for any resulting damages, and any such liability
could exceed the Company's resources. The Company may be required to incur
significant costs to comply with environmental laws and regulations in the
future. Current or future environmental laws or regulation may have a material
adverse effect on the Company's business, financial condition and results of
operation.

PRODUCT LIABILITY EXPOSURE. The use of the Company's product candidates during
testing or after approval entails an inherent risk of adverse effects which
could expose the Company to product liability claims. There can be no assurance
that the Company would have sufficient resources to satisfy any liability
resulting from these claims. The Company currently has $5.0 million of product
liability insurance for products which were in clinical testing. Although there
can be no assurance that such coverage will be adequate in scope to protect the
Company in the event of a successful product liability claim, the Company
believes that this level of coverage is sufficient. Further, there can be no
assurance that the Company will be able to maintain such insurance or obtain
general product liability insurance on reasonable terms and at an acceptable
cost once the Company begins commercial production of its proposed products or
that such insurance will be in sufficient amounts to provide the Company with
adequate coverage against potential liabilities.

VOLATILITY OF STOCK PRICE. The market prices for securities of biotechnology
companies have been volatile. The market price for the Common Stock has
fluctuated significantly since public trading commenced in 1989, and it is
likely that the market price will continue to fluctuate in the future.
Announcements of technological innovations or new commercial products by the
Company or its competitors, developments concerning proprietary rights,
including patents and litigation matters, publicity regarding actual or
potential medical results relating to products under development by the Company
or its competitors, regulatory developments in both the United States and
foreign countries, public concern as to the safety of biotechnology products and
economic and other external factors, including the outbreak or material
escalation of hostilities or other calamity or crisis, as well as
period-to-period fluctuations in financial results, may have a significant
impact on the Company's business and on the market price of the Common Stock.
Sales of substantial amounts of the Common Stock in the public market may also
have an adverse impact on the market price of the Common Stock.

ABSENCE OF DIVIDENDS. The Company has not paid any cash dividends on its capital
stock since inception. Furthermore, the Company does not anticipate paying cash
dividends in the foreseeable future.

   
SHARES ELIGIBLE FOR FUTURE SALE. Sales of substantial amounts of Common Stock in
the public market could have an adverse effect on the price of the Common Stock
and on the ability of the Company to raise additional capital when needed. In
addition to the shares offered hereby, approximately 25,019,127 shares of Common
Stock were freely tradable on the open market as of October 7, 1998. Of such
shares, approximately 874,270 shares are eligible for sale pursuant to Rule 144
of the Securities Act. Also, as of October 7, 1998, options to purchase
2,642,979 shares of Common Stock were outstanding pursuant to the Company's
stock option plans and options to purchase 1,346,292 of such shares were vested
and could be exercised at any time prior to their respective expiration dates.
As of October 7, 1998, 26,738 shares of Common Stock were issuable upon the
exercise of warrants issued in connection with a capital lease financing in
March 1994, 2,102,299 shares of Common Stock were issuable upon the exercise of
warrants issued to date in connection with the Company's March 1996 Financing
(see below) and 2,597,117 shares of Common Stock were issuable upon the exercise
of warrants issued to date in connection with the Company's October 1996 Private
Placement (see below). Also, as of October 7, 1998, warrants to purchase shares
of Common Stock equal to $6,067,000 invested to that 
    

                                       -7-
   10
date in the Company's subsidiary, ATI, by BioChem have been issued, and
2,823,528 shares of Common Stock were issuable upon the exercise of warrants
issued to that date in connection with the Company's December 1997 Private
Placement (see below). 

In March 1996, the Company issued $5.0 million principal amount convertible
debentures (the "Debentures") in a private placement (the "March 1996
Financing"). As part of the March 1996 Financing, the Company issued a $2.5
million principal amount debenture (the "First Debenture") on March 25, 1996. In
June 1996, the First Debenture, together with interest thereon, was converted
into shares of Common Stock, and warrants (the "First Warrant") to purchase
509,000 shares of Common Stock (the "First Warrant Shares") at an exercise price
of $4.00 per share were issued. The First Warrants expire in March 2001. In June
1996, a second $2.5 million convertible debenture (the "Second Debenture") was
issued and then converted into 2,500 shares of Series A Convertible Preferred
Stock ("Series A Stock") in October 1996. As of January 5, 1998, all 2,500
shares of Series A Stock plus accrued interest thereon had been converted into
2,676,235 shares of the Company's Common Stock. In June 1996, the Company issued
additional warrants to purchase 500,000 shares of the Company's Common Stock
(the "Additional Warrants") in connection with the conversion of the First
Debenture into Common Stock. The Additional Warrants have an exercise price
equal to $6.00 per share and expire in March 2001. Additionally, warrants to
purchase 250,000 shares of the Company's Common Stock were issued as finder's
fees in connection with the issuance of the Debentures. Upon conversion of the
Series A Stock, the holder received warrants (the "Second Warrants") to purchase
a number of shares of Common Stock equal to 50% of the number of shares issuable
upon conversion of the Series A Stock. The Second Warrants are exercisable at
$4.00 per share and expire five years after the date of issuance. As of January
5, 1998, warrants to purchase 1,338,117 shares of the Company's Common Stock
were issued on conversion of the Series A Stock. There can be no assurance,
however, that any or all of the warrants will be exercised, or that the Company
will receive any proceeds from such exercise. The Company has registered for
resale 3,877,000 shares of Common Stock in connection with the Series A Stock,
the First and Second Warrants, the Additional Warrant and the Warrants issued as
a finder's fee.

In October 1996, the Company sold $3.0 million of Series B Convertible Preferred
Stock ("Series B Stock") under a financing agreement it entered into in October
1996 ("the October 1996 Private Placement"). As of February 4, 1997, all 3,000
shares of the 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 (the
"October 1996 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. There can be no assurance that any or all of the
warrants will be exercised, or that the Company will receive any proceeds from
such exercise. The Company has registered for resale 1,916,666 shares of Common
Stock in connection with the Series B Stock and October 1996 Warrants.

In January 1997, the Company sold $3.0 million of Series C Convertible Preferred
Stock ("Series C Stock") in connection with the October 1996 Private Placement.
As of August 1, 1997, all 3,000 shares of the Series C Stock plus accrued
dividends thereon had been converted into 2,719,738 shares of the Company's
Common Stock. In connection with the Series C Stock, 1,147,754 warrants to
purchase Common Stock were issued to the investor ("the April 1997 Warrants").
These warrants are exercisable at $2.31 per share and expire in April, 2002.
There can be no assurance that any or all of the warrants will be exercised, or
that the Company will receive any proceeds from such exercise. The Company has
registered for resale 6,000,000 shares of Common Stock in connection with the
Series C Stock and April 1997 Warrants.

In June 1997, the Company sold $1.0 million of Series D Convertible Preferred
Stock ("Series D Stock") in connection with the October 1996 Private Placement.
As of October 21, 1997, all 1,000 shares of the Series D Stock plus accumulated
dividends thereon had been converted into 1,001,387 shares of the Company's
Common Stock. In connection with the Series D Stock, 454,545 warrants to
purchase Common Stock were issued to the investor ("the September 1997
Warrants"). These warrants are exercisable at $1.94 per share and expire in
September 2002. There can be no assurance that any or all of the warrants will
be exercised, or that the Company will receive any proceeds from such exercise.
The Company has registered for resale 2,757,862 shares of Common Stock in
connection with the Series D Stock and September 1997 Warrants.
   

In July 1997, ATI entered into a collaboration agreement with BioChem. The
agreement grants 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. The agreement also covers
the development of new screens in two areas. Under the agreement, BioChem will
invest a total of $11.125 million in non-voting convertible preferred stock of
ATI in a series of private placements over a three-year period to be used
exclusively to fund research conducted under the collaboration during a
three-year research term. As of October 7, 1998, $6.067 million has been paid
under the agreement. The balance of $5.058 million will be paid in equal
quarterly installments of $843,000. The preferred stock is convertible into ATI
common stock at any time after three years from the date of first issuance of
    

                                       -8-
   11

such stock, 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. As part of this
agreement, BioChem also receives warrants to purchase shares of ImmunoGen Common
Stock equal to the amount invested in ATI during the three-year research term.
These warrants will be exercisable for a number of shares of ImmunoGen Common
Stock determined by dividing the amount of BioChem's investment in ATI by the
market price of the 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. Consequently, BioChem's ability to convert all
of its ImmunoGen warrants into ImmunoGen Common Stock is limited to a total of
20% of the total number of shares of ImmunoGen 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
shareholder approval for such conversion is obtained, if required, or unless the
Company has obtained a waiver of that requirement. The exercise price in
connection with the warrants is payable either in cash or shares of ATI
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. In the event that ATI common stock does not become publicly traded, the
Company expects that BioChem will use its shares of ATI preferred stock, in lieu
of cash, to exercise the warrants. BioChem's obligation to provide additional
financing to ATI each quarter is subject to satisfaction of specified
conditions, including a condition that ATI maintain sufficient cash and other
resources to allow it to continue its planned operations (other than performance
of its obligations under a certain Research Agreement) for a minimum period of
time. 

In December 1997, the Company entered into an agreement to sell $3.0 million of
its Series E Convertible Preferred Stock ("Series E Stock") to an institutional
investor. As of July 13, 1998, all $3.0 million had been received. The Series E
Stock will be convertible into Common Stock at the end of a two-year holding
period at $1.0625 per share. Under the terms of the agreement, the investor
received warrants equal to 100% of the number of shares of Common Stock
issuable on conversion of the Series E Stock. As of July 13, 1998, warrants to
purchase 2,823,528 shares of Common Stock had been issued. These warrants have
an exercise price of $2.125 per share and expire in 2004 as to 941,176 warrants
and 2005 as to 1,882,352 warrants. There can be no assurance that any or all of
the warrants will be exercised, or that the Company will receive any proceeds
from such exercise. The holders of the Series E Stock have certain rights to
register Common Stock issued in connection with the sale of the Series E Stock
under the Securities Act for sale to the public, subject to certain conditions
and limitations. These registration rights can be exercised for a period of two
years following the first issuance of registrable securities. Only one such
request for registration of Common Stock can be made with respect to each sale
of Series E Stock. In addition, the Company issued 75,000 shares of Common
Stock to a third party as a finder's fee in connection with the Series E Stock
transaction. 

Proceeds from all of the private placements discussed in this Section have been 
used to fund working capital requirements. Proceeds from the BioChem
transaction were used to fund a discrete ATI research program, not to fund
ImmunoGen programs.

Certain affiliates of the Company hold approximately 792,769 shares of Common
Stock (the "Registrable Securities") and are entitled to certain rights to
register such shares under the Securities Act for sale to the public, pursuant
to a Registration Rights Agreement by and among the Company and the holders of
Registrable Securities, as amended (the "Registration Rights Agreement"). The
holders of Registrable Securities include Aeneas Venture Corporation. Such
holders have the right to require the Company, on not more than two occasions,
whether or not the Company proposes to register any of its Common Stock for
sale, to register all or part of their shares for sale to the public under the
Securities Act, subject to certain conditions and limitations. In addition,
holders of Registrable Securities may require the Company to register all or
part of their shares on Form S-3 (or a successor short Form of registration) if
the Company then qualifies for use of such form, subject to certain conditions
and limitations. The Registration Rights Agreement was amended on October 9,
1991 to limit the circumstances pursuant to which the registration rights
granted thereunder may be transferred to third parties and to amend certain
procedural requirements. 

In addition, pursuant to registration rights agreements between the Company and
holders of the Series A Stock, Series B Stock, Series C Stock and Series D
Stock (the "Preferred Stock") and/or related warrants (the "Warrants"), such
holders are entitled to rights to require the Company to register for resale to
the public under the Securities Act all shares of Common Stock issued or
issuable to such holders on conversion of the Preferred Stock and /or exercise
of the Warrants. The number of shares and warrants issued to the preferred
shareholder is determined according to a predetermined formula. As of July 16,
1998, approximately 4,699,416 shares of Common Stock are beneficially held by
six such holders. 

   
As part of the agreement entered into in July 1997 between the Company, its
subsidiary, ATI, and BioChem (the "holder"), the holder receives warrants to
purchase shares of the Company's Common Stock equal to the amount invested in
ATI over a three-year period. These warrants become exercisable at the end of
the three-year period at the then current market price of the Common Stock.
Pursuant to a registration rights agreement between the Company and the holder,
at the end of the three-year period the holder is entitled to certain rights to
require the Company to register for sale to the public under the Securities Act
all registrable securities. As of October 7, 1998, amounts to purchase Common
Stock equal to the $6.067 million invested as of that date have been issued to
the holder.
    

   
RISK ASSOCIATED WITH THE YEAR 2000. Many computer systems were not designed to
handle any dates beyond the year 1999 and, therefore, computer hardware and
software will need to be modified prior to the year 2000 in order to remain
functional; this is the so-called "Year 2000" problem. The Company does not
believe that it has material exposure with respect to Year 2000 issues. However,
the failure by the Company to convert systems on a timely basis, or a conversion
by the Company that is incompatible with other information systems, could have a
material effect on its business, financial condition and results of operations.
Further, the Company is unable to ascertain the extent to which the Year 2000
issues will affect its clinical suppliers, investment custodians,
telecommunications providers, third-party research and testing vendors or other
third-party hardware and software products. Though not considered likely, the
failure of a major supplier or vendor with Year 2000 problems to convert its
systems on a timely basis, or a conversion that is incompatible with the
Company's information systems, could also have a material adverse effect on the
Company's business, financial condition and results of operations. To date, the
Company has not sent questionnaires or sought certifications from third parties
with respect to their Year 2000 compliance and is considering whether or not
this is appropriate, given the nature of the Company's operations.
    

   
The Company, in conjunction with its information systems consultant, has 
performed an initial evaluation of the impact of the Year 2000 issues on the 
Company's information systems and has determined that it will be required to 
modify or replace certain accounting and administrative software applications 
such that dates beyond June 30, 1999, the beginning of the Company's fiscal 
year 2000, will be appropriately recognized. The Company has been assured that 
commercially produced compliant software packages are readily available. All 
remediations are planned to be completed before the end of fiscal year 1999. 
The Company is not currently able to estimate the total expense it may incur in 
evaluating and remediating any Year 2000 issues, but does not expect those 
expenses to be material. Costs and expenses of evaluating and remediating Year 
2000 issues will be expensed as they are incurred.
    

DILUTION. Dilution is likely to occur upon conversion of the Company's
outstanding preferred stock, and also upon the exercise of outstanding stock
options and warrants. See "Shares Eligible for Future Sale."

                                       -9-
   12

                                   THE COMPANY


ImmunoGen develops pharmaceuticals, primarily for the treatment of cancer. In
March 1997, the Company discontinued development of its then lead product
candidate, Oncolysin B, based on preliminary data from the Phase III clinical
trial. The Company intends to focus its resources on potential products under
development for the treatment of colorectal cancer and small-cell lung cancer,
now in research and preclinical development, and on drug development based on
proprietary apoptosis screens.

Through its majority-owned subsidiary, ATI, the Company is developing additional
technology platforms based on the regulation of programmed cell death, or
apoptosis. ATI is applying its understanding of how apoptotic pathways are
triggered in cells to identify product candidates for the treatment of cancer
and viral infections, two targets where inhibition of apoptosis is recognized as
an essential element of the disease. ATI has identified several key proteins
which play a role in the regulation of apoptosis in cancer cells and viruses
and, using these, has developed proprietary screens with which to identify leads
for drug development.

The Company was organized in 1981 as a Massachusetts corporation. The Company's
principal offices are located at 333 Providence Highway, Norwood, Massachusetts
02062, and its telephone number is (781) 769-4242.

                              SELLING STOCKHOLDERS

The shares of Common Stock offered hereby by the Selling Stockholders were
issued by the Company to the Selling Stockholders pursuant to a Stock Purchase
Agreement dated as of January 11, 1993 between the Company, Dr. Stuart
Schlossman, Dana Farber and ATI.

The number of shares registered in the Registration Statement of which this
Prospectus is a part and the number of shares offered hereby have been
determined by agreement between the Company and the Selling Stockholders.
   

The following table sets forth information with respect to the beneficial
ownership of the Common Stock by the Selling Stockholders as of October 7,
1998, and as adjusted to reflect the sale of the Common Stock offered hereby by
the Selling Stockholders. 
    


SHARES NUMBER OF SHARES OWNED PRIOR TO SHARES BEING OWNED AFTER SELLING STOCKHOLDER OFFERING OFFERED OFFERING(1) - ------------------- -------------- ------------ ----------- NUMBER PERCENT(2) ------ ------- Stuart F. Schlossman, M.D.(3) 237,713 237,713 0 * Stuart Schlossman 1994 Irrevocable Trust dated as of 9/26/94 - Robert L. Schlossman Share 118,856 118,856 0 * Stuart Schlossman 1994 Irrevocable Trust dated as of 9/26/94 - Peter E. Schlossman Share 118,856 118,856 0 *
* Less than 1% (1) Assumes the sale of all shares offered hereby to unaffiliated third parties. (2) Based on 25,494,552 shares of Common Stock outstanding on October 7, 1998. (3) Dr. Stuart Schlossman disclaims beneficial ownership of shares owned by the trusts. -10- 13 PLAN OF DISTRIBUTION The 475,425 shares of Common Stock of the Company offered hereby may be offered and sold from time to time by the Selling Stockholders, or by pledgees, donees, transferees or other successors in interest. Such offers and sales may be made from time to time on one or more exchanges or in the over-the-counter market, or otherwise, at prices and on terms then prevailing or at prices related to the then-current market price, or in negotiated transactions. The methods by which the shares may be sold may include, but not be limited to, the following: (a) a block trade in which the broker or dealer so engaged will attempt to sell the shares as agent but may position and resell a portion of the block as principal to facilitate the transaction; (b) purchases by a broker or dealer as principal and resale by such broker or dealer for its account; (c) an exchange distribution in accordance with the rules of such exchange; (d) ordinary brokerage transactions and transactions in which the broker solicits purchasers; (e) privately negotiated transactions; (f) short sales; and (g) a combination of any such methods of sale. In effecting sales, brokers or dealers engaged by the Selling Stockholders may arrange for other brokers or dealers to participate. Brokers or dealers may receive commissions or discounts from the Selling Stockholders or from the purchasers in amounts to be negotiated immediately prior to the sale. The Selling Stockholders may also sell such shares in accordance with Rule 144 under the Securities Act. From time to time the Selling Stockholders may engage in short sales, short sales against the box, puts and calls and other transactions in securities of the Company or derivatives thereof, and may sell and deliver the shares in connection therewith. From time to time Selling Stockholders may pledge their shares pursuant to the margin provisions of their respective customer agreements with their respective brokers. Upon a default by a Selling Stockholder, the broker may offer and sell the pledged shares of Common Stock from time to time. The Company has agreed to use its best efforts to maintain the effectiveness of the registration of the shares being offered hereunder until October 2000 or such earlier date when all of the shares being offered hereunder have been sold or may be sold without volume or other restrictions pursuant to Rule 144 or Rule 144A under the Securities Act, as determined by counsel to the Company pursuant to a written opinion letter. The Selling Stockholders and any brokers participating in such sales may be deemed to be underwriters within the meaning of the Securities Act. There can be no assurance that the Selling Stockholders will sell any or all of the shares of Common Stock offered hereunder. All proceeds from any such sales will be the property of the Selling Stockholders who will bear the expense of underwriting discounts and selling commissions, if any, and its own legal fees. LEGALITY OF COMMON STOCK The validity of the shares of Common Stock offered hereby is being passed upon for the Company by Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., Boston, Massachusetts. EXPERTS The financial statements incorporated in this Prospectus by reference to the Company's Annual Report on Form 10-K for the fiscal year ended June 30, 1998 have been so incorporated in reliance on the report (which includes an explanatory paragraph concerning uncertainties surrounding the Company's ability to continue as a going concern) of PricewaterhouseCoopers LLP, independent accountants, given on the authority of said firm as experts in auditing and accounting. -11- 14 INCORPORATION OF CERTAIN INFORMATION BY REFERENCE The following documents filed with the Commission are incorporated herein by reference: (a) The Company's Annual Report on Form 10-K for the fiscal year ended June 30, 1998. (b) The description of the Company's capital stock contained in the Company's registration statement on Form 8-A under the 1934 Act (File No. 0-17999), including amendments or reports filed for the purpose of updating such description. All reports and other documents subsequently filed by the Company with the Commission pursuant to Sections 13(a), 13(c), 14 and 15(d) of the 1934 Act, prior to the filing of a post-effective amendment which indicates that all securities covered by this Prospectus have been sold or which deregisters all such securities then remaining unsold, shall be deemed to be incorporated by reference herein and to be a part hereof from the date of the filing of such reports and documents. -12- 15 PART II. INFORMATION NOT REQUIRED IN PROSPECTUS ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION. The following expenses incurred in connection with the sale of the securities being registered will be borne by the Registrant. Other than the SEC registration fee, the amounts stated are estimates. SEC Registration Fee $ 245.44 Legal Fees and Expenses 10,000.00 Accounting Fees and Expenses 7,500.00 Miscellaneous 15,500.00 ----------- TOTAL $ 33,245.44 ===========
The Selling Stockholder will bear the expense of its own legal counsel and miscellaneous fees and expenses, if any. ITEM 15. INDEMNIFICATION OF DIRECTORS AND OFFICERS. Article 6(d) of the Registrant's Restated Articles of Organization provides as follows: "(d) The liability of the Directors of the Corporation shall be limited to the fullest extent permitted by Section 13(b)(1 1/2) of the Massachusetts Business Corporation Law." Section 6.6 of the Registrant's By-Laws provides as follows: "Section 6.6 Indemnification of Officers, Directors, and Members of the Scientific Advisory Board. The corporation shall indemnify and hold harmless each person, now or hereafter an officer or Director of the corporation, or a member of the Scientific Advisory Board, from and against any and all claims and liabilities to which he may be or become subject by reason of his being or having been an officer, Director of member of the Scientific Advisory Board of the corporation or by reason of his alleged acts or omissions as an officer, Director or member of the Scientific Advisory Board of the corporation, and shall indemnify and reimburse each such officer, Director and member of the Scientific Advisory Board against and for any and all legal and other expenses reasonably incurred by him in connection with any such claims and liabilities, actual or threatened, whether or not at or prior to the time which so indemnified, held harmless and reimbursed he has ceased to be an officer, Director or member of the Scientific Advisory Board of the corporation, except with respect to any matter as to which such officer, Director or member of the Scientific Advisory Board of the corporation shall have been adjudicated in any proceeding not to have acted in good faith in the reasonable belief that his action was in the best interest of the corporation; provided, however, that prior to such final adjudication the corporation may compromise and settle any such claims and liabilities and pay such expenses, if such settlement or payment or both appears, in the judgment of a majority of those members of the Board of Directors who are not involved in such matters, to be for the best interest of the corporation as evidenced by a resolution to that effect adopted after receipt by the corporation of a written opinion of counsel for the corporation, that, based on the facts available to such counsel, such officer, Director or member of the Scientific Advisory Board of the corporation has not been guilty of acting in a manner that would prohibit indemnification. Such indemnification may include payment by the corporation of expenses incurred in defending a civil or criminal action proceeding in advance of the final disposition of such action or proceeding, upon receipt of an undertaking by the person indemnified to repay such payment if he shall be adjudicated not to be entitled to indemnification under this section. The corporation shall similarly indemnify and hold harmless persons who serve at its express written request as directors or officers of another organization in which the corporation owns shares or of which it is a creditor. II-1 16 The right of indemnification herein provided shall be in addition to and not exclusive of any other rights to which any officer, Director or member of the Scientific Advisory Board of the corporation, or any such persons who serve at its request as aforesaid, may otherwise be lawfully entitled. As used in this Section, the terms "officer," "Director," and "member of the Scientific Advisory Board" include their respective heirs, executors, and administrators. ITEM 16. EXHIBITS. EXHIBIT NUMBER DESCRIPTION ------ ----------- 4.1 Article 4 of the Restated Articles of Organization of the Registrant (previously filed as Exhibit No. 3.1 to the Registrant's Registration Statement on Form S-1, File No. 33-38883, and incorporated herein by reference) 4.2 Form of Common Stock Certificate (previously filed as Exhibit No. 4.2 to the Registrant's Registration Statement on Form S-1, File No. 33-31219, and incorporated herein by reference) 5* Opinion of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., with respect to the legality of the securities being registered 23.1 Consent of PricewaterhouseCoopers LLP 23.2 Consent of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. (included in Exhibit 5) 24* Power of Attorney 99.1* Stockholders' Agreement dated as of January 11, 1993, by and among the Registrant, Dana-Farber Cancer Institute, Inc., Dr. Stuart Schlossman and Apoptosis Technology, Inc. * Previously filed. ITEM 17. UNDERTAKINGS. A. RULE 415 OFFERING The undersigned Registrant hereby undertakes: (1) To file, during any period in which offers or sales are being made, a post-effective amendment to this registration statement: (i) To include any prospectus required by Section 10(a)(3) of the Securities Act; (ii) To reflect in the prospectus any facts or events arising after the effective date of the registration statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the Form of prospectus filed with the Commission pursuant to Rule 424(b) of Regulation C under the Securities Act if, in the aggregate, the changes in volume and price represent no more than a 20% change in the maximum aggregate offering price set forth in the "Calculation of Registration Fee" table in the effective registration statement. (iii) To include any material information with respect to the plan of distribution not previously disclosed in the registration statement or any material change to such information in the registration statement; II-2 17 PROVIDED, HOWEVER, that paragraphs (1)(i) and (1)(ii) do not apply if the registration statement is on Form S-3 or Form S-8, and the information required to be included in a post-effective amendment by those paragraphs is contained in periodic reports filed by the Registrant pursuant to Section 13 or Section 15(d) of the 1934 Act that are incorporated by reference in the registration statement. (2) That, for the purpose of determining any liability under the Securities Act, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof. (3) To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering. B. FILINGS INCORPORATING SUBSEQUENT EXCHANGE ACT DOCUMENTS BY REFERENCE The undersigned Registrant hereby undertakes that, for purposes of determining any liability under the Securities Act, each filing of the Registrant's annual report pursuant to section 13(a) or section 15(d) of the 1934 Act (and, where applicable, each filing of an employee benefit plan's annual report pursuant to section 15(d) of the 1934 Act) that is incorporated by reference in the registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof. C. REQUEST FOR ACCELERATION OF EFFECTIVE DATE OR FILING OF REGISTRATION STATEMENT ON FORM S-8 Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the Registrant pursuant to the foregoing provisions, or otherwise, the Registrant has been advised that in the opinion of the Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the Registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the Registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue. II-3 18 SIGNATURES Pursuant to the requirements of the Securities Act, as amended, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and has duly caused this Amendment No. 3 to the Form S-3 Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in Norwood, Massachusetts on October 16, 1998. IMMUNOGEN, INC. By: /s/ Mitchel Sayare --------------------------------- Mitchel Sayare, Chairman of the Board, Chief Executive Officer and President --------------- Pursuant to the requirements of the Securities Act of 1933, as amended, this Amendment No. 3 to the Form S-3 Registration Statement has been signed below by the following persons in the capacities and on the dates indicated.
SIGNATURE TITLE DATE * Chairman of the Board of Directors, October 16, 1998 ----------------------- Chief Executive Officer and President Mitchel Sayare (principal executive officer) /s/ Kathleen A. Carroll Vice President, Finance and Administration, October 16, 1998 ----------------------- Treasurer (principal financial officer Kathleen A. Carroll and principal accounting officer) * Executive Vice President, October 16, 1998 ----------------------- Science and Technology and Director Walter A. Blattler * Director October 16, 1998 ----------------------- Michael R. Eisenson * Director October 16, 1998 ----------------------- Stuart F. Feiner * Director October 16, 1998 ----------------------- David W. Carter *By: /s/ Kathleen A. Carroll ----------------------- Attorney-in-Fact
II-4
   1

                                                                EXHIBIT 23.1




                      CONSENT OF INDEPENDENT ACCOUNTANTS


        We consent to the incorporation by reference in this Registration
Statement of ImmunoGen, Inc. on Form S-3 to register 475,425 shares of common
stock of our report (which includes an explanatory paragraph concerning
uncertainties surrounding the Company's ability to continue as a going
concern), dated July 29, 1998, on our audits of the consolidated financial
statements of ImmunoGen, Inc. as of June 30, 1997 and 1998 and for each of the
three years in the period ended June 30, 1998, which report is included in the
Company's 1998 Annual Report on Form 10-K.

        We also consent to the reference to our Firm in the Registration
Statement under the caption "Experts".



                                        /s/ PricewaterhouseCoopers LLP
                                        ----------------------------------
                                        PricewaterhouseCoopers LLP



Boston, Massachusetts
October 16, 1998