<PAGE>   1
                       SECURITIES AND EXCHANGE COMMISSION

                             Washington, D.C. 20549

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

                                    FORM 8-K

                                 CURRENT REPORT

                         PURSUANT TO SECTION 13 OR 15(D)
                     OF THE SECURITIES EXCHANGE ACT OF 1934

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

Date of Report (Date of earliest event reported):  March 21, 1996
                                                   --------------

                                 IMMUNOGEN, INC.
                                 ---------------
             (Exact name of registrant as specified in its charter)


Massachusetts                           0-17999               04-2726691
- -------------                           -------               ----------
(State or other jurisdiction         (Commission           (IRS Employer
of incorporation)                     File Number)          Identification No.)


                128 Sidney Street, Cambridge, Massachusetts 02139
                -------------------------------------------------

       Registrant's telephone number, including area code: (617) 661-9312
                                                           --------------


<PAGE>   2


ITEM 5.   OTHER EVENTS. 
- -------   -------------
        
        On March 21, 1996, the Registrant publicly disseminated a press release
announcing that the Registrant plans to issue $5 million of convertible
debentures pursuant to Regulation D of the Securities Act of 1933 in two        
installments: $2.5 million of debentures on or about March 25, 1996 and, as
part of the same transaction, the remaining $2.5 million immediately upon
approval by the Company's Shareholders of an increase in the Company's
authorized common stock. The information contained in the press release is
incorporated herein by reference and filed as Exhibit 99.1 hereto.


ITEM 7.   FINANCIAL STATEMENTS AND EXHIBITS.
- -------   ----------------------------------

(c)      Exhibits.

         99.1     The Registrant's Press Release dated March 21, 1996.

         99.2     Securities Purchase Agreement dated as of March 15, 1996 by 
                  and among the Registrant and Capital Ventures International.

         99.3     Registration Rights Agreement dated as of March 15, 1996 by
                  and among the Registrant and Capital Ventures International.

         99.4     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
                  (Exhibit 99.2 hereto)


                                       2

<PAGE>   3


                                   SIGNATURES

         Pursuant to the requirements of the Securities Exchange Act of 1934,
the Registrant has duly caused this report to be signed on its behalf by the
undersigned hereunto duly authorized.


                                           ImmunoGen, Inc.
                                           ---------------
                                           (Registrant)





Date: March 29, 1996                       /s/ Frank J. Pocher
      --------------                       -------------------
                                           Frank J. Pocher
                                           Vice President and
                                           principal financial officer



                                       3

<PAGE>   4


<TABLE>

                                  EXHIBIT INDEX
                                  -------------

<CAPTION>
Exhibit                                                         Sequential
Number         Description                                     Page Number
- ------         -----------                                     -----------
<S>            <C>                                                  <C>

99.1           The Registrant's Press Release                        5
               dated March 21, 1996.

99.2           Securities Purchase Agreement                         7
               dated as of March 15, 1996 by
               and among the Registrant and
               Capital Ventures International

99.3           Registration Rights Agreement                        46
               dated as of March 15, 1996 by
               and among the Registrant and
               Capital Ventures International

99.4           Letter Agreement dated as of                         63
               March 21, 1996 by and among the
               Registrant and Capital Ventures
               International regarding the
               Securities Purchase Agreement
               dated as of March 15, 1996
               (Exhibit 99.2 hereto)
</TABLE>




                                       4





<PAGE>   1



                        CONTACT:  MITCHEL SAYARE
                                       CHAIRMAN, CHIEF EXECUTIVE OFFICER
                                       (617) 661-9312


                              FOR IMMEDIATE RELEASE
                              ---------------------

               IMMUNOGEN SIGNS AGREEMENT FOR $5 MILLION FINANCING

CAMBRIDGE, MASS., MARCH 21, 1996--IMMUNOGEN, INC. (NASDAQ: IMGN) TODAY ANNOUNCED
PLANS TO ISSUE $5 MILLION OF CONVERTIBLE DEBENTURES TO AN UNDISCLOSED INVESTOR
PURSUANT TO REGULATION D OF THE SECURITIES ACT OF 1933, AS AMENDED. THE
TRANSACTION WILL TAKE PLACE IN TWO INSTALLMENTS: $2.5 MILLION OF DEBENTURES WILL
BE ISSUED ON OR ABOUT MARCH 25, 1996, AND THE REMAINING $2.5 MILLION OF
DEBENTURES WILL BE ISSUED IMMEDIATELY UPON APPROVAL BY THE COMPANY'S
SHAREHOLDERS OF AN INCREASE IN ITS AUTHORIZED COMMON STOCK.

"DECLINING CASH LEVELS HAVE PLACED CONSTRAINTS ON THE COMPANY'S OPERATIONS FOR
SOME TIME," STATED CHAIRMAN AND CHIEF EXECUTIVE OFFICER, MITCHEL SAYARE. "THE
COMPANY HAS TAKEN A VARIETY OF MEASURES TO REDUCE EXPENDITURES, INCLUDING A
RESTRUCTURING IN DECEMBER 1994 AND THE ASSIGNMENT IN JANUARY OF THIS YEAR OF THE
FACILITY AND EQUIPMENT LEASES FOR OUR ANTIBODY MANUFACTURING PLANT IN CANTON,
MASSACHUSETTS. WE ARE PLEASED TO SEE THE POSITIVE IMPACT OF THESE ACTIONS ON OUR
BOTTOM LINE. NOW, THE COMPLETION OF THIS FINANCING TRANSACTION WILL GIVE
IMMUNOGEN ADDITIONAL
 TIME AND FLEXIBILITY TO PURSUE ITS STRATEGIC OBJECTIVES."

THE DEBENTURES WILL EARN INTEREST AT A RATE OF NINE PERCENT PER ANNUM AND WILL
BE CONVERTIBLE INTO SHARES OF IMMUNOGEN COMMON STOCK AT ANY TIME ACCORDING TO A
PREDETERMINED DISCOUNT FORMULA. IF THE CONVERSION TAKES PLACE MORE THAN 80 DAYS
FOLLOWING THE SALE OF THE FIRST DEBENTURE, THE HOLDER ALSO WILL RECEIVE WARRANTS
TO PURCHASE ADDITIONAL SHARES OF COMMON STOCK EQUAL TO ONE-HALF OF THE NUMBER OF
SHARES ISSUABLE UPON CONVERSION OF THE DEBENTURES. THESE WARRANTS WOULD BE
EXERCISABLE AT $4.00 PER SHARE AND EXPIRE FIVE YEARS AFTER THE DATE OF
CONVERSION.

IMMUNOGEN, INC., DEVELOPS INNOVATIVE BIOPHARMACEUTICALS, PRIMARILY FOR CANCER
TREATMENT. THE COMPANY HAS CREATED POTENT "IMMUNOCONJUGATES" CONSISTING OF
TOXINS OR DRUGS COUPLED TO MONOCLONAL ANTIBODIES FOR DELIVERY TO AND DESTRUCTION
OF CANCER CELLS. IMMUNOGEN'S FIRST PRODUCT, ONCOLYSIN(R)B, IS BEING TESTED IN A
LARGE-SCALE, PHASE III CLINICAL TRIAL FOR THE TREATMENT OF LYMPHOMA SUBSEQUENT
TO AUTOLOGOUS BONE MARROW TRANSPLANTATION.

                                       ###


                                       6



<PAGE>   1

                         SECURITIES PURCHASE AGREEMENT

     SECURITIES PURCHASE AGREEMENT (this "Agreement"), dated as of March 15,
1996 by and among IMMUNOGEN, INC., a Massachusetts corporation, with
headquarters located at 128 Sidney Street, Cambridge, MA 02139 (the "Company"),
and the undersigned (the "Buyer").

     WHEREAS:

     A. The Company and the Buyer are executing and delivering this Agreement in
reliance upon the exemption from securities registration afforded by Rule 506
under Regulation D ("Regulation D") as promulgated by the United States
Securities and Exchange Commission (the "SEC") under the Securities Act of 1933,
as amended (the "1933 Act");

     B. The Buyer wishes to purchase, in the amounts and upon the terms and
conditions stated in this Agreement, convertible debentures of the Company in
the aggregate principal amount of Five Million Dollars ($5,000,000), convertible
into (i) shares of Common Stock, par value $.01 per share, of the Company (the
"Common Stock") and (ii) under certain circumstances, warrants (the "Warrants"),
in the form attached hereto as Exhibit "A-1", to acquire a number of shares of
Common Stock equal to 50% of the number of shares of Common Stock issuable upon
conversion of such debentures, upon the terms and subject to the conditions set
forth
 in such debentures; and

     C. Contemporaneous with the execution and delivery of this Agreement, the
parties hereto are executing and delivering a Registration Rights Agreement, in
the form attached hereto as Exhibit "A-2" (the "Registration Rights Agreement"),
pursuant to which the Company has agreed to provide certain registration rights
under the 1933 Act and the rules and regulations promulgated thereunder, and
applicable state securities laws;

     NOW THEREFORE, the Company and the Buyer hereby agree as follows:

1.   PURCHASE AND SALE OF DEBENTURES.

     a. Purchase of Debentures. The Company shall issue and sell to the Buyer
and the Buyer shall purchase from the Company convertible debentures of the
Company, in the aggregate principal amount of Five Million Dollars ($5,000,000),
having the terms and conditions set forth in the form of debenture attached
hereto as Exhibit "A" (collectively, together with any debenture(s) issued in
replacement thereof in accordance with the terms thereof, the "Debentures") for
an aggregate purchase price equal to Five Million Dollars ($5,000,000). The
issuance, sale and purchase of the Debentures shall take place in two (2)
separate closings, the first of which is hereinafter referred to as the "First
Closing" and the second of which is hereinafter referred to as the "Second
Closing." Subject to the satisfaction (or waiver) of the conditions thereto set
forth in Sections 6 and 7 below (i) at the First Closing, the Company shall
issue and sell to the Buyer and the Buyer shall purchase from the Company a
Debenture, in the principal amount of Two Million Five Hundred Thousand Dollars
($2,500,000) for a purchase price equal to Two Million Five Hundred Thousand
Dollars ($2,500,000) and (ii) at the Second Closing, the Company shall issue and
sell and the Buyer shall purchase from the Company a 


                                       8

<PAGE>   2

Debenture, in the principal amount of Two Million Five Hundred Thousand Dollars
($2,500,000) for a purchase price equal to Two Million Five Hundred Thousand
Dollars ($2,500,000).

     b. Form of Payment. On each Closing Date,(i) the Buyer shall pay the
purchase price for the Debenture to be issued and sold at the applicable closing
(the "Purchase Price") by wire transfer of Two Million Five Hundred Thousand
United States Dollars ($2,500,000) to the Company, in accordance with the
Company's written wiring instructions, against delivery of a Debenture in the
principal amount of Two Million Five Hundred Thousand Dollars ($2,500,000), and
(ii) the Company shall deliver such Debenture, duly executed on behalf of the
Company, to the Buyer, against delivery of such Purchase Price.

     c. Closing Dates. Subject to the satisfaction (or waiver) of the conditions
thereto set forth in Section 6 and Section 7 below, the date and time of the
issuance and sale of each of the Debentures pursuant to this Agreement (the
"Closing Dates") shall be (i) in the case of the First Closing, 12:00 noon
Eastern Standard Time on the first business day following notification of
satisfaction (or waiver) of each of the conditions to such closing set forth in
Section 6(a)(iii) and Section 7(a)(iii) below and (ii) in the case of the Second
Closing, 12:00 noon Eastern Standard Time, three business days following
notification of satisfaction (or waiver) of each of the conditions to such
closing set forth in Section 6(b) and 7(b) below (subject, in each case, to a
two (2) business day grace period at either party's option), or, in each case,
such other mutually agreed upon time. The closings shall occur on the Closing
Dates at the offices of Klehr, Harrison, Harvey, Branzburg & Ellers, 1401 Walnut
Street, Philadelphia, Pennsylvania 19102.

2.   BUYER'S REPRESENTATIONS AND WARRANTIES 
     The Buyer represents and warrants to the Company that:

     a. Investment Purpose. The Buyer is purchasing the Debentures, the shares
of Common Stock issuable upon conversion thereof (the "Conversion Shares"), the
Warrants and the shares of Common Stock issuable upon exercise of the Warrants
(the "Warrant Shares") for its own account for investment only and not with a
present view towards the public sale or distribution thereof, except pursuant to
sales registered under the 1933 Act.

     b. Accredited Investor Status. The Buyer is an "accredited investor" as
that term is defined in Rule 501(a) of Regulation D.

     c. Reliance on Exemptions. The Buyer understands that the Debentures are
being offered and sold to it in reliance upon specific exemptions from the
registration requirements of United States federal and state securities laws and
that the Company is relying upon the truth and accuracy of, and the Buyer's
compliance with, the representations, warranties, agreements, acknowledgments
and understandings of the Buyer set forth herein in order to determine the
availability of such exemptions and the eligibility of the Buyer to acquire the
Debentures.

     d. Information. The Buyer and its advisors, if any, have been furnished
with all materials relating to the business, finances and operations of the
Company and materials relating to the offer and sale of the Debentures which
have been requested by the Buyer or its advisors. The 


                                       9

<PAGE>   3

Buyer and its advisors, if any, have been afforded the opportunity to ask
questions of the Company and have received complete and satisfactory answers to
any such inquiries. The Buyer understands that its investment in the Debentures
involves a high degree of risk.

     e. Governmental Review. The Buyer understands that no United States federal
or state agency or any other government or governmental agency has passed upon
or made any recommendation or endorsement of the Debentures.

     f. Transfer or Resale. The Buyer understands that (i) except as provided in
the Registration Rights Agreement, the Debentures, the Conversion Shares, the
Warrants and the Warrant Shares (collectively, the "Securities") have not been
and are not being registered under the 1933 Act or any state securities laws,
and may not be transferred unless (a) subsequently registered thereunder, or (b)
the Buyer shall have delivered to the Company an opinion of counsel to the
effect that the Securities to be sold or transferred may be sold or transferred
pursuant to an exemption from such registration or (c) sold pursuant to Rule 144
promulgated under the 1933 Act (or a successor rule); (ii) any sale of such
Securities made in reliance on Rule 144 may be made only in accordance with the
terms of said Rule and further, if said Rule is not applicable, any resale of
such Securities under circumstances in which the seller (or the person through
whom the sale is made) may be deemed to be an underwriter (as that term is
defined in the 1933 Act) may require compliance with some other exemption under
the 1933 Act or the rules and regulations of the SEC thereunder; and (iii)
neither the Company nor any other person is under any obligation to register
such Securities under the 1933 Act or any state securities laws or to comply
with the terms and conditions of any exemption thereunder (in each case, other
than pursuant to the Registration Rights Agreement).

     g. Legends. The Buyer understands that the Debentures, Warrants and, until
such time as the Conversion Shares and Warrant Shares have been registered under
the 1933 Act, as contemplated by the Registration Rights Agreement, the
certificates for the Conversion Shares and Warrant Shares, may bear a
restrictive legend in substantially the following form (and a stop-transfer
order may be placed against transfer of the certificates for such Securities):

     "The securities represented by this certificate have not been registered
under the Securities Act of 1933, as amended. The securities have been acquired
for investment and may not be sold, transferred or assigned in the absence of an
effective registration statement for the securities under said Act, or an
opinion of counsel, in form, substance and scope reasonably acceptable to the
Company, that registration is not required under said Act or unless sold
pursuant to Rule 144 under said Act."

     The legend set forth above shall be removed and the Company shall issue a
certificate without such legend to the holder of any Security upon which it is
stamped, if, unless otherwise required by state securities laws, (a) the sale of
such security is registered under the 1933 Act, or (b) such holder provides the
Company with an opinion of counsel, in form, substance and scope reasonably
accepted to the Company, to the effect that a public sale or transfer of such
security may be made without registration under the 1933 Act or (c) such holder
provides the Company with reasonable assurances that such Security can be sold
pursuant to Rule 144 under the 1933 


                                       10

<PAGE>   4

Act (or a successor rule thereto) without any restriction as to the number of
securities acquired as of a particular date that can then be immediately sold.
The Buyer agrees to sell all Securities, including those represented by a
certificate(s) from which the legend has been removed, in compliance with
applicable securities law.

     h. Authorization; Enforcement. This Agreement has been duly and validly
authorized, executed and delivered on behalf of the Buyer and is a valid and
binding agreement of the Buyer enforceable in accordance with its terms, except
as such enforceability may be limited by applicable bankruptcy, insolvency,
reorganization, moratorium, liquidation, fraudulent conveyance or similar laws
relating to, or affecting generally, the enforcement of creditors' rights and
remedies or by other equitable principles of general application and subject to
the limitation that the indemnification provisions of the Registration Rights
Agreement may be unenforceable as a matter of public policy.

     i. Residency. The Buyer is a resident of the Cayman Islands..

3.   REPRESENTATIONS AND WARRANTIES OF THE COMPANY.

     The Company represents and warrants to the Buyer that:

     a. Organization and Qualification. Each of the Company and its subsidiaries
is a corporation duly organized and existing in good standing under the laws of
the jurisdiction in which it is incorporated, except, in the case of any such
subsidiaries, as would not have a Material Adverse Effect (as defined below),
and has the requisite corporate power to own its properties and to carry on its
business as now being conducted. Each of the Company and its subsidiaries is
duly qualified as a foreign corporation to do business and is in good standing
in every jurisdiction in which the nature of the business conducted by it makes
such qualification necessary and where the failure so to qualify would have a
Material Adverse Effect. "Material Adverse Effect" means any material adverse
effect on the operations, properties financial condition or prospects of the
Company and its subsidiaries taken as a whole.

     b. Authorization; Enforcement. (i) The Company has the requisite corporate
power and authority to enter into and perform this Agreement, the Registration
Rights Agreement, the Debentures and the Warrants, and to issue the Securities,
in accordance with the terms hereof and thereof, (ii) the execution and delivery
of this Agreement, the Registration Rights Agreement, the Debentures and the
Warrants by the Company and the consummation by it of the transactions
contemplated hereby and thereby (including without limitation the issuance of
the Debentures and the Warrants and the issuance and reservation for issuance of
the Conversion Shares and Warrant Shares issuable upon conversion or exercise
thereof) have been duly authorized by the Company's Board of Directors and no
further consent or authorization of the Company, its Board or Directors, or its
stockholders is required, except, in the case of the transactions to be
consummated at the Second Closing, stockholder approval of an increase in the
authorized number of shares of Common Stock of the Company, as contemplated by
Section 6(b) and 7(b) below, (iii) this Agreement has been duly executed and
delivered by the Company, and (iv) this Agreement constitutes, and upon
execution and delivery by the Company of the Registration 


                                       11

<PAGE>   5

Rights Agreement, the Debentures and the Warrants, each of such instruments will
constitute, a valid and binding obligation of the Company enforceable against
the Company in accordance with its terms, except as such enforceability may be
limited by applicable bankruptcy, insolvency, reorganization, moratorium,
liquidation, fraudulent conveyance or similar laws relating to, or affecting
generally, the enforcement of creditors' rights and remedies or by other
equitable principles of general application and subject to the limitation that
the indemnification provisions of the Registration Rights Agreement may be
unenforceable as a matter of public policy.

     c. Capitalization. As of March 15, 1996, the authorized capital stock of
the Company consists of (i) 20,000,000 shares of Common Stock of which
15,526,357 shares are issued and outstanding, 419,467 shares are reserved for
issuance pursuant to the Company's stock option plan, 26,738 shares are reserved
for issuance pursuant to warrants granted to Aberlyn Capital Management and
3,900,000 shares are reserved for issuance upon conversion of the Debenture and
exercise of the Warrants (subject to adjustment pursuant to the Company's
covenant set forth in Section 4(h) below), and (ii) 277,080 shares of Redeemable
Convertible Preferred Stock, $.01 par value, of which no shares are issued and
outstanding. All of such outstanding shares have been validly issued and are
fully paid and nonassessable. No shares of capital stock of the Company are
subject to preemptive rights or any other similar rights of the stockholders of
the Company or any liens or encumbrances. Except as disclosed in Schedule 3(c),
as of the effective date of this Agreement, (i) there are no outstanding
options, warrants, scrip, rights to subscribe to, calls or commitments of any
character whatsoever relating to, or securities or rights convertible into, any
shares of capital stock of the Company or any of its subsidiaries, or
arrangements by which the Company or any of its subsidiaries is or may become
bound to issue additional shares of capital stock of the Company or any of its
subsidiaries, and (ii) there are no agreements or arrangements under which the
Company or any of its subsidiaries is obligated to register the sale of any of
its or their securities under the 1933 Act (except the Registration Rights
Agreement). The Company has furnished to the Buyer true and correct copies of
the Company's Articles of Organization as in effect on the date hereof
("Articles of Organization") and the Company's By-laws, as in effect on the date
hereof (the "By-laws"). The Company shall provide the Buyer with a written
update of this representation signed by the Company's Chief Executive or Chief
Financial Officer on behalf of the Company as of the Closing Date.

     d. Issuance of Shares. The Conversion Shares and Warrant Shares are duly
authorized and, upon issuance in accordance with the terms of the Debentures and
Warrants, as applicable, shall be validly issued, fully paid and non-assessable,
and free from all taxes, liens and charges with respect to the issue thereof.
The term Conversion Shares includes the shares of Common Stock issuable upon
conversion of the Debentures, including without limitation, such additional
shares, if any, as are issuable as a result of the events described in Section
2(c) of the Registration Rights Agreement, Section 2.3 of the Debentures and
Article III of the Debentures.

     e. No Conflicts. The execution, delivery and performance of this Agreement,
the Registration Rights Agreement, the Debentures and the Warrants by the
Company and the consummation by the Company of the transactions contemplated
hereby and thereby (including without limitation, the issuance and reservation
for issuance of the Conversion Shares and 


                                       12


<PAGE>   6

Warrant Shares) will not (i) result in a violation of the Articles of
Organization or By-laws or (ii) conflict with, or constitute a default (or an
event which with notice or lapse of time or both would become a default) under,
or give to others any rights of termination, amendment, acceleration or
cancellation of, any agreement, indenture or instrument to which the Company or
any of its subsidiaries is a party, or result in a violation of any law, rule,
regulation, order, judgment or decree (including federal and state securities
laws and regulations) applicable to the Company or any of its subsidiaries or by
which any property or asset of the Company or any of its subsidiaries is bound
or affected (except for such conflicts, defaults, terminations, amendments,
accelerations, cancellations and violations as would not, individually or in the
aggregate, have a Material Adverse Effect). The businesses of the Company and
its subsidiaries are not being conducted, and shall not be conducted through the
later of (i) the latest maturity date of the Debentures and (ii) the expiration
or exercise of all of the Warrants, in violation of any law, ordinance or
regulation of any governmental entity, except for possible violations which
either singly or in the aggregate do not have a Material Adverse Effect. Except
as specifically contemplated by this Agreement and as required under the 1933
Act and any applicable state securities laws, the Company is not required to
obtain any consent, authorization or order of, or make any filing or
registration with, any court or governmental agency or any regulatory or self
regulatory agency in order for it to execute, deliver or perform any of its
obligations under this Agreement in accordance with the terms hereof.

     f. SEC Documents, Financial Statements. Since December 31, 1994, the
Company has filed all reports, schedules, forms, statements and other documents
required to be filed by it with the SEC pursuant to the reporting requirements
of the Exchange Act of 1934, as amended (the "1934 Act") (all of the foregoing
filed prior to the date hereof and all exhibits included therein and financial
statements and schedules thereto and documents (other than exhibits)
incorporated by reference therein, being hereinafter referred to herein as the
"SEC Documents"). The Company has delivered to the Buyer true and complete
copies of the SEC Documents, except for such exhibits, schedules and
incorporated documents. As of their respective dates, the SEC Documents complied
in all material respects with the requirements of the 1934 Act and the rules and
regulations of the SEC promulgated thereunder applicable to the SEC Documents,
and none of the SEC Documents, at the time they were filed with the SEC,
contained any untrue statement of a material fact or omitted to state a material
fact required to be stated therein or necessary in order to make the statements
therein, in light of the circumstances under which they were made, not
misleading. As of their respective dates, the financial statements of the
Company included in the SEC Documents complied as to form in all material
respects with applicable accounting requirements and the published rules and
regulations of the SEC with respect thereto. Such financial statements have been
prepared in accordance with generally accepted accounting principles,
consistently applied, during the periods involved (except (i) as may be
otherwise indicated in such financial statements or the notes thereto, or (ii)
in the case of unaudited interim statements, to the extent they may include
footnotes or may be condensed or summary statements) and fairly present in all
material respects the consolidated financial position of the Company and its
consolidated subsidiaries as of the dates thereof and the consolidated results
of their operations and cash flows for the periods then ended (subject, in the
case of unaudited statements, to normal year-end audit adjustments). Except as
set forth in the financial statements of the Company included in the SEC
documents, the Company has no liabilities, contingent or 


                                       13


<PAGE>   7

otherwise, other than (i) liabilities incurred in the ordinary course of
business subsequent to December 31, 1995 and (ii) obligations under contracts
and commitments incurred in the ordinary course of business and not required
under generally accepted accounting principles to be reflected in such financial
statements, which, individually or in the aggregate, are not material to the
financial condition or operating results of the Company.

     g. Absence of Certain Changes. Since December 31, 1995 there has been no
material adverse change and no material adverse development in the business,
properties, operations, financial condition, results of operations or prospects
of the Company, except as disclosed in the documents referred to in Section 2(d)
hereof or in the SEC Documents and continued utilization of cash resources since
December 31, 1995.

     h. Absence of Litigation. There is no action, suit, proceeding, inquiry or
investigation before or by any court, public board, government agency,
self-regulatory organization or body pending or, to the knowledge of the Company
or any of its subsidiaries, threatened against or affecting the Company or any
of its subsidiaries except suits having an aggregate amount in dispute of not
more than $1,000.

     i. Disclosure. All information relating to or concerning the Company set
forth in this Agreement is true and correct in all material respects and the
Company has not omitted to state any material fact necessary in order to make
the statements made herein, in light of the circumstances under which they are
made, not misleading.

     j. Absence of Events of Default. No Event of Default, as defined in the
Debenture, and no event which, with the giving of notice or the passage of time
or both, would become an Event of Default has occurred and is continuing.

     k. Acknowledgment Regarding Buyer's Purchase of Debentures. The Company
acknowledges and agrees that the Buyer is acting solely in the capacity of an
arm's length counterparty with respect to this Agreement and the transactions
contemplated hereby. The Company further acknowledges that the Buyer is not
acting as a financial advisor or fiduciary of the Company (or in any similar
capacity) with respect to this Agreement and the transactions contemplated
hereby and any advice given by the Buyer in connection with this Agreement and
the transactions contemplated hereby is merely incidental to the Buyer's
purchase of the Debentures. The Company further represents to the Buyer that its
decision to enter into this Agreement has been based solely on the independent
evaluation of the Company and its representatives.

4.   COVENANTS.

     a. Best Efforts. The parties shall use their best efforts timely to satisfy
each of the conditions described in Section 6 and 7 of this Agreement.

     b. Form D; Blue Sky Laws. The Company agrees to file a Form D with respect
to the Securities as required under Regulation D and to provide a copy thereof
to the Buyer promptly 


                                       14


<PAGE>   8

after such filing. The Company shall, on or before each Closing Date, take such
action as the Company shall reasonably determine is necessary to qualify the
Securities for, or obtain exemption for the Securities for, sale to the Buyer at
the applicable closing pursuant to this Agreement under applicable securities or
"blue sky" laws of the states of the United States, and shall provide evidence
of any such action so taken to the Buyer on or prior to such Closing Date.

     c. Reporting Status. So long as the Buyer beneficially owns any of the
Securities, the Company shall file all reports required to be filed with the SEC
pursuant to the 1934 Act, and the Company shall not terminate its status as an
issuer required to file reports under the 1934 Act even if the 1934 Act or the
rules and regulations thereunder would permit such termination.

     d. Use of Proceeds. The Company shall not use the proceeds from the sale of
the Debentures for anything other than the Company's internal working capital
purposes and shall not, directly or indirectly, use such proceeds for any loan
to or investment in any other corporation, partnership, enterprise or other
person.

     e. Additional Equity Capital; Right of First Refusal. The Company agrees
that, during the period beginning on the date hereof and ending eighty (80) days
following the Closing Date in respect of the Second Closing, the Company will
not, without the prior written consent of the Buyer, negotiate or contract with
any party to obtain additional equity financing (including debt financing with
an equity component) in any form ("Future Offerings"). The Company will not
conduct any Future Offering during the period beginning on the date hereof and
ending one (1) year after the Closing Date in respect of the First Closing
unless it shall have first delivered to the Buyer, at least seven (7) days prior
to the closing of such Future Offering, written notice describing the proposed
Future Offering, including the terms and conditions thereof, and providing the
Buyer an option during such seven (7) day period to purchase all or any portion
of the securities being offered in the Future Offering on the same terms as
contemplated by such Future Offering (the limitations referred to in this and
the immediately preceding sentence are collectively referred to as the "Capital
Raising Limitation"). The Capital Raising Limitation shall not apply to any
transaction involving the Company's commercial banking arrangements or issuances
of securities in connection with a merger, consolidation or sale of assets, or
in connection with any strategic partnership or joint venture (the primary
purpose of which is not to raise equity capital), or in connection with the
disposition of a business, product or license by the Company or exercise of
options by employees, consultants or directors. The Capital Raising Limitation
also shall not apply to the issuance of securities pursuant to an underwritten
public offering or upon exercise or conversion of the Company's options,
warrants or other convertible securities outstanding as of March 1, 1996 or to
the grant of additional options or warrants, or the issuance of additional
securities, under any Company stock option or restricted stock plan.

     f. Expenses. The Company shall pay all expenses incurred in connection with
the negotiation, preparation, execution, delivery and performance of this
Agreement and the other agreements to be executed in connection herewith,
including, without limitation, Buyer's attorneys' fees and expenses. The
Company's obligation to pay Buyer's expenses under this Section 4(f) shall be
limited to Ten Thousand Dollars ($10,000).


                                       15


<PAGE>   9

     g. Financial Information. The Company agrees to send the following reports
to the Buyer until the Buyer transfers, assigns, or sells all of the Securities:
(i) within ten (10) days after the filing with the SEC, a copy of its Annual
Report on Form 10-K, its Quarterly Reports on Form 10-Q and any Current Reports
on Form 8-K; and (ii) within two (2) days after release, copies of all press
releases issued by the Company or any of its subsidiaries.

     h. Reservation of Shares. The Company shall at all times have authorized,
and reserved for the purpose of issuance, a sufficient number of shares of
Common Stock to provide for the conversion of the outstanding Debentures and
issuance of the Conversion Shares in connection therewith and the exercise of
the Warrants issuable upon conversion thereof and the issuance of the Warrant
Shares in connection therewith. In that regard (i) at the time of the First
Closing, the Company shall have at least 3,900,000 shares of Common Stock
reserved for issuance upon conversion of the Debentures and the exercise of the
Warrants issuable upon conversion thereof and (ii) at the time of the Second
Closing, the Company shall have at least 8,900,000 shares of Common Stock
reserved for issuance upon conversion of the Debentures and the exercise of the
Warrants (subject to proportionate adjustment based on the Conversion Price (as
defined in the Debentures) of the Debentures and the Exercise Price (as defined
in the Warrants) of the Warrants, as appropriate; provided that the number of
shares reserved may not be decreased without the consent of the Buyer, which
consent will not be unreasonably withheld).

     i. Listing. The Company shall promptly secure the listing of the Conversion
Shares and Warrant Shares upon each national securities exchange or automated
quotation system, if any, upon which shares of Common Stock are then listed
(subject to official notice of issuance) and shall maintain, so long as any
other shares of Common Stock shall be so listed, such listing of all Conversion
Shares from time to time issuable upon conversion of the Debentures and Warrant
Shares issuable upon exercise of the Warrants.

     j. Corporate Existence. The Company shall, until the earlier of maturity
and repayment of all of the Debentures or the earlier conversion of all of the
Debentures, maintain its corporate existence, except in the event of a merger,
consolidation or sale of all or substantially all of the Company's assets, as
long as the surviving or successor entity in such transaction (i) assumes the
Company's obligations hereunder and under the agreements and instruments entered
into in connection herewith and (ii) is a publicly traded corporation whose
Common Stock is listed for trading on the NASDAQ-NMS or another national
securities exchange or automated quotation system.

     k. Buyer Trading Restrictions. The Buyer covenants that it does not
currently have and will not establish, prior to the earlier of (i) the
effectiveness of the registration statement to be filed pursuant to the
Registration Rights Agreement and (ii) seventy five (75) days following the
Closing Date in respect of the First Closing, any short position (directly or
indirectly) in the Company's Common Stock. In addition, Buyer agrees not to sell
a number of shares of Common Stock of the Company (through short positions or
otherwise) in excess of the greater of (i) 20% of the weekly volume (calculated
based on the five (5) trading day period immediately prior to the date of any
sale) in any given five (5) trading day period and (ii) on any given day, 20% of
the volume on such day. Unintentional violations of the limitation set forth in
the immediately 


                                       16


<PAGE>   10

preceding sentence shall not be a breach of this Agreement to the extent such
violations occur infrequently and are not substantially in excess of such
limitation. In order for the Company to track Buyer's compliance with this
Section 4(j), the Buyer shall provide the Company with notice of its daily sales
of Common Stock within ten (10) business days of the end of each month in which
any such sales occur.

5.   TRANSFER AGENT INSTRUCTIONS.

     The Company shall instruct its transfer agent to issue certificates,
registered in the name of the Buyer or its nominee, for the Conversion Shares
and Warrant Shares in such amounts as specified from time to time by the Buyer
to the Company. Prior to registration of the Conversion Shares and Warrant
Shares under the 1933 Act, all such certificates shall bear the restrictive
legend specified in Section 2(g) of this Agreement. The Company warrants that no
instruction other than such instructions referred to in this Section 5, and stop
transfer instructions to give effect to Section 2(f) hereof, in the case of the
Conversion Shares and Warrant Shares, prior to registration of the Conversion
Shares and Warrant Shares under the 1933 Act, will be given by the Company to
its transfer agent and that the Securities shall otherwise be freely
transferable on the books and records of the Company as and to the extent
provided in this Agreement and the Registration Rights Agreement. Nothing in
this Section shall affect in any way the Buyer's obligations and agreement set
forth in Section 2(g) hereof to comply with all applicable securities laws upon
resale of the Securities. If the Buyer provides the Company with an opinion of
counsel, reasonably satisfactory to the Company in form, substance and scope,
that registration of a resale by the Buyer of any of the Securities is not
required under the 1933 Act, the Company shall permit the transfer, and, in the
case of the Conversion Shares and Warrant Shares promptly instruct its transfer
agent to issue one or more certificates in such name and in such denominations
as specified by the Buyer.

6.   CONDITIONS TO THE COMPANY'S OBLIGATION TO SELL.

     The obligation of the Company hereunder to issue and sell the Debentures at
each of the First Closing and the Second Closing, as applicable, is subject to
the satisfaction, at or before the Closing Date in respect of such closing, of
each of the following conditions thereto, provided that these conditions are for
the Company's sole benefit and may be waived by the Company at any time in its
sole discretion:

     a. With respect to the First Closing and the Second Closing:

        (i) The parties shall have executed this Agreement and the Registration
Rights Agreement, and delivered the same to each other.

        (ii) The Buyer shall have delivered the Purchase Price in accordance 
with Section 1(b) above.

        (iii) The National Association of Securities Dealers ("NASD") shall have
granted to the Company an exemption from the requirements of Section 6(i) of
Part III of Schedule D of the NASD Bylaws and the ten (10) day notice period in
connection therewith shall have been satisfied.


                                       17


<PAGE>   11

        (iv) The representations and warranties of the Buyer shall be true and
correct in all material respects as of the date when made and as of the Closing
Date as though made at that time (except for representations and warranties that
speak as of a specific date), and the Buyer shall have performed, satisfied and
complied in all material respects with the covenants, agreements and conditions
required by this Agreement to be performed, satisfied or complied with by the
Buyer at or prior to the Closing Date.

     b. With respect to the Second Closing:

        (i) The Company's authorized number of shares of Common Stock shall have
been duly and validly increased to at least 30,000,000.

7.   CONDITIONS TO THE BUYER'S OBLIGATION TO PURCHASE.

     The obligation of the Buyer hereunder to purchase the Debenture is subject
to the satisfaction, at or before the Closing Date, of each of the following
conditions, provided that these conditions are for the Buyer's sole benefit and
may be waived by the Buyer at any time in its sole discretion:

     a. With respect to the First Closing and the Second Closing:

        (i) The parties shall have executed this Agreement and the Registration
Rights Agreement, and delivered the same to each other.

        (ii) The Company shall have executed the Debenture and delivered it to
the Buyer in accordance with Section 1(b) above.

        (iii) The NASD shall have granted to the Company an exemption from the
requirements of Section 6(i) of Part III of Schedule D of the NASD Bylaws and
the ten (10) day notice period in connection therewith shall have been satisfied
within fifteen (15) days of the date hereof.

        (iv) The Common Stock shall be authorized for quotation on the NASDAQ
National Market ("NASDAQ-NMS"), and trading in the Common Stock (or on
NASDAQ-NMS generally) shall not have been suspended by the SEC or NASDAQ.

        (v) The representations and warranties of the Company shall be true and
correct in all material respects as of the date when made and as of the Closing
Date as though made at that time (except for representations and warranties that
speak as of a specific date) and the Company shall have performed, satisfied and
complied in all material respects with the covenants, agreements and conditions
required by this Agreement to be performed, satisfied or complied with by the
Company at or prior to the Closing Date. The Buyer shall have received a
certificate, executed by the chief executive officer of the Company, dated as of
the Closing Date, to the foregoing effect and as to such other matters as may be
reasonably requested by the Buyer.

        (vi) The Buyer shall have received an opinion of the Company's counsel,
dated as of the Closing Date, in form, scope and substance reasonably
satisfactory to the Buyer and in substantially the same form as Exhibit "B"
attached hereto. 

        (vii) The Buyer shall have received the officer's certificate described
in Section 3(c) above, dated as of the Closing Date.


                                       18


<PAGE>   12

     b. With respect to the Second Closing:

        (i) The Company's authorized number of shares of Common Stock shall have
been duly and validly increased to at least 30,000,000 within 120 days following
the Closing Date in respect of First Closing.

8.   GOVERNING LAW; MISCELLANEOUS.

     a. Governing Law. This Agreement shall be governed by and interpreted in
accordance with the laws of the Commonwealth of Massachusetts without regard to
the principles of conflict of laws.

     b. Counterparts. This Agreement may be executed in two or more
counterparts, all of which shall be considered one and the same agreement and
shall become effective when counterparts have been signed by each party and
delivered to the other party.

     c. Headings. The headings of this Agreement are for convenience of
reference and shall not form part of, or affect the interpretation of, this
Agreement.

     d. Severability. If any provision of this Agreement shall be invalid or
unenforceable in any jurisdiction, such invalidity or unenforceability shall not
affect the validity or enforceability of the remainder of this Agreement or the
validity or enforceability of this Agreement in any other jurisdiction.

     e. Entire Agreement; Amendments. This Agreement and the instruments
referenced herein contain the entire understanding of the parties with respect
to the matters covered herein and therein and, except as specifically set forth
herein or therein, neither the Company nor the Buyer makes any representation,
warranty, covenant or undertaking with respect to such matters. No provision of
this Agreement may be waived or amended other than by an instrument in writing
signed by the party to be charged with enforcement.

     f. Notices. Any notices required or permitted to be given under the terms
of this Agreement shall be sent by mail or delivered personally or by courier
and shall be effective five days after being placed in the mail, if mailed, or
upon receipt, if delivered personally or by courier, in each case addressed to a
party. The addresses for such communications shall be:

If to the Company:
         IMMUNOGEN, INC.
         128 Sidney Street
         Cambridge, MA 02139
         Attention: Frank J. Pocher, Chief Financial Officer

With copy to:
         Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
         One Financial Center


                                       19


<PAGE>   13

         Boston, MA 02111
         Telecopy: (617) 542-2241
         Attention: Jonathan L. Kravetz, Esq.

If to the Buyer:
         Capital Ventures International
         c/o Arbit, Inc.
         1900 Market Street
         Philadelphia, PA  19102
         Telecopy:  (215) 963-3379
         Attention: Steve Katznelson

With copy to:
         Gary S. Kaminsky, Esq.
         c/o Arbit, Inc.
         1900 Market Street
         Philadelphia, PA  19102
         Telecopy:  (215) 656-8758

And:
         Klehr, Harrison, Harvey, Branzburg & Ellers
         1401 Walnut Street
         Philadelphia, PA  19102
         Telecopy:  (215) 568-5725
         Attention:  Wayne D. Bloch, Esq.

Each party shall provide notice to the other party of any change in address.

     g. Successors and Assigns. This Agreement shall be binding upon and inure
to the benefit of the parties and their successors and assigns. Neither the
Company nor the Buyer shall assign this Agreement or any rights or obligations
hereunder without the prior written consent of the other (which consent shall
not be unreasonably withheld). Notwithstanding the foregoing, the Buyer may
assign its rights hereunder to any of its "affiliates," as that term is defined
under the 1934 Act, without the consent of the Company.

     h. Third Party Beneficiaries. This Agreement is intended for the benefit of
the parties hereto and their respective permitted successors and assigns, and is
not for the benefit of, nor may any provision hereof be enforced by, any other
person.

     i. Survival. The representations and warranties of the Company and the
agreements and covenants set forth in Sections 4, 5 and 8 shall survive the
closings. The Company agrees to indemnify and hold harmless the Buyer for loss
or damage arising as a result of or related to any breach or alleged breach by
the Company of any of its representations set forth in Section 3 hereof,
including advancement of expenses as they are incurred.


                                       20


<PAGE>   14

     j. Publicity. The Company and the Buyer shall have the right to approve
before issuance any press releases, SEC or NASD filings, or any other public
statements with respect to the transactions contemplated hereby; provided,
however, that the Company shall be entitled, without the prior approval of the
Buyer, to make any press release or SEC or NASD filings with respect to such
transactions as is required by applicable law and regulations (although the
Buyer shall be consulted by the Company in connection with any such press
release prior to its release and shall be provided with a copy thereof).

     k. Further Assurances. Each party shall do and perform, or cause to be done
and performed, all such further acts and things, and shall execute and deliver
all such other agreements, certificates, instruments and documents, as the other
party may reasonably request in order to carry out the intent and accomplish the
purposes of this Agreement and the consummation of the transactions contemplated
hereby.

     l. Termination. In the event that the First Closing shall not have occurred
on or before fifteen (15) business days from the date hereof, unless the parties
agree otherwise, this Agreement shall terminate at the close of business on such
date.

     IN WITNESS WHEREOF, the Buyer and the Company have caused this Agreement to
be duly executed.

IMMUNOGEN, INC.


By:    /s/ Mitchel Sayare
Name:  Mitchel Sayare
Its:   Chairman and Chief Executive Officer

CAPITAL VENTURES INTERNATIONAL


By:
/s/ Steve Katznelson
Name:  Steve Katznelson
Its:   Vice President, Arbit, Inc.



                                       21


<PAGE>   15

                                                                  EXHIBIT A to
                                                           Securities Purchase
                                                                     Agreement

THE SECURITIES REPRESENTED BY THIS CERTIFICATE AND THE SECURITIES ISSUABLE UPON
CONVERSION THEREOF HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED. THE SECURITIES HAVE BEEN ACQUIRED FOR INVESTMENT AND MAY NOT BE SOLD,
TRANSFERRED OR ASSIGNED IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT
FOR THE SECURITIES UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR AN OPINION
OF COUNSEL IN FORM, SUBSTANCE AND SCOPE REASONABLE ACCEPTABLE TO THE BORROWER
THAT REGISTRATION IS NOT REQUIRED UNDER SAID ACT OR UNLESS SOLD PURSUANT TO RULE
144 UNDER SAID ACT. ANY SUCH SALE, ASSIGNMENT OR TRANSFER MUST ALSO COMPLY WITH
APPLICABLE STATE SECURITIES LAWS.

                              CONVERTIBLE DEBENTURE

_______________, 1996                                               $2,500,000

         FOR VALUE RECEIVED, IMMUNOGEN, INC., a Massachusetts corporation
(hereinafter called the "Borrower") hereby promises to pay to the order of
CAPITAL VENTURES INTERNATIONAL or registered assigns (the "Holder") the sum of
Two Million Five Hundred Thousand Dollars ($2,500,000.00), on ____________,
2000, and to pay interest on the unpaid principal balance hereof at the rate of
nine percent (9%) per annum from ______, 1996 (the "Issue Date") until the same
becomes due and payable, whether at maturity or upon acceleration or by
prepayment or otherwise. Any amount of principal of or interest on this
Debenture which is not paid when due shall bear interest at the rate of ten
percent (10%) per annum from the due date thereof until the same is paid.
Interest shall commence accruing on the Issue Date and, to the extent not
converted in accordance with the provisions of Article II below, shall be
payable in arrears on the date the principal amount in respect of which it has
accrued is paid, whether at maturity or upon acceleration or by prepayment or
otherwise. All payments of principal and interest (to the extent not converted
in accordance with the terms hereof) shall be made in lawful money of the United
States of America. All payments shall be made at such address as the Holder
shall hereafter give to the Borrower by written notice made in accordance with
the provisions of this Debenture.

     The following terms shall apply to this Debenture:
     ARTICLE I . PREPAYMENT

     1.1 Prepayment. So long as no Event of Default (as defined herein) shall
have occurred and be continuing and so long as the closing bid price of the
Common Stock (as defined in Section 2.1 below) is, both on the date of receipt
of the Prepayment Notice (as defined herein) and on the Prepayment Date (as
defined herein), and has been for at least five (5) consecutive trading days
prior thereto, at least $_______ [200% of closing bid price on the Issue Date,
at anytime following March ___, 1998, the Borrower shall have the right,
exercisable on not less


                                       22


<PAGE>   16

than 120 days prior written notice to the Holder, to prepay this Debenture, in
whole or in any part of not less than $500,000 principal amount (or such lesser
principal amount as shall remain unpaid at the time of exercise of such right),
in accordance with this Section 1.1. Any notice of prepayment (a "Prepayment
Notice") shall be delivered to the Holder at its registered address appearing on
the records of the Borrower and shall state (1) that the Borrower is exercising
its right to prepay all or a portion of the principal amount of this Debenture,
(2) the principal amount to be prepaid and (3) the date of prepayment. On the
date fixed for prepayment (the "Prepayment Date"), the Borrower shall make
payment of the Prepayment Amount (as hereinafter defined) to or upon the order
of the Holder as specified by the Holder in writing to the Borrower at least one
business day prior to the prepayment date. If the Borrower exercises its right
to prepay all or a portion of this Debenture, the Borrower shall make payment to
the Holder or upon the order of the Holder of an amount equal to the sum of (1)
the principal amount of this Debenture to be prepaid (the "Prepayment Amount"),
plus (2) in each case, accrued and unpaid interest on the principal amount being
prepaid to the Prepayment Date. Upon the prepayment of less than the entire
unpaid principal amount of this Debenture, a new Debenture containing the same
date and provisions as this Debenture shall be issued by the Borrower to the
Holder for the principal balance of this Debenture which shall not have been
prepaid. Notwithstanding anything to the contrary contained in this Section 1.1,
the Holder shall at all times maintain the right to convert all or any part of
the outstanding and unpaid principal amount of this Debenture in accordance with
Article II below and any amounts so converted after receipt of a Prepayment
Notice and prior to the Prepayment date of the amounts set forth in such notice
(which period shall be at least 119 days) shall be deducted from the principal
amount which is otherwise subject to prepayment pursuant to such notice.

     ARTICLE II. CONVERSION AND PURCHASE RIGHTS

     2.1 Conversion Right. The Holder shall have the right from and after
the date of this Debenture and then at any time on or prior to the day this
Debenture is paid in full (whether or not the Borrower has sent a Prepayment
Notice to the Holder pursuant to Article I hereof), to convert at any time all
or from time to time any part of the outstanding and unpaid principal amount of
this Debenture of at least $100,000, or such lesser amount as shall remain
unpaid at the time of the conversion, into (i) fully paid and non-assessable
shares of Common Stock, par value $.01 per share of the Borrower as such stock
exists on the date of issuance of this Debenture, or any shares of capital
stock of Borrower into which such stock shall hereafter be changed or
reclassified (the "Common Stock") at the conversion price determined as
provided herein (the "Conversion Price") and (ii) if such conversion takes
place after the eightieth (80th) day following the Closing Date in respect of
the First Closing under the Securities Purchase Agreement (as defined in
Section 2.4 below), warrants (the "Warrants"), in the form attached as Exhibit
A-1 to the Securities Purchase Agreement (as hereinafter defined), to acquire a
number of shares of Common Stock equal to 50% of the number of shares of Common
Stock issuable upon such conversion; provided, however, that in no event shall
the Holder be entitled to convert any portion of this Debenture in excess of
that portion of this Debenture upon conversion of which the sum of (1) the
number of shares of Common Stock beneficially owned by the Holder and its
affiliates (other than shares of Common Stock which may be deemed beneficially
owned through the ownership of the unconverted 


                                       23


<PAGE>   17

portion of this Debenture and unexercised Warrants (which contain a provision   
substantially identical to this proviso)) and (2) the number of shares of
Common Stock issuable upon the conversion of the portion of this Debenture
with respect to which the determination of this proviso is being made, would
result in beneficial ownership by the Holder and its affiliates of more than
9.9% of the outstanding shares of Common Stock. For purposes of the proviso to
the immediately preceding sentence, beneficial ownership shall be determined in
accordance with Section 13(d) of the Securities Exchange Act of 1934, as
amended, and Regulation 13 D-G thereunder, except as otherwise provided in
clause (1) of such proviso. Upon the surrender of this Debenture, accompanied
by a Notice of Conversion of Convertible Debenture in the form attached hereto
as Exhibit A, properly completed and duly executed by the Holder (a "Conversion
Notice"), the Borrower shall issue and, within two (2) business days (the
"Deadline") after such surrender of this Debenture with the Conversion Notice,
deliver to or upon the order of the Holder (1) that number of shares of Common
Stock and Warrants for the portion of the Debenture converted as shall be
determined in accordance herewith and (2) a new Debenture in the form hereof
for the balance of the principal amount hereof, if any. Without in any way
limiting the Holder's right to pursue other remedies, including actual damages
and/or equitable relief, the parties agree that if delivery of the Common Stock
and Warrants issuable upon conversion of this Debenture is more than one (1)
business day after the Deadline (other than a failure due to the circumstances
described in Section 2.3 below, which failure shall be governed by such
Section) the Borrower shall pay to the Holder $150 per day in cash, for the
first day beyond the Deadline and $500 per day for each day thereafter that the
Borrower fails to deliver such Common Stock and Warrants. Such cash amount
shall be paid to Holder by the fifth day of the month following the month in
which it has accrued or, at the option of the Holder (by written notice to the
Borrower by the first day of the month following the month in which it has
accrued), shall be added to the principal amount of this Debenture, in which
event interest shall accrue thereon in accordance with the terms of this
Debenture and such additional principal amount shall be convertible into Common
Stock in accordance with the terms of this Debenture.

     The number of shares of Common Stock to be issued upon each conversion of
this Debenture shall be determined by dividing (i) the sum of (A) that portion
of the principal amount of the Debenture to be converted plus (B) the
"Conversion Date Interest" (as defined below), by (ii) the Conversion Price in
effect on the date the Conversion Notice is delivered to the Borrower by the
Holder. The number of Warrants to be issued upon each conversion of this
Debenture occurring after the eightieth (80th) day following the date of
issuance of this Debenture shall be 50% of the number of shares of Common Stock
issuable upon such conversion in accordance with the immediately preceding
sentence. Conversion Date Interest means the product of (i) the principal amount
of the Debenture to be converted, multiplied by (ii) a fraction (A) the
numerator of which is the number of days elapsed since the date of issuance of
this Debenture and (B) the denominator of which is 365, multiplied by (iii) .09.

     2.2 Conversion Price. The Conversion Price shall be the lesser of (i) the
Applicable Percentage (as hereinafter defined) of the average of the closing bid
prices for the Common Stock on the NASDAQ National Market, or on the principal
securities exchange or other securities market on which the Common Stock is then
being traded, for the five (5) consecutive 


                                       24


<PAGE>   18

Trading Days (as defined below) ending one Trading Day prior to the date the
Conversion Notice is sent by the Holder to the Borrower via facsimile (the
"Conversion Date"), and (ii) $_______ [the closing bid price for the Common
Stock on the date of issuance of this Debenture] (subject to equitable
adjustments for stock splits, stock dividends, combinations, recapitalization,
reclassifications and similar events). "Trading Day" shall mean any day on which
the Common Stock is traded for any period on the NASDAQ National Market, or on
the principal securities exchange or other securities market on which the Common
Stock is then being traded. Applicable Percentage means (i) 100%, if the
Conversion Date is within forty (40) days after the Closing Date in respect of
the First Closing under the Securities Purchase Agreement (the "First Closing
Date"), (ii) 90%, if the Conversion Date is within eighty (80) days, but more
than forty (40) days, after the First Closing Date, and (iii) 85%, if the
Conversion Date is more than eighty (80) days after the First Closing Date.

     2.3 Authorized Shares. The Borrower covenants that during the period the
conversion right exists, the Borrower will reserve from its authorized and
unissued Common Stock a sufficient number of shares to provide for the issuance
of Common Stock upon the full conversion of this Debenture (including, without
limitation, the shares of Common Stock issuable upon exercise of Warrants
issuable upon such full conversion). As of the date of issuance of this
Debenture, [3,900,000/8,900,000] authorized and unissued shares of Common Stock
have been duly reserved for issuance upon conversion of this Debenture (and
issuance of the Warrants). The Borrower represents that upon issuance, such
shares will be duly and validly issued, fully paid and non-assessable. The
Borrower (i) acknowledges that it has irrevocably instructed its transfer agent
to issue certificates for the Common Stock issuable upon conversion of this
Debenture (and exercise of the Warrants) and (ii) agrees that its issuance of
this Debenture shall constitute full authority to its officers and agents who
are charged with the duty of executing stock certificates and Warrants to
execute and issue the necessary certificates for shares of Common Stock and
Warrants upon the conversion of this Debenture (and to execute and issue the
necessary certificates for shares of Common Stock upon exercise of the
Warrants).

If, at any time a Holder of this Debenture submits a Notice of Conversion, the
Borrower does not have sufficient authorized but unissued shares of Common Stock
available to effect such conversion (and exercise of the Warrants) in accordance
with the provisions of this Article II (a "Conversion Default"), the Borrower
shall issue to the Holder all of the shares of Common Stock which are available
to effect such conversion. The portion of this Debenture which the Holder
included in its Conversion Notice and which exceeds the amount which is then
convertible into available shares of Common Stock and Warrants exercisable for
Common Stock (the "Excess Amount") shall, notwithstanding anything to the
contrary contained herein, not be convertible into Common Stock and Warrants in
accordance with the terms hereof until (and at the Holder's option at any time
after) the date additional shares of Common Stock are authorized by the
Borrower, at which time the Conversion Price in respect thereof shall be the
lower of (i) the Conversion Price on the Conversion Default Date (as defined
below) and (ii) the Conversion Price on the Conversion Date thereafter elected
by the Holder in respect thereof. The Borrower shall pay to the Holder payments
("Conversion Default Payments") for a Conversion Default in the amount of
(N/365) x the "Default Rate" (as defined below) x the Excess Amount on the first
day of the Conversion Default (the "Conversion Default Date"), where N = the
number of days 


                                       25


<PAGE>   19

from the Conversion Default Date to the date (the "Authorization Date") that
the Borrower authorizes a sufficient number of shares of Common Stock to effect
conversion of the full outstanding principal balance of this Debenture and the
Warrants. The Default Rate means .10 for the first forty-five (45) days
following the Conversion Default Date and .24 for the period thereafter until
the Authorization Date. The Borrower shall use all commercially reasonable
efforts to authorize a sufficient number of shares of Common Stock as soon as
practicable following a Conversion Default. The Borrower shall send notice to
the Holder of the authorization of additional shares of Common Stock, the
Authorization Date and the amount of Holder's accrued Conversion Default
Payments. The accrued Conversion Default Payments for each calendar month shall
be paid in cash or shall be convertible into Common Stock at the Market Price
(as defined in the Warrants), at the Holder's option, as follows:

     (a) In the event Holder elects to take such payment in cash, cash payment
shall be made to Holder by the fifth day of the month following the month in
which it has accrued; and

     (b) In the event Holder elects to take such payment in Common Stock, the
Holder may convert such payment amount into Common Stock at the Market Price (as
in effect at the time of Conversion) at any time after the fifth day of the
month following the month in which it has accrued in accordance with the terms
of this Article II.

     Nothing herein shall limit the Holder's right to pursue actual damages (to
the extent in excess of the Conversion Default Payments) due to the Borrower's
failure to maintain a sufficient number of authorized shares of Common Stock.

     2.4 Method of Conversion. Except as otherwise provided in this Debenture or
agreed by the Holder, this Debenture may be converted by the Holder in whole at
any time or in part (provided such partial conversion is at least $100,000) from
time to time by (i) submitting to the Borrower a Conversion Notice (by facsimile
dispatched on the Conversion Date and confirmed by U.S. mail or overnight mail
service sent within two Trading Days thereafter) and (ii) surrendering this
Debenture with the mailed confirmation of the Conversion Notice at the principal
office of the Borrower. Upon partial exercise of the conversion rights provided
hereby, a new Debenture containing the same date and provisions as this
Debenture shall be issued by the Borrower to the Holder for the principal
balance of this Debenture which shall not have been converted. This Debenture
has been issued pursuant to a Securities Purchase Agreement, dated as of March
15, 1996, between the Borrower and the original Holder of this Debenture (the
"Securities Purchase Agreement"). By its acceptance of this Debenture, each
Holder agrees to be bound by the terms of the Securities Purchase Agreement.
This Debenture has been issued by the Borrower pursuant to the exemption from
registration under the Act provided by Regulation D thereunder.

     2.5 Concerning the Shares and Warrants. The shares of Common Stock and
Warrants issuable upon conversion of this Debenture and the shares of Common
Stock issuable upon exercise of the Warrants may not be sold or transferred
unless either (i) they first shall have been registered under the Act and
applicable state securities laws or (ii) the Borrower shall have been furnished
with an opinion of legal counsel to the effect that such sale or transfer is
exempt from the registration requirements of the Act or (iii) unless sold
pursuant to Rule 144 under the Act. 


                                       26


<PAGE>   20

Except as otherwise provided in the Securities Purchase Agreement, each
certificate for shares of Common Stock and Warrant issuable upon conversion of
this Debenture and the shares of Common Stock issuable upon exercise of the
Warrants that have not been so registered and that have not been sold pursuant
to an exemption that permits removal of the legend, shall bear a legend
substantially in the following form, as appropriate:

     THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
UNDER THE SECURITIES ACT OF 1933, AS AMENDED. THE SECURITIES HAVE BEEN ACQUIRED
FOR INVESTMENT AND MAY NOT BE SOLD, TRANSFERRED OR ASSIGNED IN THE ABSENCE OF AN
EFFECTIVE REGISTRATION STATEMENT FOR THE SECURITIES UNDER THE SECURITIES ACT OF
1933, AS AMENDED, OR AN OPINION OF COUNSEL IN FORM, SUBSTANCE AND SCOPE
REASONABLY ACCEPTABLE TO THE BORROWER THAT REGISTRATION IS NOT REQUIRED UNDER
SAID ACT OR UNLESS SOLD PURSUANT TO RULE 144 UNDER SAID ACT. ANY SUCH SALE,
ASSIGNMENT OR TRANSFER MUST ALSO COMPLY WITH APPLICABLE STATE SECURITIES LAWS.

     Upon the request of a holder of a certificate representing any shares of
Common Stock or Warrants issuable upon conversion of this Debenture or shares of
Common Stock issuable upon exercise of the Warrants, the Borrower shall remove
the foregoing legend from the certificate or issue to such holder a new
certificate therefor free of any transfer legend, if (i) with such request, the
Borrower shall have received either (A) an opinion of counsel, reasonably
satisfactory to the Borrower in form, substance and scope, to the effect that
any such legend may be removed from such certificate, or (B) satisfactory
representations from the holder that such holder is eligible to immediately sell
all of the Common Stock or Warrants issuable upon conversion of the Debenture or
shares of Common Stock issuable upon exercise of the Warrants (to the extent
such securities are deemed to have been acquired on the same date) pursuant to
Rule 144 (or a successor rule) or (ii) in the case of the Common Stock issuable
upon conversion of this Debenture or exercise of the Warrants, a registration
statement under the Act covering such securities is in effect. Nothing in this
Debenture shall (i) limit the Borrower's obligation under the Registration
Rights Agreement, dated as of March 15, 1996, by and among the Company and the
other signatories thereto (the "Registration Rights Agreement") or (ii) affect
in any way the Holder's obligations to comply with applicable securities laws
upon the resale of the securities referred to herein.

     2.6 Effect of Merger, Consolidation, etc. If at anytime when this Debenture
is issued and outstanding, there shall be any merger, consolidation, exchange of
shares, recapitalization, reorganization, or other similar event, as a result of
which shares of Common Stock of the Borrower shall be changed into the same or a
different number of shares of another class or classes of stock or securities of
the Borrower or another entity, or in case of any sale or conveyance of all or
substantially all of the assets of the Borrower other than in connection with a
plan of complete liquidation of the Borrower, then the Holder of this Debenture
shall thereafter have the right to receive upon conversion of this Debenture,
upon the bases and upon the terms and conditions specified herein and in lieu of
the shares of Common Stock immediately theretofore issuable upon conversion,
such stock, securities or assets which the Holder would 


                                       27


<PAGE>   21

have been entitled to receive in such transaction had this Debenture been
converted immediately prior to such transaction, and in any such case
appropriate provisions shall be made with respect to the rights and interests of
the Holder of this Debenture to the end that the provisions hereof (including,
without limitation, provisions for adjustment of the Conversion Price and of the
number of shares issuable upon conversion of the Debenture) shall thereafter be
applicable, as nearly as maybe practicable in relation to any securities or
assets thereafter deliverable upon the exercise hereof. The Borrower shall not
effect any transaction described in this Section 2.6 unless (a) it first gives,
to the extent practical, forty-five (45) days prior written notice (but in any
event at least fifteen (15) business days prior written notice) of such merger,
consolidation, exchange of shares, recapitalization, reorganization or other
similar event or sale of assets (during which time the Holder shall be entitled
to convert this Debenture) and (b) the resulting successor or acquiring entity
(if not the Borrower) assumes by written instrument the obligations of this
Section 2.6.

     ARTICLE III.  EVENTS OF DEFAULT

     If of any of the following events of default (each, an "Event of Default" )
shall occur:

     3.1 Failure to Pay Principal or Interest. The Borrower fails (a) to pay the
principal hereof when due, whether at maturity, upon a Prepayment Date, upon
acceleration or otherwise or (b) to pay any installment of interest hereon when
due and, in the case of this clause (b) only, such failure continues for a
period of five (5) days after the due date thereof;

     3.2 Conversion and the Shares and Warrants. The Borrower fails to issue
shares of Common Stock or Warrants to the Holder upon exercise by the Holder of
the conversion rights of the Holder in accordance with the terms of this
Debenture (for a period of at least 120 days, if such failure is solely as a
result of the circumstances governed by Section 2.3 and the Borrower is using
all commercially reasonable efforts to authorize a sufficient number of shares
of Common Stock as soon as practicable), fails to transfer any certificate for
shares of Common Stock or Warrants issued to the Holder upon conversion of this
Debenture and when required by this Debenture or the Registration Rights
Agreement (or shares of Common Stock issuable upon exercise of the Warrants in
accordance with the Warrants), or fails to remove any restrictive legend on any
certificate or any shares of Common Stock or Warrants issued to the Holder upon
conversion of this Debenture as and when required by this Debenture, the
Securities Purchase Agreement or the Registration Rights Agreement (or shares of
Common Stock issuable upon exercise of the Warrants in accordance with the
Warrants) and any such failure shall continue uncured for three (3) business
days after the Borrower shall have been notified thereof in writing by the
Borrower;

     3.3 Breach of Covenant. The Borrower breaches any material covenant or
other material term or condition of this Debenture (other than as specifically
provided in Sections 3.1 and 3.2 hereof), the Securities Purchase Agreement, the
Registration Rights Agreement or the Warrants and such breach continues for a
period of ten business (10) days after written notice thereof to the Borrower
from the Holder, it being understood that the failure of the Borrower to obtain
effectiveness with the Securities and Exchange Commission of the Registration
Statement within 


                                       28


<PAGE>   22

the 60-day period specified in Section 2(c) of the Registration Rights Agreement
for any reason other than the failure of the Borrower to amend such Registration
Statement as specified in Section 2(a) of the Registration Rights Agreement or
to use its best efforts to cause such Registration Statement to become effective
within such period and to remain effective during the Registration Period (as
defined therein), without more, shall not constitute an Event of Default;

     3.4 Breach of Representations and Warranties. Any representation or
warranty of the Borrower made herein or in any agreement, statement or
certificate given in writing pursuant hereto or in connection herewith
(including, without limitation, the Securities Purchase Agreement, the
Registration Rights Agreement and the Warrants), shall be false or misleading in
any material respect when made and the breach of which would have a material
adverse effect on the Borrower or the prospects of the Borrower or a material
adverse effect on the Holder or the rights of the Holder with respect to this
Debenture or the shares of Common Stock issuable upon conversion of this
Debenture;

     3.5 Receiver or Trustee. The Borrower or any subsidiary of the Borrower
shall make an assignment for the benefit of creditors, or apply for or consent
to the appointment of a receiver or trustee for it or for a substantial part of
its property or business; or such a receiver or trustee shall otherwise be
appointed;

     3.6 Judgments. Any money judgment, writ or similar process shall be entered
or filed against the Borrower or any subsidiary of the Borrower or any of its
property or other assets for more than $500,000, and shall remain unvacated,
unbonded or unstayed for a period of twenty (20) days unless otherwise consented
to by the Holder, which consent will not be unreasonably withheld; or

     3.7 Bankruptcy. Bankruptcy, insolvency, reorganization or liquidation
proceedings or other proceedings for relief under any bankruptcy law or any law
for the relief of debtors shall be instituted by or against the Borrower or any
subsidiary of the Borrower.

     Then upon the occurrence and during the continuation of any Event of
Default specified in Section 3.1, 3.2, 3.3, 3.4 or 3.6, at the option of the
Holder hereof, the Borrower shall, and upon the occurrence of any event of
default specified in Section 3.5 or 3.7, the Borrower shall, pay to the Holder
an amount equal to the product of (1) the sum of (x) the then outstanding
principal amount of this Debenture plus (y) accrued and unpaid interest on the
unpaid principal amount of this Debenture to the date of payment multiplied by
(2) 125% (the "Default Amount") and all other amounts payable hereunder shall
immediately become due and payable, all without demand, presentment or notice,
all of which hereby are expressly waived, together with all costs, including,
without limitation, legal fees and expenses, of collection, and the Holder shall
be entitled to exercise all other rights and remedies available at law or in
equity.

     If the Borrower fails to pay the Default Amount within 5 business days of
written notice that such amount is due and payable, then the Holder shall have
the right at any time, so long as the Borrower remains in default, to require
the Borrower, upon written notice, to immediately issue (in accordance with the
terms of Article II), in lieu of the Default Amount, the number of 


                                       29


<PAGE>   23

shares of Common Stock of the Borrower equal to the Default Amount divided by
the Conversion Price then in effect.

     ARTICLE IV. MISCELLANEOUS

     4.1 Failure or Indulgency Not waiver. No failure or delay on the part of
the Holder in the exercise of any power, right or privilege hereunder shall
operate as a waiver thereof, nor shall any single or partial exercise of any
such power, right or privilege preclude other or further exercise thereof or of
any other right, power or privileges. All rights and remedies existing hereunder
are cumulative to, and not exclusive of, any rights or remedies otherwise
available.

     4.2 Notices. Any notice herein required or permitted to be given shall be
in writing and may be personally served or delivered by courier or sent by
United States mail and shall be deemed to have been given upon receipt if
personally served (which shall include telephone line facsimile transmission) or
sent by courier or three (3) days after being deposited in the United States
mail, certified, with postage pre-paid and properly addressed, if sent by mail.
For the purposes hereof, the address of the Holder shall be as shown on the
records of the Borrower; and the address of the Borrower shall be Capital
Ventures International, c/o Arbit, 1900 Market Street, Philadelphia,
Pennsylvania 19102, Attention: Steve Katznelson (facsimile number 215-963-3379).
Both the Holder and the Borrower may change the address for service by service
of written notice to the other as herein provided.

     4.3 Amendment Provision. This Debenture and any provision hereof may only
be amended by an instrument in writing signed by the Borrower and the Holder.
The term "Debenture" and all reference thereto, as used throughout this
instrument, shall mean this instrument as originally executed, or if later
amended or supplemented, then as so amended or supplemented.

     4.4 Assignability. This Debenture shall be binding upon the Borrower and
its successors and assigns, and shall inure to be the benefit of the Holder and
its successors and assigns; provided, however, that so long as no Event of
Default has occurred, this Debenture shall only be transferable in whole or in
increments of $500,000 to "Accredited Investors" (as defined in Rule 501(a)
under the Securities Act); provided further, however, no "Subject Holder" (as
defined below) may sell or otherwise transfer the Debentures, except (i) to the
Borrower or to a stockholder or a group of stockholders who immediately prior
to the sale control a majority of the Borrower's voting shares (a "Controlling
Stockholder" or "Controlling Group", as applicable); (ii) to an affiliate of
such Holder; (iii) in connection with any merger, consolidation, reorganization
or sale of more than 50% of the outstanding Common Stock of the Borrower (a
"Reorganization"); (iv) in a registered public offering or a public sale
pursuant to Rule 144 or other applicable exemption from the registration
requirements of the Securities Act (or any successor rule or regulation); or
(v) in a private sale (otherwise than to the Borrower, to a Controlling
Stockholder or a Controlling Group, to an affiliate of such Holder, or in a
Reorganization), provided that the Holder shall not sell or otherwise transfer
during any ninety (90) day period a portion(s) of the Debentures which, if
converted into Common Stock and Warrants at the time of the transfer, would
represent, in the aggregate, beneficial ownership by the transferee(s) of more
than 9.9% percent of the Common Stock then outstanding.  Subject Holder means
any Holder who, but for the proviso set forth in the first paragraph of Section
2.3, would beneficially own 10% or more of the outstanding Common Stock of the
Borrower.  For purposes of this paragraph, "Debentures" means this Debenture and
the other Debenture issuable pursuant to the Securities Purchase Agreement.

     4.5 Cost of Collection. If default is made in the payment of this
Debenture, the Borrower shall pay the Holder hereof costs of collection,
including reasonable attorneys' fees.

     4.6 Governing Law. This Debenture shall be governed by the internal laws of
the Commonwealth of Massachusetts, without regard to the principles of conflict
of laws.

     4.7 Damages Shares. The shares of Common Stock that may be issuable to the
Holder pursuant to Section 2.3 and Article III hereof and pursuant to Section
2(C) of the Registration Rights Agreement ("Damages Shares") shall be treated as
Common Stock issuable upon conversion of this Debenture for all purposes hereof
and shall be subject to all of the limitations and afforded all of the rights of
the other shares of Common Stock issuable hereunder, including 


                                       30


<PAGE>   24

without limitation, the right to be included in the Registration Statement filed
pursuant to the Registration Rights Agreement. For purposes of calculating
interest payable on the outstanding principal amount hereof, amounts convertible
into Damages Shares ("Damages Amounts") shall not bear interest but must be
converted prior to the conversion of any outstanding principal amount hereof,
until the outstanding Damages Amounts is zero.

     4.8 Denominations. At the request of the Holder, upon surrender of this
Debenture, the Borrower shall promptly issue new Debentures in the aggregate
outstanding principal amount hereof, in the form hereof, in such denominations
of at least $100,000 as the Holder shall request.

     IN WITNESS WHEREOF, Borrower has caused this Debenture to be signed in its
name by its duly authorized officer this ___ day of March, 1996.

IMMUNOGEN, INC.



By:_______________________________
Name:
Title:

                                       31


<PAGE>   25

                              NOTICE OF CONVERSION                    Exhibit A
                            OF CONVERTIBLE DEBENTURE
TO:

     (1) Pursuant to the terms of the attached Convertible Debenture (the
"Debenture"), the undersigned hereby elects to convert $          principal
amount of the Debenture into shares of Common Stock [and Warrants] of ImmunoGen,
Inc., a Delaware corporation (the "Borrower"). Capitalized terms used herein and
not otherwise defined herein have the respective meanings provided in the
Debenture.

     (2) Please issue a certificate or certificates for the number of shares of
Common Stock [and Warrants] into which such principal amount of the Debenture is
convertible in the name(s) specified immediately below or, if additional space
is necessary, on an attachment hereto:

                Name                          Name
               Address                       Address
              SS or Tax ID Number           SS or Tax ID Number

     (3) In the event of partial exercise, please reissue an appropriate
Debenture(s) for the principal balance which shall not have been converted.
Capitalized terms used in this Notice of Conversion and not otherwise defined
herein shall have the respective meanings provided in the Debenture.

     (4) If the shares of Common Stock [and Warrants] issuable upon conversion
of the Debenture and the shares of Common Stock issuable upon conversion of the
Warrants have not been registered under the Securities Act of 1933, as amended
(the "Act"), the undersigned represents and warrants that (i) such shares of
Common Stock [and Warrants] are being acquired for the account of the
undersigned for investment, and not with a present view to, or for resale in
connection with, the distribution thereof, and that the undersigned has no
present intention of distributing or reselling such securities, in each case,
other than pursuant to a registration statement under the Act and (ii) the
undersigned is an "accredited investor" as defined in Regulation D under the
Act. The undersigned further agrees that (A) the sale of such securities shall
not be sold or transferred unless either (i) they first shall have been
registered under the Act and applicable state securities laws or (ii) the
Borrower first shall have been furnished with either (x) an opinion of legal
counsel to the effect that such sale or transfer is exempt from the registration
requirements of the Act or (y) satisfactory representations from the undersigned
that the undersigned may immediately sell all of such securities (to the extent
such securities are deemed to have been acquired on the same date) pursuant to
Rule 144 (or a successor thereto) and (B) the Borrower may place a legend on the
certificate(s) for such securities to that effect and place a stop transfer
restriction in its records relating to such securities. Nothing in this Notice
of Conversion shall limit the Borrower's obligation under Section 3 of the
Registration Rights Agreement.

Date
Signature of Registered Holder
(must be signed exactly as name appears in the Debenture. The signature must be
guaranteed by a member firm of the NYSE or the NASD or by a commercial bank or
trust Borrower having an office in the U.S.)

                                       32


<PAGE>   26

                                                                 EXHIBIT A-1 to
                                                                     Securities
                                                                       Purchase
                                                                      Agreement

THIS WARRANT AND THE SHARES ISSUABLE UPON THE EXERCISE OF THIS WARRANT HAVE NOT
BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED. EXCEPT AS
OTHERWISE SET FORTH HEREIN OR IN A SECURITIES PURCHASE AGREEMENT DATED AS OF
MARCH 15, 1996, NEITHER THIS WARRANT NOR ANY OF SUCH SHARES MAY BE SOLD, OFFERED
FOR SALE, ASSIGNED, TRANSFERRED, OR OTHERWISE DISPOSED OF IN THE ABSENCE OF
REGISTRATION UNDER SUCH ACT OR AN OPINION OF COUNSEL THAT REGISTRATION IS NOT
REQUIRED UNDER SUCH ACT OR UNLESS SOLD PURSUANT TO RULE 144 UNDER SUCH ACT. ANY
SUCH SALE, ASSIGNMENT OR TRANSFER MUST ALSO COMPLY WITH APPLICABLE STATE
SECURITIES LAWS.

Right to Purchase  _______ Shares of Common Stock, par value $.01 per share

                                 IMMUNOGEN, INC.
                             STOCK PURCHASE WARRANT

     THIS CERTIFIES THAT, for value received, _________________________ or its
registered assigns, is entitled to purchase from IMMUNOGEN, INC., a
Massachusetts corporation (the "Company"), at any time or from time to time
during the period specified in Paragraph 2 hereof, ____________________
(_______) fully paid and nonassessable shares of the Company's Common Stock, par
value $.01 per share (the "Common Stock"), at an exercise price of $4.00 per
share (the "Exercise Price"). The term "Warrant Shares", as used herein, refers
to the shares of Common Stock purchasable hereunder. The Warrant Shares and the
Exercise Price are subject to adjustment as provided in Paragraph 4 hereof. The
term Warrants means this Warrant and the other warrants of the Company issued
upon conversion of the Convertible Debenture issued pursuant to the Securities
Purchase Agreement (as hereinafter defined).

     This Warrant is subject to the following terms, provisions, and conditions:

     1. Manner of Exercise; Issuance of Certificates; Payment for Shares.
Subject to the provisions hereof, this Warrant may be exercised by the holder
hereof, in whole or in part, by the surrender of this Warrant, together with a
completed exercise agreement in the form attached hereto (the "Exercise
Agreement"), to the Company during normal business hours on any business day at
the Company's principal executive offices (or such other office or agency of the
Company as it may designate by notice to the holder hereof), and upon (i)
payment to the Company in cash, by certified or official bank check or by wire
transfer for the account of the Company of the Exercise Price for the Warrant
Shares specified in the Exercise Agreement or (ii) if the resale of the Warrant
Shares by the holder is not then registered pursuant to an effective
registration statement under the Securities Act of 1933, as amended (the
"Securities Act"), delivery to the Company of a written notice of an election to
effect a "Cashless Exercise" (as defined in Section 11(c) below) for the Warrant
Shares specified in the Exercise Agreement. The 


                                       33


<PAGE>   27

Warrant Shares so purchased shall be deemed to be issued to the holder hereof or
such holder's designee, as the record owner of such shares, as of the close of
business on the date on which this Warrant shall have been surrendered, the
completed Exercise Agreement shall have been delivered, and payment shall have
been made for such shares as set forth above. Certificates for the Warrant
Shares so purchased, representing the aggregate number of shares specified in
the Exercise Agreement, shall be delivered to the holder hereof within a
reasonable time, not exceeding three (3) business days, after this Warrant shall
have been so exercised. The certificates so delivered shall be in such
denominations as may be requested by the holder hereof and shall be registered
in the name of such holder or such other name as shall be designated by such
holder. If this Warrant shall have been exercised only in part, then, unless
this Warrant has expired, the Company shall, at its expense, at the time of
delivery of such certificates, deliver to the holder a new Warrant representing
the number of shares with respect to which this Warrant shall not then have been
exercised.

     Notwithstanding anything in this Warrant to the contrary, in no event shall
the Holder of this Warrant be entitled to exercise a number of Warrants (or
portions thereof) in excess of the number of Warrants (or portions thereof) upon
exercise of which the sum of (i) the number of shares of Common Stock
beneficially owned by the Holder and its affiliates (other than shares of Common
Stock which may be deemed beneficially owned through the ownership of the
unexercised Warrants and portion of the unconverted Debentures (as defined
below)) and (ii) the number of shares of Common Stock issuable upon exercise of
the Warrants (or portions thereof) with respect to which the determination
described herein is being made, would result in beneficial ownership by the
Holder and its affiliates of more than 9.9% of the outstanding shares of Common
Stock. For purposes of the immediately preceding sentence, beneficial ownership
shall be determined in accordance with Section 13(d) of the Securities Exchange
Act of 1934, as amended, and Regulation 13D-G thereunder, except as otherwise
provided in clause (i) thereof.

     2. Period of Exercise. This Warrant is exercisable at any time or from time
to time on or after the date on which this Warrant is issued and delivered upon
conversion of the Debenture dated _________, 1996 (together with the other
Debenture(s) issued pursuant thereto, the "Debentures"), issued pursuant to the
terms of that certain Securities Purchase Agreement, dated as of March 15, 1996,
by and among the Company and the Buyer listed on the execution page thereof (the
"Securities Purchase Agreement"), and before 5:00 p.m., New York City time on
the fifth (5th) anniversary of the date of issuance (the "Exercise Period").

     3. Certain Agreements of the Company. The Company hereby covenants and
agrees as follows:

        (a) Shares to be Fully Paid. All Warrant Shares will, upon issuance in
accordance with the terms of this Warrant, be validly issued, fully paid, and
nonassessable and free from all taxes, liens, and charges with respect to the
issue thereof.

        (b) Reservation of Shares. During the Exercise Period, the Company shall
at all times have authorized, and reserved for the purpose of issuance upon
exercise of this Warrant, a sufficient number of shares of Common Stock to
provide for the exercise of this Warrant. 


                                       34


<PAGE>   28

        (c) Listing. The Company shall promptly secure the listing of the shares
of Common Stock issuable upon exercise of the Warrant upon each national
securities exchange or automated quotation system, if any, upon which shares of
Common Stock are then listed (subject to official notice of issuance upon
exercise of this Warrant) and shall maintain, so long as any other shares of
Common Stock shall be so listed, such listing of all shares of Common Stock from
time to time issuable upon the exercise of this Warrant; and the Company shall
so list on each national securities exchange or automated quotation system, as
the case may be, and shall maintain such listing of, any other shares of capital
stock of the Company issuable upon the exercise of this Warrant if and so long
as any shares of the same class shall be listed on such national securities
exchange or automated quotation system.

        (d) Certain Actions Prohibited. The Company will not, by amendment of
its charter or through any reorganization, transfer of assets, consolidation,
merger, dissolution, issue or sale of securities, or any other voluntary action,
avoid or seek to avoid the observance or performance of any of the terms to be
observed or performed by it hereunder, but will at all times in good faith
assist in the carrying out of all the provisions of this Warrant and in the
taking of all such action as may reasonably be requested by the holder of this
Warrant in order to protect the exercise privilege of the holder of this Warrant
against dilution or other impairment, consistent with the tenor and purpose of
this Warrant. Without limiting the generality of the foregoing, the Company (i)
will not increase the par value of any shares of Common Stock receivable upon
the exercise of this Warrant above the Exercise Price then in effect, and (ii)
will take all such actions as may be necessary or appropriate in order that the
Company may validly and legally issue fully paid and nonassessable shares of
Common Stock upon the exercise of this Warrant.

        (e) Successors and Assigns. This Warrant will be binding upon any entity
succeeding to the Company by merger, consolidation, or acquisition of all or
substantially all the Company's assets.

     4. Antidilution Provisions. During the Exercise Period, the Exercise Price
and the number of Warrant Shares shall be subject to adjustment from time to
time as provided in this Paragraph 4.

     In the event that any adjustment of the Exercise Price as required herein
results in a fraction of a cent, such Exercise Price shall be rounded up to the
nearest cent.

        (a) Adjustment of Exercise Price and Number of Shares upon Issuance of
Common Stock. Except as otherwise provided in Paragraphs 4(c) and 4(e) hereof,
if and whenever on or after the date of issuance of this Warrant, the Company
issues or sells, or in accordance with Paragraph 4(b) hereof is deemed to have
issued or sold, any shares of Common Stock for no consideration or for a
consideration per share (before deduction of reasonable expenses or commissions
or underwriting discounts or allowances in connection therewith) less than the
Market Price (as hereinafter defined) on the date of issuance (a "Dilutive
Issuance"), then immediately upon the Dilutive Issuance, the Exercise Price will
be reduced to a price determined by multiplying the Exercise Price in effect
immediately prior to the Dilutive Issuance by a fraction, (i) the numerator of
which is an amount equal to the sum of (x) the number of shares of Common Stock
Deemed 


                                       35


<PAGE>   29

Outstanding (as hereinafter defined) immediately prior to the Dilutive Issuance,
plus (y) the aggregate consideration, calculated as set forth in Section 4(b)
hereof, received by the Company upon such Dilutive Issuance, divided by the
Market Price in effect immediately prior to the Dilutive Issuance, and (ii) the
denominator of which is the total number of shares of Common Stock Deemed
Outstanding immediately after the Dilutive Issuance.

        (b) Effect on Exercise Price of Certain Events. For purposes of
determining the adjusted Exercise Price under Paragraph 4(a) hereof, the
following will be applicable:

            (i) Issuance of Rights or Options. If the Company in any manner
issues or grants any warrants, rights or options, whether or not immediately
exercisable, to subscribe for or to purchase Common Stock or other securities
convertible into or exchangeable for Common Stock ("Convertible Securities")
(such warrants, rights and options to purchase Common Stock or Convertible
Securities are hereinafter referred to as "Options") and the price per share for
which Common Stock is issuable upon the exercise of such Options is less than
the Market Price on the date of issuance, then the maximum total number of
shares of Common Stock issuable upon the exercise of all such Options will, as
of the date of the issuance or grant of such Options, be deemed to be
outstanding and to have been issued and sold by the Company for such price per
share. For purposes of the preceding sentence, the "price per share for which
Common Stock is issuable upon the exercise of such Options" is determined by
dividing (i) the total amount, if any, received or receivable by the Company as
consideration for the issuance or granting of all such Options, plus the minimum
aggregate amount of additional consideration, if any, payable to the Company
upon the exercise of all such Options, plus, in the case of Convertible
Securities issuable upon the exercise of such Options, the minimum aggregate
amount of additional consideration payable upon the conversion or exchange
thereof at the time such Convertible Securities first become convertible or
exchangeable, by (ii) the maximum total number of shares of Common Stock
issuable upon the exercise of all such Options (assuming full conversion of
Convertible Securities, if applicable). No further adjustment to the Exercise
Price will be made upon the actual issuance of such Common Stock upon the
exercise of such Options or upon the conversion or exchange of Convertible
Securities issuable upon exercise of such Options. 

            (ii) Issuance of Convertible Securities. If the Company in any
manner issues or sells any Convertible Securities, whether or not immediately
convertible (other than where the same are issuable upon the exercise of
Options) and the price per share for which Common Stock is issuable upon such
conversion or exchange is less than the Market Price on the date of issuance,
then the maximum total number of shares of Common Stock issuable upon the
conversion or exchange of all such Convertible Securities will, as of the date
of the issuance of such Convertible Securities, be deemed to be outstanding and
to have been issued and sold by the Company for such price per share. For the
purposes of the preceding sentence, the "price per share for which Common Stock
is issuable upon such conversion or exchange" is determined by dividing (i) the
total amount, if any, received or receivable by the Company as consideration for
the issuance or sale of all such Convertible Securities, plus the minimum
aggregate amount of additional consideration, if any, payable to the Company
upon the conversion or exchange thereof at the time such Convertible Securities
first become convertible or exchangeable, by (ii) the maximum total number of
shares of Common Stock issuable upon the conversion or exchange of all such
Convertible Securities. No further adjustment to the Exercise Price will be 


                                       36


<PAGE>   30

made upon the actual issuance of such Common Stock upon conversion or exchange
of such Convertible Securities.

            (iii) Change in Option Price or Conversion Rate. If there is a
change at any time in (i) the amount of additional consideration payable to the
Company upon the exercise of any Options; (ii) the amount of additional
consideration, if any, payable to the Company upon the conversion or exchange of
any Convertible Securities; or (iii) the rate at which any Convertible
Securities are convertible into or exchangeable for Common Stock (other than
under or by reason of provisions designed to protect against dilution), the
Exercise Price in effect at the time of such change will be readjusted to the
Exercise Price which would have been in effect at such time had such Options or
Convertible Securities still outstanding provided for such changed additional
consideration or changed conversion rate, as the case may be, at the time
initially granted, issued or sold. 

            (iv) Treatment of Expired Options and Unexercised Convertible
Securities. If, in any case, the total number of shares of Common Stock issuable
upon exercise of any Option or upon conversion or exchange of any Convertible
Securities is not, in fact, issued and the rights to exercise such Option or to
convert or exchange such Convertible Securities shall have expired or
terminated, the Exercise Price then in effect will be readjusted to the Exercise
Price which would have been in effect at the time of such expiration or
termination had such Option or Convertible Securities, to the extent outstanding
immediately prior to such expiration or termination (other than in respect of
the actual number of shares of Common Stock issued upon exercise or conversion
thereof), never been issued. 

            (v) Calculation of Consideration Received. If any Common Stock,
Options or Convertible Securities are issued, granted or sold for cash, the
consideration received therefor for purposes of this Warrant will be the amount
received by the Company therefor, before deduction of reasonable commissions,
underwriting discounts or allowances or other reasonable expenses paid or
incurred by the Company in connection with such issuance, grant or sale. In case
any Common Stock, Options or Convertible Securities are issued or sold for a
consideration part or all of which shall be other than cash, the amount of the
consideration other than cash received by the Company will be the fair value of
such consideration, except where such consideration consists of securities, in
which case the amount of consideration received by the Company will be the
Market Price thereof as of the date of receipt. In case any Common Stock,
Options or Convertible Securities are issued in connection with any merger or
consolidation in which the Company is the surviving corporation, the amount of
consideration therefor will be deemed to be the fair value of such portion of
the net assets and business of the non-surviving corporation as is attributable
to such Common Stock, Options or,Convertible Securities, as the case may be. The
fair value of any consideration other than cash or securities will be determined
in good faith by the Board of Directors of the Company. 

            (vi) Exceptions to Adjustment of Exercise Price. No adjustment to
the Exercise Price will be made (i) upon the exercise of any warrants, options
or convertible securities issued and outstanding on the March 4, 1996; (ii) upon
the grant or exercise of any stock or options which may hereafter be granted or
exercised under any employee benefit plan of the Company now existing or to be
implemented in the future, so long as the issuance of such stock or options is
approved by a majority of the independent members of the Board of Directors of
the Company or a majority of the members of a committee of independent directors
established for such purpose; or (iii) upon the exercise of the Warrants or
conversion of the Debenture. 


                                       37


<PAGE>   31

        (c) Subdivision or Combination of Common Stock. If the Company at any
time subdivides (by any stock split, stock dividend, recapitalization,
reorganization, reclassification or otherwise) the shares of Common Stock
acquirable hereunder into a greater number of shares, then, after the date of
record for effecting such subdivision, the Exercise Price in effect immediately
prior to such subdivision will be proportionately reduced. If the Company at any
time combines (by reverse stock split, recapitalization, reorganization,
reclassification or otherwise) the shares of Common Stock acquirable hereunder
into a smaller number of shares, then, after the date of record for effecting
such combination, the Exercise Price in effect immediately prior to such
combination will be proportionately increased.

        (d) Adjustment in Number of Shares. Upon each adjustment of the Exercise
Price pursuant to the provisions of this Paragraph 4, the number of shares of
Common Stock issuable upon exercise of this Warrant shall be adjusted by
multiplying a number equal to the Exercise Price in effect immediately prior to
such adjustment by the number of shares of Common Stock issuable upon exercise
of this Warrant immediately prior to such adjustment and dividing the product so
obtained by the adjusted Exercise Price.

          (e) Consolidation, Merger or Sale. In case of any consolidation of the
Company with, or merger of the Company into any other corporation, or in case of
any sale or conveyance of all or substantially all of the assets of the Company
other than in connection with a plan of complete liquidation of the Company,
then as a condition of such consolidation, merger or sale or conveyance,
adequate provision will be made whereby the holder of this Warrant will have the
right to acquire and receive upon exercise of this Warrant in lieu of the shares
of Common Stock immediately theretofore acquirable upon the exercise of this
Warrant, such shares of stock, securities or assets as may be issued or payable
with respect to or in exchange for the number of shares of Common Stock
immediately theretofore acquirable and receivable upon exercise of this Warrant
had such consolidation, merger or sale or conveyance not taken place. In any
such case, the Company will make appropriate provision to insure that the
provisions of this Paragraph 4 hereof will thereafter be applicable as nearly as
may be in relation to any shares of stock or securities thereafter deliverable
upon the exercise of this Warrant. The Company will not effect any
consolidation, merger or sale or conveyance unless prior to the consummation
thereof, the successor corporation (if other than the Company) assumes by
written instrument the obligations under this Paragraph 4 and the obligations to
deliver to the holder of this Warrant such shares of stock, securities or assets
as, in accordance with the foregoing provisions, the holder may be entitled to
acquire.

        (f) Distribution of Assets. In case the Company shall declare or make
any distribution of its assets to holders of Common Stock as a partial
liquidating dividend, by way of return of capital or otherwise, then, after the
date of record for determining stockholders entitled to such distribution, but
prior to the date of distribution, the holder of this Warrant shall be entitled
upon exercise of this Warrant for the purchase of any or all of the shares of
Common Stock subject hereto, to receive the amount of such assets which would
have been payable to the holder had such holder been the holder of such shares
of Common Stock on the record date for the determination of stockholders
entitled to such distribution.


                                       38


<PAGE>   32

        (g) Notice of Adjustment. Upon the occurrence of any event which
requires any adjustment of the Exercise Price, then, and in each such case, the
Company shall give notice thereof to the holder of this Warrant, which notice
shall state the Exercise Price resulting from such adjustment and the increase
or decrease in the number of Warrant Shares purchasable at such price upon
exercise, setting forth in reasonable detail the method of calculation and the
facts upon which such calculation is based. Such calculation shall be certified
by the chief financial officer of the Company.

        (h) Minimum Adjustment of Exercise Price. No adjustment of the Exercise
Price shall be made in an amount of less than 1% of the Exercise Price in effect
at the time such adjustment is otherwise required to be made, but any such
lesser adjustment shall be carried forward and shall be made at the time and
together with the next subsequent adjustment which, together with any
adjustments so carried forward, shall amount to not less than 1% of such
Exercise Price.

        (i) No Fractional Shares. No fractional shares of Common Stock are to be
issued upon the exercise of this Warrant, but the Company shall pay a cash
adjustment in respect of any fractional share which would otherwise be issuable
in an amount equal to the same fraction of the Market Price of a share of Common
Stock on the date of such exercise.

        (j) Other Notices. In case at any time:

            (i) the Company shall declare any dividend upon the Common Stock
payable in shares of stock of any class or make any other distribution (other
than dividends or distributions payable in cash out of retained earnings) to the
holders of the Common Stock;

            (ii) the Company shall offer for subscription pro rata to the
holders of the Common Stock any additional shares of stock of any class or other
rights;

            (iii) there shall be any capital reorganization of the Company, or
reclassification of the Common Stock, or consolidation or merger of the Company
with or into, or sale of all or substantially all its assets to, another
corporation or entity; or
              

            (iv) there shall be a voluntary or involuntary dissolution,
liquidation or winding-up of the Company; then, in each such case, the Company
shall give to the holder of this Warrant (a) notice of the date on which the
books of the Company shall close or a record shall be taken for determining the
holders of Common Stock entitled to receive any such dividend, distribution, or
subscription rights or for determining the holders of Common Stock entitled to
vote in respect of any such reorganization, reclassification, consolidation,
merger, sale, dissolution, liquidation or winding-up and (b) in the case of any
such reorganization, reclassification, consolidation, merger, sale, dissolution,
liquidation or winding-up, notice of the date (or, if not then known, a
reasonable approximation thereof by the Company) when the same shall take place.
Such notice shall also specify the date on which the holders of Common Stock
shall be entitled to receive such dividend, distribution, or subscription rights
or to exchange their Common Stock for stock or other securities or property
deliverable upon such reorganization, reclassification, consolidation, merger,
sale, dissolution, liquidation, or winding-up, as the case may be. Such notice
shall be given at least 30 days prior to the record date or the date on which
the Company's books are closed in respect thereto. Failure to give any such
notice or any defect therein shall not affect the validity of the proceedings
referred to in clauses (i), (ii), (iii) and (iv) above. 


                                       39


<PAGE>   33

        (k) Certain Events. If any event occurs of the type contemplated by the
adjustment provisions of this Paragraph 4 but not expressly provided for by such
provisions, the Company will give notice of such event as provided in Paragraph
4(g) hereof, and the Company's Board of Directors will make an appropriate
adjustment in the Exercise Price and the number of shares of Common Stock
acquirable upon exercise of this Warrant so that the rights of the Holder shall
be neither enhanced nor diminished by such event. 

        (l) Certain Definitions.

            (i) "Common Stock Deemed Outstanding" shall mean the number of
shares of Common Stock actually outstanding (not including shares of Common
Stock held in the treasury of the Company), plus (x) pursuant to Paragraph
4(b)(i) hereof, the maximum total number of shares of Common Stock issuable upon
the exercise of Options, as of the date of such issuance or grant of such
Options, if any, and (y) pursuant to Paragraph 4(b)(ii) hereof, the maximum
total number of shares of Common Stock issuable upon conversion or exchange of
Convertible Securities, as of the date of issuance of such Convertible
Securities, if any.

            (ii) "Market Price," as of any date, (i) means the average of the
last reported sale prices for the shares of Common Stock as reported by the
National Association of Securities Dealers Automated Quotation National Market
System ("NASDAQ-NMS") for the five (5) trading days immediately preceding such
date, or (ii) if the NASDAQ-NMS is not the principal trading market for the
shares of Common Stock, the average of the last reported sale prices on the
principal trading market for the Common Stock during the same period, or (iii)
if market value cannot be calculated as of such date on any of the foregoing
bases, the Market Price shall be the average fair market value as reasonably
determined in good faith by the Board of Directors of the Company. The manner of
determining the Market Price of the Common Stock set forth in the foregoing
definition shall apply with respect to any other security in respect of which a
determination as to market value must be made hereunder.

            (iii) "Common Stock," for purposes of this Paragraph 4, includes the
Common Stock, par value $.01 per share, and any additional class of stock of the
Company having no preference as to dividends or distributions on liquidation,
provided that the shares purchasable pursuant to this Warrant shall include only
shares of Common Stock, par value $.01 per share, in respect of which this
Warrant is exercisable, or shares resulting from any subdivision or combination
of such Common Stock, or in the case of any reorganization, reclassification,
consolidation, merger, or sale of the character referred to in Paragraph 4(e)
hereof, the stock or other securities or property provided for in such
Paragraph.

     5. Issue Tax. The issuance of certificates for Warrant Shares upon the
exercise of this Warrant shall be made without charge to the holder of this
Warrant or such shares for any issuance tax or other costs in respect thereof,
provided that the Company shall not be required to pay any tax which may be
payable in respect of any transfer involved in the issuance and delivery of any
certificate in a name other than the holder of this Warrant.

     6. No Rights or Liabilities as a Shareholder. This Warrant shall not
entitle the holder hereof to any voting rights or other rights as a shareholder
of the Company. No provision of this Warrant, in the absence of affirmative
action by the holder hereof to purchase Warrant Shares, and no mere enumeration
herein of the rights or privileges of the holder hereof, shall give rise to 


                                       40


<PAGE>   34

any liability of such holder for the Exercise Price or as a shareholder of the
Company, whether such liability is asserted by the Company or by creditors of
the Company.

     7. Transfer, Exchange, and Replacement of Warrant.

        (a) Restriction on Transfer. This Warrant and the rights granted to the
holder hereof are transferable, in whole or in part, upon surrender of this
Warrant, together with a properly executed assignment in the form attached
hereto, at the office or agency of the Company referred to in Paragraph 7(e)
below, provided, however, that any transfer or assignment shall be subject to
the conditions set forth in Paragraph 7(f) hereof and to the applicable
provisions of the Securities Purchase Agreement. Until due presentment for
registration of transfer on the books of the Company, the Company may treat the
registered holder hereof as the owner and holder hereof for all purposes, and
the Company shall not be affected by any notice to the contrary. Notwithstanding
anything to the contrary contained herein, the registration rights described in
Paragraph 8 are assignable only in accordance with the provisions of that
certain Registration Rights Agreement, dated as of March 15, 1996, by and among
the Company and the other signatories thereto (the "Registration Rights
Agreement").

        (b) Warrant Exchangeable for Different Denominations. This Warrant is
exchangeable, upon the surrender hereof by the holder hereof at the office or
agency of the Company referred to in Paragraph 7(e) below, for new Warrants of
like tenor representing in the aggregate the right to purchase the number of
shares of Common Stock which may be purchased hereunder, each of such new
Warrants to represent the right to purchase such number of shares as shall be
designated by the holder hereof at the time of such surrender.

        (c) Replacement of Warrant. Upon receipt of evidence reasonably
satisfactory to the Company of the loss, theft, destruction, or mutilation of
this Warrant and, in the case of any such loss, theft, or destruction, upon
delivery of an indemnity agreement reasonably satisfactory in form and amount to
the Company, or, in the case of any such mutilation, upon surrender and
cancellation of this Warrant, the Company, at its expense, will execute and
deliver, in lieu thereof, a new Warrant of like tenor.

        (d) Cancellation; Payment of Expenses. Upon the surrender of this
Warrant in connection with any transfer, exchange, or replacement as provided in
this Paragraph 7, this Warrant shall be promptly canceled by the Company. The
Company shall pay all taxes (other than securities transfer taxes) and all other
expenses (other than legal expenses, if any, incurred by the Holder or
transferees) and charges payable in connection with the preparation, execution,
and delivery of Warrants pursuant to this Paragraph 7.

        (e) Register. The Company shall maintain, at its principal executive
offices (or such other office or agency of the Company as it may designate by
notice to the holder hereof), a register for this Warrant, in which the Company
shall record the name and address of the person in whose name this Warrant has
been issued, as well as the name and address of each transferee and each prior
owner of this Warrant.


                                       41


<PAGE>   35

(f) Exercise or Transfer Without Registration. If, at the time of the surrender
of this Warrant in connection with any exercise, transfer, or exchange of this
Warrant, this Warrant (or, in the case of any exercise, the Warrant Shares
issuable hereunder), shall not be registered under the Securities Act and under
applicable state securities or blue sky laws, the Company may require, as a
condition of allowing such exercise, transfer, or exchange, (i) that the holder
or transferee of this Warrant, as the case may be, furnish to the Company a
written opinion of counsel, which opinion and counsel are acceptable to the
Company, to the effect that such exercise, transfer, or exchange may be made
without registration under said Act and under applicable state securities or
blue sky laws (ii) that the holder or transferee execute and deliver to the
Company an investment letter in form and substance acceptable to the Company,
(iii) that the transferee be an "accredited investor" as defined in Rule 501(a)
promulgated under the Securities Act and (iv) that, upon such transfer, the
transferee beneficially own Registrable Securities (as defined in the
Registration Rights Agreement) having an aggregate Market Price of at least
$500,000; provided that no such opinion, letter, status as an "accredited
investor" or minimum Market Price shall be required in connection with a
transfer pursuant to Rule 144 under the Securities Act. No "Subject Holder" (as
defined below) may sell or otherwise transfer Warrants, except (i) to the
Company or to a stockholder or a group of stockholders who immediately prior to
the sale control a majority of the Company's voting shares (a "Controlling
Stockholder" or "Controlling Group", as applicable); (ii) to an affiliate of 
such holder; (iii) in connection with any merger, consolidation,
reorganization or sale of more than 50% of the outstanding Common Stock of the 
Company (a "Reorganization"); (iv) in a registered public offering or a public
sale pursuant to Rule 144 or other applicable exemption from the registration
requirements of the Securities Act (or any successor rule or regulation); or
(v) in a private sale (otherwise than to the Company, to a Controlling
Stockholder or a Controlling Group, to an affiliate of such holder, or in a
Reorganization), provided that the holder shall not sell or otherwise transfer
during any ninety (90) day period a portion(s) of the Warrants which, if
converted into Common Stock at the time of the transfer, would represent, in
the aggregate, beneficial ownership by the transferee(s) of more than 9.9%
percent of the Common Stock then outstanding.  Subject Holder means any holder
who, but for the second paragraph of Section 1 hereof, would beneficially own   
10% or more of the outstanding Common Stock of the Company. The first holder of
this Warrant, by taking and holding the same, represents to the Company that
such holder is acquiring this Warrant for investment and not with a view to the
distribution thereof.

     8. Registration Rights. The initial holder of this Warrant (and certain
assignees thereof) is entitled to the benefit of such registration rights in
respect of the Warrant Shares as are set forth in Section 2 of the Registration
Rights Agreement.

     9. Notices. All notices, requests, and other communications required or
permitted to be given or delivered hereunder to the holder of this Warrant shall
be in writing, and shall be personally delivered, or shall be sent by certified
or registered mail or by recognized overnight mail courier, postage prepaid and
addressed, to such holder at the address shown for such holder on the books of
the Company, or at such other address as shall have been furnished to the
Company by notice from such holder. All notices, requests, and other
communications required or permitted to be given or delivered hereunder to the
Company shall be in writing, and shall be personally delivered, or shall be sent
by certified or registered mail or by recognized overnight mail courier, postage
prepaid and addressed, to the office of the Company at 128 Sidney Street,
Cambridge, Massachusetts 02139, Attention: Frank J. Pocher, Chief Financial
Officer, or at such other address as shall have been furnished to the holder of
this Warrant by notice from the Company. Any such notice, request, or other
communication may be sent by facsimile, but shall in such case be subsequently
confirmed by a writing personally delivered or sent by certified or registered
mail or by recognized overnight mail courier as provided above. All notices,
requests, and other communications shall be deemed to have been given either at
the time of the receipt thereof by the person entitled to receive such notice at
the address of such person for purposes of this Paragraph 9, or, if mailed by
registered or certified mail or with a recognized overnight mail courier upon
deposit with the United States Post Office or such overnight mail courier, if
postage is prepaid and the mailing is properly addressed, as the case may be.

     10. Governing Law. THIS WARRANT SHALL BE GOVERNED BY AND CONSTRUED AND
ENFORCED IN ACCORDANCE WITH THE INTERNAL LAWS OF 


                                       42


<PAGE>   36

THE COMMONWEALTH OF MASSACHUSETTS WITHOUT REGARD TO THE BODY OF LAW CONTROLLING
CONFLICTS OF LAW.

     11. Miscellaneous.

        (a) Amendments. This Warrant and any provision hereof may only be
amended by an instrument in writing signed by the Company and the holder hereof.

        (b) Descriptive Headings. The descriptive headings of the several
paragraphs of this Warrant are inserted for purposes of reference only, and
shall not affect the meaning or construction of any of the provisions hereof.

        (c) Cashless Exercise. Notwithstanding anything to the contrary
contained in this Warrant, if the resale of the Warrant Shares by the holder is
not then registered pursuant to an effective registration statement under the
Securities Act, this Warrant may be exercised by presentation and surrender of
this Warrant to the Company at its principal executive offices with a written
notice of the holder's intention to effect a cashless exercise, including a
calculation of the number of shares of Common Stock to be issued upon such
exercise in accordance with the terms hereof (a "Cashless Exercise"). In the
event of a Cashless Exercise, in lieu of paying the Exercise Price in cash, the
holder shall surrender this Warrant for that number of shares of Common Stock
determined by multiplying the number of Warrant Shares to which it would
otherwise be entitled by a fraction, the numerator of which shall be the
difference between the then current Market Price per share of the Common Stock
and the Exercise Price, and the denominator of which shall be the then current
Market Price per share of Common Stock.

     IN WITNESS WHEREOF, the Company has caused this Warrant to be signed by its
duly authorized officer.

IMMUNOGEN, INC.




By: ________________________
Name:___________________
Title:____________________


Agreed to and Accepted



By:_______________________          , Initial Holder

Dated as of ___________, 199__

                                       43


<PAGE>   37

                           FORM OF EXERCISE AGREEMENT

Dated:  ________, ____.

To:_____________________________

     The undersigned, pursuant to the provisions set forth in the within
Warrant, hereby agrees to purchase ________ shares of Common Stock covered by
such Warrant, and makes payment herewith in full therefor at the price per share
provided by such Warrant in cash or by certified or official bank check in the
amount of, [or, if the resale of such Common Stock by the undersigned is not
currently registered pursuant to an effective registration statement under the
Securities Act of 1933, as amended, by surrender of securities issued by the
Company (including a portion of the Warrant) having a market value (in the case
of a portion of this Warrant, determined in accordance with Section 11(c) of the
Warrant) equal to] $_________. Please issue a certificate or certificates for
such shares of Common Stock in the name of and pay any cash for any fractional
share to:


Name:________________________________

Signature:___________________________

Address:_____________________________

        _____________________________

Note: The above signature should correspond exactly with the name on the face of
the within Warrant, and, if said number of shares of Common Stock shall not be
all the shares purchasable under the within Warrant, a new Warrant is to be
issued in the name of said undersigned covering the balance of the shares
purchasable thereunder less any fraction of a share paid in cash.



                                       44


<PAGE>   38

                               FORM OF ASSIGNMENT

     FOR VALUE RECEIVED, the undersigned hereby sells, assigns, and transfers
all the rights of the undersigned under the within Warrant, with respect to the
number of shares of Common Stock covered thereby set forth hereinbelow, to:

Name of Assignee                   Address                      No of Shares


, and hereby irrevocably constitutes and appoints ____________________________
as agent and attorney-in-fact to transfer said Warrant on the books of the
within-named corporation, with full power of substitution in the premises.


Dated: _____________________, ____,


In the presence of

__________________


Name: ____________________________


Signature: _______________________

Title of Signing Officer or Agent (if any):

__________________________________________


Address:  ________________________ 


          ________________________ 

Note: The above signature should correspond exactly with the name on the face of
the within Warrant.



                                       45



<PAGE>   1

                                                               EXHIBIT A-2  to
                                                                    Securities
                                                                      Purchase
                                                                     Agreement

                          REGISTRATION RIGHTS AGREEMENT

     REGISTRATION RIGHTS AGREEMENT (this "Agreement"), dated as of March 15,
1996 by and among IMMUNOGEN, INC., a Massachusetts corporation, with
headquarters located at 128 Sidney Street, Cambridge, Massachusetts 02139 (the
"Company"), and the undersigned (together with its affiliates and any assignee
or transferee of all of its rights hereunder, the "Initial Investor").

     WHEREAS:

     A. In connection with the Securities Purchase Agreement by and among the
parties of even date herewith (the "Securities Purchase Agreement"), the Company
has agreed, upon the terms and subject to the conditions contained therein, to
issue and sell to the Initial Investor a Convertible Debenture (the
"Debentures") that is convertible into (i) shares (the "Conversion Shares") of
the Company's common stock (the "Common Stock") and (ii) warrants (the
"Warrants") to acquire a number of shares of Common Stock (the "Warrant Shares")
equal to 50% of the Conversion Shares issuable upon conversion of the Debenture,
upon the terms and subject to the conditions of such Debenture; and

     B. To induce the Initial Investor to execute and deliver the Securities
Purchase Agreement, the Company has agreed
 to provide certain registration
rights under the Securities Act of 1933, as amended, and the rules and
regulations thereunder, or any similar successor statute (collectively, the
"1933 Act"), and applicable state securities laws;

     NOW, THEREFORE, in consideration of the premises and the mutual covenants
contained herein and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the Company and each of the
Initial Investors hereby agree as follows:

     1. DEFINITIONS.

        a. As used in this Agreement, the following terms shall have the
following meanings:

           (i) "Investor" means the Initial Investor and any transferee or
assignee who agrees to become bound by the provisions of this Agreement in
accordance with Section 9 hereof.

           (ii) "register," "registered," and "registration" refer to a
registration effected by preparing and filing a Registration Statement or
Statements in compliance with the 1933 Act and pursuant to Rule 415 under the
1933 Act or any successor rule providing for offering securities on a continuous
basis ("Rule 415"), and the declaration or ordering of effectiveness of such
Registration Statement by the United States Securities and Exchange Commission
(the "SEC").


                                       47


<PAGE>   2

           (iii) "Registrable Securities" means the Conversion Shares
(including, without limitation, the Damages Shares, as defined in Section 4.7 of
the Debenture) and the Warrant Shares.

           (iv) "Registration Statement" means a registration statement of the
Company under the 1933 Act.

        b. Capitalized terms used herein and not otherwise defined herein shall
have the respective meanings set forth in the Securities Purchase Agreement.

     2. REGISTRATION.

        a. Mandatory Registration. The Company shall prepare, and, on or prior
to the date which is fifteen (15) days after the date of the closing under the
Securities Purchase Agreement (the "Closing Date"), file with the SEC a
Registration Statement on Form S-3 (or, if Form S-3 is not then available, on
such form of Registration Statement as is then available to effect a
registration of the Registrable Securities, subject to the consent of the
Initial Investor, which consent will not be unreasonably withheld) covering the
resale of the Registrable Securities, which Registration Statement, to the
extent allowable under the 1933 Act and the Rules promulgated thereunder
(including Rule 416), shall state that such Registration Statement also covers
such indeterminate number of additional shares of Common Stock as may become
issuable upon conversion of the Debenture (i) to prevent dilution resulting from
stock splits, stock dividends or similar transactions or (ii) by reason of
changes in the Conversion Price of the Debenture in accordance with the terms
thereof or (iii) by reason of the issuance of Damages Shares. The Registration
Statement (and each amendment or supplement thereto, and each request for
acceleration of effectiveness thereof) shall be provided to (and subject to the
reasonable approval of) the Initial Investor and its counsel prior to its filing
or other submission.

        b. Underwritten Offering. If any offering pursuant to a Registration
Statement pursuant to Section 2(a) hereof involves an underwritten offering, the
Investors who hold a majority in interest of the Registrable Securities subject
to such underwritten offering, with the consent of the Initial Investor, shall
have the right to select one legal counsel and an investment banker or bankers
and manager or managers to administer the offering, which investment banker or
bankers or manager or managers shall be reasonably satisfactory to the Company.

        c. Payments by the Company. If (i) the Registration Statement covering
the Registrable Securities required to be filed by the Company pursuant to
Section 2(a) hereof is not declared effective by the SEC within sixty (60) days
after the Closing Date (other than by reason of any act or failure to act by the
Investors or, in the case of the Warrant Shares, any rule, regulation or
position of the SEC that limits the Company's ability to register such shares
until the Warrants are issued) or if, after the Registration Statement has been
declared effective by the SEC, sales cannot be made pursuant to the Registration
Statement (by reason of stop order, or the Company's failure to update the
Registration Statement), or (ii) the Common Stock is not listed or included for
quotation on the NASDAQ National Market (the "NASDAQ-NMS"), the New York Stock
Exchange (the "NYSE") the American Stock Exchange (the "AMEX") or the NASDAQ
Small Cap ("NASDAQ Small Cap"), then the Company will make payments to the


                                       48

<PAGE>   3

Investors in such amounts and at such times as shall be determined pursuant to
this Section 2(c) as partial relief for the damages to the Investors by reason
of any such delay in or reduction of their ability to sell the Registrable
Securities (which remedy shall not be exclusive of any other remedies available
at law or in equity). The Company shall pay to each holder of Registerable
Securities an amount equal to the then outstanding principal amount of the
Debenture ("Outstanding Principal Amount") multiplied by one and one-half
hundredths (.015) (with such fraction increasing to two-hundredths (.02) for all
periods following the 90-day period after the Closing Date, if the Registration
Statement is not then effective and thereafter remaining at such level in
respect of any subsequent payments as described in clauses (ii) and (iii) below
of this sentence) times the sum of: (i) the number of months (prorated for
partial months) after the end of such 60-day period and prior to the date the
Registration Statement is declared effective by the SEC, provided, however, that
there shall be excluded from such period any delays which are attributable to
changes required by the Investors in the Registration Statement, including,
without limitation, changes to the plan of distribution, or to the failure of
the Investors to conduct their review of the registration statement pursuant to
Section 2(a) above in a reasonably prompt manner; (ii) the number of months
(prorated for partial months) that sales cannot be made pursuant to the
Registration Statement after the Registration Statement has been declared
effective; and (iii) the number of months (prorated for partial months) that the
Common Stock is not listed or included for quotation on the NASDAQ-NMS, NYSE,
AMEX or NASDAQ Small Cap after the Registration Statement has been declared
effective. (For example, if the Registration Statement becomes effective one (1)
month after the end of such 60 day period, the Company would pay $15,000 for
each $1,000,000 of Outstanding Principal Amount until any subsequent adjustment;
if thereafter, sales could not be made pursuant to the Registration Statement
for an additional period of one (1) month, the Company would pay an additional
$20,000 for each $1,000,000 of Outstanding Principal Amount.) Such amounts shall
be paid in cash or, at each Investor's option, may be convertible into Common
Stock at the "Market Price" (as defined in the Warrants). Any shares of Common
Stock issued upon conversion of such amounts shall be Registrable Securities. If
the Investor desires to convert the amounts due hereunder into Registrable
Securities it shall so notify the Company in writing within two (2) business
days of the date on which such amounts are first payable in cash and such
amounts shall be so convertible (pursuant to the mechanics set forth under
Article II of the Debenture), beginning on the last day upon which the cash
amount would otherwise be due in accordance with the following sentence.
Payments of cash pursuant hereto shall be made within five (5) days after the
end of each period that gives rise to such obligation, provided that, if any
such period extends for more than thirty (30) days, interim payments shall be
made for each such thirty (30) day period.

        d. Piggy-Back Registrations. If at any time prior to the expiration of
the Registration Period (as hereinafter defined) the Company shall file with the
SEC a Registration Statement relating to an offering for its own account or the
account of others under the 1933 Act of any of its equity securities (other than
on Form S-4 or Form S-8 or their then equivalents relating to equity securities
to be issued solely in connection with any acquisition of any entity or business
or equity securities issuable in connection with stock option or other employee
benefit plans) the Company shall send to each Investor who is entitled to
registration rights under this Section 2(d) written notice of such determination
and, if within fifteen (15) days after the effective date of 


                                       49


<PAGE>   4

such notice, such Investor shall so request in writing, the Company shall
include in such Registration Statement all or any part of the Registrable
Securities such Investor requests to be registered, except that if, in
connection with any underwritten public offering for the account of the Company
the managing underwriter(s) thereof shall impose a limitation on the number of
shares of Common Stock which may be included in the Registration Statement
because, in such underwriter(s)' judgment, marketing or other factors dictate
such limitation is necessary to facilitate public distribution, then the Company
shall be obligated to include in such Registration Statement only such limited
portion of the Registrable Securities with respect to which such Investor has
requested inclusion hereunder. Any exclusion of Registrable Securities shall be
made pro rata among the Investors seeking to include Registrable Securities, in
proportion to the number of Registrable Securities sought to be included by such
Investors; provided, however, that the Company shall not exclude any Registrable
Securities unless the Company has first excluded all outstanding securities, the
holders of which are not entitled to inclusion of such securities in such
Registration Statement or are not entitled to pro rata inclusion with the
Registrable Securities; and provided, further, however, that, after giving
effect to the immediately preceding proviso, any exclusion of Registrable
Securities shall be made pro rata with holders of other securities having the
right to include such securities in the Registration Statement other than
holders of securities entitled to inclusion of their securities in such
Registration Statement by reason of demand registration rights. No right to
registration of Registrable Securities under this Section 2(d) shall be
construed to limit any registration required under Section 2(a) hereof. If an
offering in connection with which an Investor is entitled to registration under
this Section 2(d) is an underwritten offering, then each Investor whose
Registrable Securities are included in such Registration Statement shall, unless
otherwise agreed by the Company, offer and sell such Registrable Securities in
an underwritten offering using the same underwriter or underwriters and, subject
to the provisions of this Agreement, on the same terms and conditions as other
shares of Common Stock included in such underwritten offering.

        e. Eligibility for Form S-3. The Company represents and warrants that it
meets the requirements for the use of Form S-3 for registration of the sale by
the Buyer and any other Investor of the Registrable Securities and the Company
shall file all reports required to be filed by the Company with the SEC in a
timely manner so as to maintain such eligibility for the use of Form S-3.

     3. OBLIGATIONS OF THE COMPANY.

     In connection with the registration of the Registrable Securities, the
Company shall have the following obligations:

        a. The Company shall prepare promptly, and file with the SEC not later
than fifteen (15) days after the Closing Date, a Registration Statement with
respect to the number of Registrable Securities provided in Section 2(a), and
thereafter to use its best efforts to cause each Registration Statement relating
to Registrable Securities to become effective as soon as possible after such
filing, and keep the Registration Statement effective pursuant to Rule 415 at
all times until such date as is the earlier of (i) the date on which all of the
Registrable Securities have been sold and (ii) the date on which the Registrable
Securities (in the opinion of counsel to the Initial Investor, in form,
substance and scope reasonably acceptable to the Company) may be 


                                       50


<PAGE>   5

immediately sold without registration (the "Registration Period"), which
Registration Statement (including any amendments or supplements thereto and
prospectuses contained therein) shall not contain any untrue statement of a
material fact or omit to state a material fact required to be stated therein, or
necessary to make the statements therein, in light of the circumstances in which
they were made, not misleading.

        b. The Company shall prepare and file with the SEC such amendments
(including post-effective amendments) and supplements to the Registration
Statement and the prospectus used in connection with the Registration Statement
as may be necessary to keep the Registration Statement effective at all times
during the Registration Period, and, during such period, comply with the
provisions of the 1933 Act with respect to the disposition of all Registrable
Securities of the Company covered by the Registration Statement until such time
as all of such Registrable Securities have been disposed of in accordance with
the intended methods of disposition by the seller or sellers thereof as set
forth in the Registration Statement.

        c. The Company shall furnish to each Investor whose Registrable
Securities are included in the Registration Statement and its legal counsel (i)
promptly after the same is prepared and publicly distributed, filed with the
SEC, or received by the Company, one copy of the Registration Statement and any
amendment thereto each preliminary prospectus and prospectus and each amendment
or supplement thereto, and, in the case of the Registration Statement referred
to in Section 2(a), each letter written by or on behalf of the Company to the
SEC or the staff of the SEC, and each item of correspondence from the SEC or the
staff of the SEC, in each case relating to such Registration Statement (other
than any portion of any thereof which contains information for which the Company
has sought confidential treatment), and (ii) such number of copies of a
prospectus, including a preliminary prospectus, and all amendments and
supplements thereto and such other documents as such Investor may reasonably
request in order to facilitate the disposition of the Registrable Securities
owned by such Investor.

        d. The Company shall use reasonable efforts to (i) register and qualify
the Registrable Securities covered by the Registration Statement under such
other securities or "blue sky" laws of such jurisdictions in the United States
as the Investors who hold a majority in interest of the Registrable Securities
being offered reasonably request, (ii) prepare and file in those jurisdictions
such amendments (including post-effective amendments) and supplements to such
registrations and qualifications as may be necessary to maintain the
effectiveness thereof during the Registration Period, (iii) take such other
actions as may be necessary to maintain such registrations and qualifications in
effect at all times during the Registration Period, and (iv) take all other
actions reasonably necessary or advisable to qualify the Registrable Securities
for sale in such jurisdictions; provided, however, that the Company shall not be
required in connection therewith or as a condition thereto to (a) qualify to do
business in any jurisdiction where it would not otherwise be required to qualify
but for this Section 3(d), (b) subject itself to general taxation in any such
jurisdiction, (c) file a general consent to service of process in any such
jurisdiction, (d) provide any undertakings that cause more than nominal expense
or burden to the Company, or (e) make any change in its charter or bylaws, which
in each case the Board of Directors of the Company determines to be contrary to
the best interests of the Company and its stockholders. 


                                       51


<PAGE>   6

        e. In the event Investors who hold a majority in interest of the
Registrable Securities being offered in the offering (with the approval of the
Initial Investor) select underwriters for the offering, the Company shall enter
into and perform its obligations under an underwriting agreement, in usual and
customary form, including, without limitation, customary indemnification and
contribution obligations, with the underwriters of such offering.

        f. As promptly as practicable after becoming aware of such event, the
Company shall notify each Investor of the happening of any event, of which the
Company has knowledge, as a result of which the prospectus included in the
Registration Statement, as then in effect, includes an untrue statement of a
material fact or omission to state a material fact required to be stated therein
or necessary to make the statements therein, in light of the circumstances under
which they were made, not misleading, and use its best efforts promptly to
prepare a supplement or amendment to the Registration Statement to correct such
untrue statement or omission, and deliver such number of copies of such
supplement or amendment to each Investor as such Investor may reasonably
request; provided, that, not more than once in any twelve month period, for up
to a period of thirty (30) days, the Company may delay the disclosure of
material non-public information concerning the Company the disclosure of which
at the time is not, in the good faith opinion of the Board of Directors of the
Company, in the best interest of the Company and, in the opinion of counsel to
the Company, otherwise required (an "Allowed Delay"); provided, further, that
the Company shall promptly (i) notify the Investors in writing of the existence
of material non-public information giving rise to an Allowed Delay and (ii)
advise the Investors in writing to cease all sales under the Registration
Statement until the end of the Allowed Delay. Notwithstanding the first proviso
of the immediately preceding sentence, the provisions of Section 2(c) shall be
applicable during the period of an Allowed Delay. Upon expiration of the Allowed
Delay, the Company shall again be bound by the first sentence of this Section
3(f) with respect to the information giving rise thereto.

        g. The Company shall use its best efforts to prevent the issuance of any
stop order or other suspension of effectiveness of a Registration Statement,
and, if such an order is issued, to obtain the withdrawal of such order at the
earliest possible moment and to notify each Investor who holds Registrable
Securities being sold (or, in the event of an underwritten offering, the
managing underwriters) of the issuance of such order and the resolution thereof.

        h. The Company shall permit a single firm of counsel designated by the
Initial Investor to review the Registration Statement and all amendments and
supplements thereto a reasonable period of time prior to their filing with the
SEC, and not file any document in a form to which such counsel reasonably
objects.

        i. The Company shall make generally available to its security holders as
soon as practical, but not later than ninety (90) days after the close of the
period covered thereby, an earnings statement (in form complying with the
provisions of Rule 158 under the 1933 Act) covering a twelve-month period
beginning not later than the first day of the Company's fiscal quarter next
following the effective date of the Registration Statement.


                                       52


<PAGE>   7

        j. At the request of the Initial Investor or any Investor or group of
Investors holding Registrable Securities having a Market Price of at least
$500,000, the Company shall furnish, on the date that Registrable Securities are
delivered to an underwriter, if any, for sale in connection with the
Registration Statement or, if such securities are not being sold by an
underwriter, on the date of effectiveness thereof (i) an opinion, dated as of
such date, from counsel representing the Company for purposes of such
Registration Statement, in form, scope and substance as is customarily given in
an underwritten public offering, addressed to the underwriters, if any, and the
Investors and (ii) solely in the case of an underwritten offering, a letter,
dated such date, from the Company's independent certified public accountants in
form and substance as is customarily given by independent certified public
accountants to underwriters in an underwritten public offering, addressed to the
underwriters.

        k. The Company shall make available for inspection by (i) any Investor,
(ii) any underwriter participating in any disposition pursuant to the
Registration Statement, (iii) one firm of attorneys and one firm of accountants
or other agents retained by the Initial Investor, (iv) one firm of attorneys and
one firm of accountants or other agents retained by all other Investors, and (v)
one firm of attorneys retained by all such underwriters (collectively, the
"Inspectors") all pertinent financial and other records, and pertinent corporate
documents and properties of the Company (collectively, the "Records"), as shall
be reasonably deemed necessary by each Inspector to enable each Inspector to
exercise its due diligence responsibility, and cause the Company's officers,
directors and employees to supply all information which any Inspector may
reasonably request for purposes of such due diligence; provided, however, that
each Inspector shall hold in confidence and shall not make any disclosure
(except to an Investor) of any Record or other information which the Company
determines in good faith to be confidential, and of which determination the
Inspectors are so notified, unless (a) the disclosure of such Records is
necessary to avoid or correct a misstatement or omission in any Registration
Statement, (b) the release of such Records is ordered pursuant to a subpoena or
other order from a court or government body of competent jurisdiction, or (c)
the information in such Records has been made generally available to the public
other than by disclosure in violation of this or any other agreement. The
Company shall not be required to disclose any confidential information in such
Records to any Inspector until and unless such Inspector shall have entered into
confidentiality agreements (in form and substance satisfactory to the Company)
with the Company with respect thereto, substantially in the form of this Section
3(k). Each Investor agrees that it shall, upon learning that disclosure of such
Records is sought in or by a court or governmental body of competent
jurisdiction or through other means, give prompt notice to the Company and allow
the Company, at its expense, to undertake appropriate action to prevent
disclosure of, or to obtain a protective order for, the Records deemed
confidential. Nothing herein shall be deemed to limit the Investor's ability to
sell Registrable Securities in a manner which is otherwise consistent with
applicable laws and regulations.

        l. The Company shall hold in confidence and not make any disclosure of
information concerning an Investor provided to the Company unless (i) disclosure
of such information is necessary to comply with federal or state securities
laws, (ii) the disclosure of such information is necessary to avoid or correct a
misstatement or omission in any Registration Statement, (iii) the release of
such information is ordered pursuant to a subpoena or other order from a court
or 


                                       53


<PAGE>   8

governmental body of competent jurisdiction, or (iv) such information has been
made generally available to the public other than by disclosure in violation of
this or any other agreement. The Company agrees that it shall, upon learning
that disclosure of such information concerning an Investor is sought in or by a
court or governmental body of competent jurisdiction or through other means,
give prompt notice to such Investor prior to making such disclosure, and allow
the Investor, at its expense, to undertake appropriate action to prevent
disclosure of, or to obtain a protective order for, such information.

        m. The Company shall use its best efforts either to (i) cause all the
Registrable Securities covered by the Registration Statement to be listed on a
national securities exchange and on each additional national securities exchange
on which securities of the same class or series issued by the Company are then
listed, if any, if the listing of such Registrable Securities is then permitted
under the rules of such exchange, or (ii) secure designation and quotation of
all the Registrable Securities covered by the Registration Statement on the
NASDAQ-NMS or, if, despite the Company's best efforts to satisfy the preceding
clause (i) or (ii), the Company is unsuccessful in satisfying the preceding
clause (i) or (ii), to secure the inclusion for quotation on the NASDAQ Small
Cap for such Registrable Securities and, without limiting the generality of the
foregoing, to arrange for at least two market makers to register with the
National Association of Securities Dealers, Inc. ("NASD") as such with respect
to such Registrable Securities.

        n. The Company shall provide a transfer agent and registrar, which may
be a single entity, for the Registrable Securities not later than the effective
date of the Registration Statement.

        o. The Company shall cooperate with the Investors who hold Registrable
Securities being offered and the managing underwriter or underwriters, if any,
to facilitate the timely preparation and delivery of certificates (not bearing
any restrictive legends) representing Registrable Securities to be offered
pursuant to the Registration Statement and enable such certificates to be in
such denominations or amounts, as the case may be, as the managing underwriter
or underwriters, if any, or the Investors may reasonably request and registered
in such names as the managing underwriter or underwriters, if any, or the
Investors may request, and, within three (3) business days after a Registration
Statement which includes Registrable Securities is ordered effective by the SEC,
the Company shall deliver, and shall cause legal counsel selected by the Company
to deliver, to the transfer agent for the Registrable Securities (with copies to
the Investors whose Registrable Securities are included in such Registration
Statement) an instruction in the form attached hereto as Exhibit 1 and an
opinion of such counsel in the form attached hereto as Exhibit 2.

        p. The Company shall take all other reasonable actions necessary to
expedite and facilitate disposition by the Investors of Registrable Securities
pursuant to the Registration Statement.

     4. OBLIGATIONS OF THE INVESTORS.

     In connection with the registration of the Registrable Securities, the
Investors shall have the following obligations:


                                       54


<PAGE>   9

        a. It shall be a condition precedent to the obligations of the Company
to complete the registration pursuant to this Agreement with respect to the
Registrable Securities of a particular Investor that such Investor shall furnish
to the Company such information regarding itself, the Registrable Securities
held by it and the intended method of disposition of the Registrable Securities
held by it as shall be reasonably required to effect the registration of such
Registrable Securities and shall execute such documents in connection with such
registration as the Company may reasonably request. At least three (3) business
days prior to the first anticipated filing date of the Registration Statement,
the Company shall notify each Investor of the information ("Requested
Information") the Company requires from each such Investor if such Investor
elects to have any of such Investor's Registrable Securities included in the
Registration Statement. If at least one (1) business day prior to the filing
date the Company has not received the Requested Information from an Investor,
then the Company shall not be responsible to such Investor for the failure to
include such Requested Information or for any inaccuracies in respect thereof.

        b. Each Investor by such Investor's acceptance of the Registrable
Securities agrees to cooperate with the Company as reasonably requested by the
Company in connection with the preparation and filing of the Registration
Statement hereunder, unless such Investor has notified the Company in writing of
such Investor's election to exclude all of such Investor's Registrable
Securities from the Registration Statement.

        c. In the event Investors holding a majority in interest of the
Registrable Securities being registered (with the approval of the Initial
Investor) determine to engage the services of an underwriter, each Investor
agrees to enter into and perform such Investor's obligations under an
underwriting agreement, in usual and customary form, including, without
limitation, customary indemnification and contribution obligations, with the
managing underwriter of such offering and take such other actions as are
reasonably required in order to expedite or facilitate the disposition of the
Registrable Securities, unless such Investor has notified the Company in writing
of such Investor's election to exclude all of such Investor's Registrable
Securities from the Registration Statement.

        d. Each Investor agrees that, upon receipt of any notice from the
Company of the happening of any event of the kind described in Section 3(f) or
3(g), such Investor will immediately discontinue disposition of Registrable
Securities pursuant to the Registration Statement covering such Registrable
Securities until such Investor's receipt of the copies of the supplemented or
amended prospectus contemplated by Section 3(f) or 3(g) and, if so directed by
the Company, such Investor shall deliver to the Company (at the expense of the
Company) or destroy (and deliver to the Company a certificate of destruction)
all copies in such Investor's possession, of the prospectus covering such
Registrable Securities current at the time of receipt of such notice.

        e. No Investor may participate in any underwritten registration
hereunder unless such Investor (i) agrees to sell such Investor's Registrable
Securities on the basis provided in any underwriting arrangements in usual and
customary form entered into by the Company, (ii) completes and executes all
questionnaires, powers of attorney, indemnities, underwriting agreements and
other documents reasonably required under the terms of such underwriting


                                       55


<PAGE>   10

arrangements, and (iii) agrees to pay its pro rata share of all underwriting
discounts and commissions and any expenses in excess of those payable by the
Company pursuant to Section 5 below.

     5. EXPENSES OF REGISTRATION.

     All reasonable expenses, other than underwriting discounts and commissions,
incurred in connection with registrations, filings or qualifications pursuant to
Sections 2 and 3, including, without limitation, all registration, listing and
qualifications fees, printers and accounting fees, the fees and disbursements of
counsel for the Company, and the reasonable fees and disbursements of one
counsel selected by the Initial Investor pursuant to Section 2(b) hereof (up to
a maximum of $10,000, inclusive of the Initial Investor's expenses payable by
the Company pursuant to Section 4(f) of the Securities Purchase Agreement),
shall be borne by the Company.

     6. INDEMNIFICATION.

     In the event any Registrable Securities are included in a Registration
Statement under this Agreement:

        a. To the extent permitted by law, the Company will indemnify, hold
harmless and defend (i) each Investor who holds such Registrable Securities,
(ii) the directors, officers, partners, employees, agents and each person who
controls any Investor within the meaning of the 1933 Act or the Securities
Exchange Act of 1934, as amended (the "1934 Act"), if any, and (iii) any
underwriter (as defined in the 1933 Act) for the Investors; and the directors,
officers, partners, employees and each person who controls any such underwriter
within the meaning of the 1933 Act or the 1934 Act, if any, (each, an
"Indemnified Person"), against any joint or several losses, claims, damages,
liabilities or expenses (collectively, together with actions, proceedings or
inquiries by any regulatory or self-regulatory organization, whether commenced
or threatened, in respect thereof, "Claims") to which any of them may become
subject insofar as such Claims arise out of or are based upon: (i) any untrue
statement or alleged untrue statement of a material fact in a Registration
Statement or the omission or alleged omission to state therein a material fact
required to be stated or necessary to make the statements therein not
misleading, (ii) any untrue statement or alleged untrue statement of a material
fact contained in any preliminary prospectus if used prior to the effective date
of such Registration Statement, or contained in the final prospectus (as amended
or supplemented, if the Company files any amendment thereof or supplement
thereto with the SEC) or the omission or alleged omission to state therein any
material fact necessary to make the statements made therein, in light of the
circumstances under which the statements therein were made, not misleading, or
(iii) any violation or alleged violation by the Company of the 1933 Act, the
1934 Act, any other law, including, without limitation, any state securities
law, or any rule or regulation thereunder relating to the offer or sale of the
Registrable Securities (the matters in the foregoing clauses (i) through (iii)
being, collectively, "Violations"). Subject to the restrictions set forth in
Section 6(c) with respect to the number of legal counsel, the Company shall
reimburse the Investors and each such underwriter or controlling person,
promptly as such expenses are incurred and are due and payable, for any
reasonable legal fees or other reasonable expenses incurred by them in
connection with 


                                       56

<PAGE>   11

investigating or defending any such Claim. Notwithstanding anything to the
contrary contained herein, the indemnification agreement contained in this
Section 6(a): (i) shall not apply to a Claim arising out of or based upon a
Violation which occurs in reliance upon and in conformity with information
furnished in writing to the Company by any Indemnified Person or underwriter for
such Indemnified Person expressly for use in connection with the preparation of
the Registration Statement or any such amendment thereof or supplement thereto,
if such prospectus was timely made available by the Company pursuant to Section
3(c) hereof; (ii) shall not apply to amounts paid in settlement of any Claim if
such settlement is effected without the prior written consent of the Company,
which consent shall not be unreasonably withheld; and (iii) with respect to any
preliminary prospectus, shall not inure to the benefit of any Indemnified Person
if the untrue statement or omission of material fact contained in the
preliminary prospectus was corrected on a timely basis in the prospectus, as
then amended or supplemented, such corrected prospectus was timely made
available by the Company pursuant to Section 3(c) hereof, and the Indemnified
Person was promptly advised in writing not to use the incorrect prospectus prior
to the use giving rise to a Violation and such Indemnified Person,
notwithstanding such advise, used it. Such indemnity shall remain in full force
and effect regardless of any investigation made by or on behalf of the
Indemnified Person and shall survive the transfer of the Registrable Securities
by the Investors pursuant to Section 9.

        b. In connection with any Registration Statement in which an Investor is
participating, each such Investor agrees severally and not jointly to indemnify,
hold harmless and defend, to the same extent and in the same manner set forth in
Section 6(a), the Company, each of its directors, each of its officers who signs
the Registration Statement, each person, if any, who controls the Company within
the meaning of the 1933 Act or the 1934 Act, any underwriter and any other
stockholder selling securities pursuant to the Registration Statement or any of
its directors or officers or any person who controls such stockholder or
underwriter within the meaning of the 1933 Act or the 1934 Act (collectively and
together with an Indemnified Person, an "Indemnified Party"), against any Claim
to which any of them may become subject, under the 1933 Act, the 1934 Act or
otherwise, insofar as such Claim arises out of or is based upon any Violation,
in each case to the extent (and only to the extent) that such Violation occurs
in reliance upon and in conformity with written information furnished to the
Company by such Investor expressly for use in connection with such Registration
Statement; and subject to Section 6(c) such Investor will reimburse any legal or
other expenses (promptly as such expenses are incurred and are due and payable)
reasonably incurred by them in connection with investigating or defending any
such Claim; provided, however, that the indemnity agreement contained in this
Section 6(b) shall not apply to amounts paid in settlement of any Claim if such
settlement is effected without the prior written consent of such Investor, which
consent shall not be unreasonably withheld; provided, further, however, that the
Investor shall be liable under this Section 6(b) for only that amount of a Claim
as does not exceed the net proceeds to such Investor as a result of the sale of
Registrable Securities pursuant to such Registration Statement. Such indemnity
shall remain in full force and effect regardless of any investigation made by or
on behalf of such Indemnified Party and shall survive the transfer of the
Registrable Securities by the Investors pursuant to Section 9. Notwithstanding
anything to the contrary contained herein, the indemnification agreement
contained in this Section 6(b) with respect to any preliminary prospectus shall
not inure to the benefit of any Indemnified Party if the untrue statement or


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<PAGE>   12

omission of material fact contained in the preliminary prospectus was corrected
on a timely basis in the prospectus, as then amended or supplemented.

        c. Promptly after receipt by an Indemnified Person or Indemnified Party
under this Section 6 of notice of the commencement of any action (including any
governmental action), such Indemnified Person or Indemnified Party shall, if a
Claim in respect thereof is to made against any indemnifying party under this
Section 6, deliver to the indemnifying party a written notice of the
commencement thereof, and the indemnifying party shall have the right to
participate in, and, to the extent the indemnifying party so desires, jointly
with any other indemnifying party similarly noticed, to assume control of the
defense thereof with counsel mutually satisfactory to the indemnifying party and
the Indemnified Person or the Indemnified Party, as the case may be; provided,
however, that an Indemnified Person or Indemnified Party shall have the right to
retain its own counsel with the fees and expenses to be paid by the indemnifying
party, if, in the reasonable opinion of counsel retained by the indemnifying
party, the representation by such counsel of the Indemnified Person or
Indemnified Party and the indemnifying party would be inappropriate due to
actual or potential differing interests between such Indemnified Person or
Indemnified Party and any other party represented by such counsel in such
proceeding. The indemnifying party shall pay for only one separate legal counsel
for the Indemnified Persons or the Indemnified Parties, as applicable, and such
legal counsel shall be selected by Investors holding a majority-in-interest of
the Registrable Securities included in the Registration Statement to which the
Claim relates (with the approval of the Initial Investor if it holds Registrable
Securities included in such Registration Statement), if the Investors are
entitled to indemnification hereunder, or the Company, if the Company is
entitled to indemnification hereunder, as applicable. The failure to deliver
written notice to the indemnifying party within a reasonable time of the
commencement of any such action shall not relieve such indemnifying party of any
liability to the Indemnified Person or Indemnified Party under this Section 6,
except to the extent that the indemnifying party is actually prejudiced in its
ability to defend such action. The indemnification required by this Section 6
shall be made by periodic payments of the amount thereof during the course of
the investigation or defense, as such expense, loss, damage or liability is
incurred and is due and payable.

     7. CONTRIBUTION.

     To the extent any indemnification by an indemnifying party is prohibited or
limited by law, the indemnifying party agrees to make the maximum contribution
with respect to any amounts for which it would otherwise be liable under Section
6 to the fullest extent permitted by law; provided, however, that (i) no
contribution shall be made under circumstances where the maker would not have
been liable for indemnification under the fault standards set forth in Section
6, (ii) no seller of Registrable Securities guilty of fraudulent
misrepresentation (within the meaning of Section 11(f) of the 1933 Act) shall be
entitled to contribution from any seller of Registrable Securities who was not
guilty of such fraudulent misrepresentation, and (iii) contribution by any
seller of Registrable Securities shall be limited in amount to the net amount of
proceeds received by such seller from the sale of such Registrable Securities.


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<PAGE>   13

     8. REPORTS UNDER THE 1934 ACT.

     With a view to making available to the Investors the benefits of Rule 144
promulgated under the 1933 Act or any other similar rule or regulation of the
SEC that may at any time permit the investors to sell securities of the Company
to the public without registration ("Rule 144"), the Company agrees to:

        a. make and keep public information available, as those terms are
understood and defined in Rule 144;

        b. file with the SEC in a timely manner all reports and other documents
required of the Company under the 1933 Act and the 1934 Act so long as the
Company remains subject to such requirements (it being understood that nothing
herein shall limit the Company's obligations under Section 4(c) of the
Securities Purchase Agreement) and the filing of such reports and other
documents is required for the applicable provisions of Rule 144; and

        c. furnish to each Investor so long as such Investor owns Registrable
Securities, promptly upon request, (i) a written statement by the Company that
it has complied with the reporting requirements of Rule 144, the 1933 Act and
the 1934 Act, (ii) a copy of the most recent annual or quarterly report of the
Company and such other reports and documents so filed by the Company, and (iii)
such other information as may be reasonably requested to permit the Investors to
sell such securities pursuant to Rule 144 without registration.

     9. ASSIGNMENT OF REGISTRATION RIGHTS.

     The rights to have the Company register Registrable Securities pursuant to
this Agreement shall be automatically assignable by the Investors to any
transferee of all or any portion of Registrable Securities (provided that such
portion has a Market Price of at least $500,000 in the aggregate or consists of
at least 200,000 shares, whichever is greater) if: (i) the Investor agrees in
writing with the transferee or assignee to assign such rights, and a copy of
such agreement is furnished to the Company within a reasonable time after such
assignment, (ii) the Company is, within a reasonable time after such transfer or
assignment, furnished with written notice of (a) the name and address of such
transferee or assignee, and (b) the securities with respect to which such
registration rights are being transferred or assigned, (iii) following such
transfer or assignment the further disposition of such securities by the
transferee or assignee is restricted under the 1933 Act and applicable state
securities laws, (iv) at or before the time the Company receives the written
notice contemplated by clause (ii) of this sentence the transferee or assignee
agrees in writing with the Company to be bound by all of the provisions
contained herein, (v) such transfer shall have been made in accordance with the
applicable requirements of the Securities Purchase Agreement, and (vi) such
transferee shall be an "accredited investor" as that term defined in Rule 501 of
Regulation D promulgated under the 1933 Act.

     10. AMENDMENT OF REGISTRATION RIGHTS.

     Provisions of this Agreement may be amended and the observance thereof may
be waived (either generally or in a particular instance and either retroactively
or prospectively), only with written consent of the Company, the Initial
Investor and Investors who hold a majority interest of 


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<PAGE>   14

the Registrable Securities. Any amendment or waiver effected in accordance with
this Section 10 shall be binding upon each Investor and the Company.

     11. MISCELLANEOUS.

        a. A person or entity is deemed to be a holder of Registrable Securities
whenever such person or entity owns of record such Registrable Securities. If
the Company receives conflicting instructions, notices or elections from two or
more persons or entities with respect to the same Registrable Securities, the
Company shall act upon the basis of instructions, notice or election received
from the registered owner of such Registrable Securities.

        b. Notices required or permitted to be given hereunder shall be in
writing and shall be deemed to be sufficiently given when personally delivered
(by hand, by courier, by telephone line facsimile transmission or other means)
or sent by certified mail, return receipt requested, properly addressed and with
proper postage pre-paid,

if to the Company:
         Immunogen, Inc.
         128 Sidney Street
         Cambridge, MA 02139
         Attention: Frank J. Pocher, Chief Financial Officer

with copy to:
         Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
         One Financial Center
         Boston, MA 02111
         Telecopier: (617) 542-2241
         Attention:  Jonathan L. Kravetz, Esq.

if to the Buyer,
         Capital Ventures International
         c/o Arbit, Inc.
         1900 Market Street
         Philadelphia, PA  19102
         Telecopy:  (215) 963-3379
         Attention: Steve Katznelson

with copy to:
         Gary Kaminsky, Esq.
         c/o Arbit, Inc.
         1900 Market Street
         Philadelphia, PA  19102
         Telecopy:  (215) 656-8758


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<PAGE>   15

and:
         Klehr, Harrison, Harvey, Branzburg & Ellers
         1401 Walnut Street
         Philadelphia, PA  19102
         Telecopy:  (215) 568-5725
         Attention:  Wayne D. Bloch, Esq.

and if to any other Investor, at such address as such Investor shall have
provided in writing to the Company, or at such other address as each such party
furnishes by notice given in accordance with this Section 11(b), and shall be
effective, when personally delivered, upon receipt and, when so sent by
certified mail, four days after deposit with the United States Postal Service.

        c. Failure of any party to exercise any right or remedy under this
Agreement or otherwise, or delay by a party in exercising such right or remedy,
shall not operate as a waiver thereof.

        d. This Agreement shall be enforced, governed by and construed in
accordance with the laws of the Commonwealth of Massachusetts applicable to
agreements made and to be performed entirely within such State. In the event
that any provision of this Agreement is invalid or unenforceable under any
applicable statute or rule of law, then such provision shall be deemed
inoperative to the extent that it may conflict therewith and shall be deemed
modified to conform with such statute or rule of law. Any provision hereof which
may prove invalid or unenforceable under any law shall not affect the validity
or enforceability of any other provision hereof.

        e. This Agreement and the Securities Purchase Agreement constitute the
entire agreement among the parties hereto with respect to the subject matter
hereof and thereof. There are no restrictions, promises, warranties or
undertakings, other than those set forth or referred to herein and therein. This
Agreement and the Securities Purchase Agreement supersede all prior agreements
and understandings among the parties hereto with respect to the subject matter
hereof and thereof.

        f. Subject to the requirements of Section 9 hereof, this Agreement shall
inure to the benefit of and be binding upon the successors and assigns of each
of the parties hereto.

        g. The headings in this Agreement are for convenience of reference only
and shall not limit or otherwise affect the meaning hereof.

        h. This Agreement may be executed in two or more counterparts, each of
which shall be deemed an original but all of which shall constitute one and the
same agreement. This Agreement, once executed by a party, may be delivered to
the other party hereto by facsimile transmission of a copy of this Agreement
bearing the signature of the party so delivering this Agreement.

        i. Each party shall do and perform, or cause to be done and performed,
all such further acts and things, and shall execute and deliver all such other
agreements, certificates, instruments and documents, as the other party may
reasonably request in order to carry out the intent 


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<PAGE>   16

and accomplish the purposes of this Agreement and the consummation of the
transactions contemplated hereby.

     IN WITNESS WHEREOF, the parties have caused this Agreement to be duly
executed.


IMMUNOGEN, INC.



By:____________________________
Name:
Its:


CAPITAL VENTURES INTERNATIONAL



By:____________________________
Name:
Its:

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<PAGE>   1


                         CAPITAL VENTURES INTERNATIONAL


March 21, 1996

ImmunoGen, Inc.
128 Sidney Street
Cambridge, MA 02139
Attention: Frank J. Pocher, Chief Financial Officer

RE: Securities Purchase Agreement, dated as of March 15, 1996 (collectively,
together with the exhibits thereto, the "Agreement")

Dear Frank:

     As per our recent conversations regarding our understanding of the terms
and provisions of the Agreement, we hereby acknowledge and agree to the
following:

     A. The fixed feature of the Conversion Price in the Debentures issued
pursuant to the First Closing and the Second Closing shall be equal to the
closing bid price of the Common Stock on NASDAQ-NMS on the Trading Day
immediately preceding the Closing Date in respect of the First Closing (subject
to adjustment as provided therein) (the "Strike Price"); and

     B. The reference in Section 1.1 of the Debentures to "200% of the closing
bid price on the issue date" shall mean the Strike Price multiplied by 200%; and

     C. If the SEC will not allow the Registrable Securities underlying the
Debenture to be issued at the Second Closing to be included in the Registration
Statement initially filed pursuant to Section 2(a) of the Registration Rights
Agreement, the Company shall amend such
 Registration Statement to include such
Registrable Securities (if such Registration Statement is not yet effective at
the time of the Second Closing) or shall file a new Registration Statement (on
the short form available therefor, if applicable) registering such Registrable
Securities, in each case, as soon as practicable, but in any event within
fifteen (15) days after the Second Closing. The provisions of Section 2(c) of
the Registration Rights Agreement shall be applicable with respect to such
obligation; and

     D. In the event the number of shares available under a Registration
Statement filed pursuant to the Registration Rights Agreement is insufficient to
cover all of the Registrable Securities issued or issuable upon conversion of
the Debentures and exercise of the Warrants, the Company shall amend the
Registration Statement, or file a new Registration Statement (on the short form
available therefor, if applicable), or both, so as to cover all of such
Registrable Securities, in each case, as soon as practicable, but in any event
within fifteen (15) days after the necessity therefor arises (based on the
market price of the Common Stock and other relevant factors on which the Company
reasonably elects to rely). The provisions of Section 2(c) of the Registration
Rights Agreement shall be applicable with respect to such obligation, with the
sixty 


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<PAGE>   2

(60) days running from the day after the date on which the Company reasonably
first determines (or should have reasonably determined) the need therefor.

     Each capitalized term used herein and not otherwise defined shall have the
meaning ascribed to it in the Agreement.

     If the foregoing is in accordance with your understanding, please so
indicate by executing the enclosed copy of this letter in the space provided
below.


CAPITAL VENTURES INTERNATIONAL



By:____________________________________

ACKNOWLEDGED AND AGREED:
IMMUNOGEN, INC.



By:____________________________



                                       65