<PAGE>

                       SECURITIES AND EXCHANGE COMMISSION
                              Washington, DC 20549

                                  Schedule 13D

                    Under the Securities Exchange Act of 1934
                               (Amendment No. __)

                                 Immunogen, Inc.
- --------------------------------------------------------------------------------
                                (Name of Issuer)

                       Common Stock, $.01 par value share
- --------------------------------------------------------------------------------
                         (Title of Class of Securities)

                                    45253H101
- --------------------------------------------------------------------------------
                                 (CUSIP Number)

                              Wayne D. Bloch, Esq.
                   Klehr, Harrison, Harvey, Branzburg & Ellers
                               1401 Walnut Street
                             Philadelphia, PA 19102
- --------------------------------------------------------------------------------
                  (Name, Address and Telephone Number of Person
                Authorized to Receive Notices and Communications)

                                 March 25, 1996
- --------------------------------------------------------------------------------
             (Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report
the acquisition which is the subject of this Schedule 13D, and is filing this
schedule because of Rule 13d-1(b)(3) or (4), check the following box / /.

Check the following box if a fee is being paid with the statement /X/. (A fee is
not required only if the reporting person: (1) has a previous statement on file
reporting beneficial ownership of more than five percent of the class of
securities described in Item 1; and (2) has filed no amendment subsequent
thereto reporting beneficial ownership of five percent or less of such class.)
(See Rule 13d-7.)

Note: Six copies of this statement, including all exhibits, should be filed with
the Commission. See Rule 13d-1(a) for other parties to whom copies are to be
sent.

                        (Continued on following pages)
                                   Page 1 of 7


<PAGE>

- ------------------                                            -----------------
CUSIP No.45253H101                                            Page 2 of 7 Pages
- ------------------                                            -----------------


- --------------------------------------------------------------------------------
1 NAME OF REPORTING PERSON
  S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

             Capital Ventures International
- --------------------------------------------------------------------------------
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                       (a) / /

                                                       (b) / /
- --------------------------------------------------------------------------------
3 SEC USE ONLY

- --------------------------------------------------------------------------------
4 SOURCE OF FUNDS*

  WC
- --------------------------------------------------------------------------------
5 CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS
  REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)

                                                          / /
- --------------------------------------------------------------------------------
6 CITIZENSHIP OR PLACE OF ORGANIZATION

  Cayman Islands
- --------------------------------------------------------------------------------
                 7       SOLE VOTING POWER

NUMBER OF
SHARES                   1,500,000**
BENEFICIALLY             -----------------------------------------------------
OWNED BY         8       SHARED VOTING POWER
EACH        
REPORTING                N/A
PERSON                   -----------------------------------------------------
WITH             9       SOLE DISPOSITIVE POWER

                         1,500,000**
                         -----------------------------------------------------

                 10      SHARED DISPOSITIVE POWER

                         N/A
- --------------------------------------------------------------------------------
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

               1,500,000**
- --------------------------------------------------------------------------------
12 CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11)
   EXCLUDES CERTAIN SHARES*                              / /
- --------------------------------------------------------------------------------
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

               8.8% ***
- --------------------------------------------------------------------------------
14 TYPE OF REPORTING PERSON*

               CO
- --------------------------------------------------------------------------------
                      *SEE INSTRUCTIONS BEFORE FILLING OUT

** Based on the closing bid price for the Common Stock on March 22, 1996 of
$2.50 as reported on The NASDAQ National Market.
*** Based on 15,526,367 shares of Common Stock outstanding on March 22, 1996.


<PAGE>


- ------------------                                            -----------------
CUSIP No.45253H101                                            Page 3 of 7 Pages
- ------------------                                            -----------------


                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  SCHEDULE 13D

                                  Statement of
                      Reporting Persons (as defined below)

                        Pursuant to Section 13(d) of the
                         Securities Exchange Act of 1934

                                  in respect of

                                 IMMUNOGEN, INC.

                  This Report filed by Capital Ventures International ("CVI") is
its initial filing on Schedule 13D with respect to the common stock, $.01 par
value per share (the "Common Stock"), of Immunogen, Inc. (the "Company").

                  The descriptions contained in this Report of certain
agreements and documents are qualified in their entirety by reference to the
completed text of such agreements and documents filed as Exhibits hereto.

Item 1. Security and Issuer

                  The class of equity securities to which this statement relates
is the Common Stock of the Company. The principal executive office of the
Company is located at 128 Sidney Street, Cambridge, MA 02139.

Item 2. Identity and Background

                  CVI is an unlimited liability company organized under the laws
of the Cayman Islands with its principal place of business and principal office
located at One Capitol Place, P.O. Box 1787GT, Grand Cayman, Cayman Islands,
BWI.

                  CVI is engaged in the purchase of securities for investment.

                  The entire share capital of CVI is owned by the following
entities in the stated percentages.

                  Mu Trading, Inc. ("Mu Trading") owns 44% of CVI and is a
Delaware corporation which does not conduct business other than as a holding
company for subsidiary operating companies engaged in the securities business.
Mu Trading's principal place of business and principal office are located at 42
Reads Way, New Castle, DE 19720. Mu Trading is owned 100% by Jeffrey Yass, a
U.S. citizen and an investor with a business address of 1900 Market Street, 6th
Floor, Philadelphia, PA 19103. Mr. Yass is the sole director and executive
officer of Mu Trading.

                  Rattan, Inc. ("Rattan") owns 14% of CVI and is a Delaware
corporation which does not conduct business other than as a holding company for
subsidiary operating companies engaged in the securities business. Rattan's
principal place of business and principal office are located at 42 Reads Way,
New Castle, DE 19720. Rattan is owned 100% by Eric Brooks, a U.S. citizen and an
investor with a business address of 1900 Market Street, 6th Floor, Philadelphia,
PA 19103. Mr. Brooks is the sole director and executive officer of Rattan.

                  Lobby, Inc. ("Lobby") owns 14% of CVI and is a Delaware
corporation which does not conduct business other than as a holding company for
subsidiary operating companies engaged in the securities business. Lobby's
principal place of business and principal office are located at 42 Reads Way,
New Castle, DE 19720. Lobby is owned 100% by Arthur Dantchik, a U.S. citizen and
an investor with a business address of 1900 Market Street, 6th Floor,
Philadelphia, PA 19103. Mr. Dantchik is the sole director and executive officer
of Lobby.




<PAGE>


- ------------------                                            -----------------
CUSIP No.45253H101                                            Page 4 of 7 Pages
- ------------------                                            -----------------


                  East Bay, Inc. ("East Bay") owns 14% of CVI and is a Delaware
corporation which does not conduct business other than as a holding company for
subsidiary operating companies engaged in the securities business. East Bay's
principal place of business and principal office are located at 42 Reads Way,
New Castle, DE 19720. East Bay is owned 100% by Andrew Frost, a U.S. citizen and
an investor with a business address of 1900 Market Street, 6th Floor,
Philadelphia, PA 19103. Mr. Frost is the sole director and executive officer of
East Bay.

                  Selt, Inc. ("Selt") owns 14% of CVI and is a Delaware
corporation which does not conduct business other than as a holding company for
subsidiary operating companies engaged in the securities business. Selt's
principal place of business and principal office are located at 42 Reads Way,
New Castle, DE 19720. Selt is owned 100% by Joel Greenberg, a U.S. citizen and
an investor with a business address of 1900 Market Street, 6th Floor,
Philadelphia, PA 19103. Mr. Greenberg is the sole director and executive officer
of Selt.

                  Each of the individuals referred to above (the "Principals")
conducts his principal business activities through Susquehanna Investment Group
("SIG") and its related companies. SIG's principal place of business and
principal office are located at 1900 Market Street, 6th Floor, Philadelphia, PA
19103. CVI's directors are comprised solely by the Principals, except for
director Ian Wight, a U.K. Citizen and a trust accountant with a business
address of RHB Trust Company, Ltd, 1 Capitol Place, Grand Cayman, Cayman
Islands, BWI. The executive officers of CVI are Richard Douglas, a U.K. Citizen
and a trust accountant, and Woodbourne Associates (Cayman) Ltd., a nominee
company located in the Cayman Islands.

                  During the last five years, neither CVI, nor to the best of
CVI's knowledge, any individual or entity named in this Item 2, has been
convicted in a criminal proceeding (excluding traffic violations or similar
misdemeanors) or has been a party to a civil proceeding of a judicial or
administrative body of competent jurisdiction and as a result of such civil
proceeding was or is subject to a judgment, decree or final order enjoining
future violations of, or prohibiting or mandating activities subject to, federal
or state securities laws or finding any violation with respect to such laws.

Item 3. Source and Amount of Funds and Other Consideration.

                  CVI is a party to a Securities Purchase Agreement with the
Company, dated as of March 15, 1996, pursuant to which CVI agreed to acquire for
purposes of investment convertible debentures in the aggregate principal amount
of $5,000,000 (the "Debentures"). The aggregate purchase price of the Debentures
is $5,000,000. Pursuant to the terms of the Securities Purchase Agreement, two
Debentures, each in an aggregate principal amount of $2,500,000 (the "First
Debenture" and the "Second Debenture", respectively) will be issued and sold to
CVI in separate closings. The first closing occurred as of March 25, 1996. The
second closing is subject to the condition that the Company obtain, within 120
days of the issuance of the First Debenture, shareholder approval of an increase
in the Company's number of authorized shares of Common Stock from 20 million
shares to no less than 30 million shares. The second closing will take place, if
at all, shortly after the Company obtains such approval.

                  The outstanding principal amount of the Debentures together
with interest accrued thereon (at a rate of 10% per annum) is convertible into
Common Stock at a price per share (the "Conversion Price") equal to the lesser
of: (i) $2.50 (the "Fixed Conversion Price") and (ii) 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 consecutive trading days ending one trading day prior to the conversion
date (subject to equitable adjustments for stock splits, stock dividends,
combinations, recapitalization, reclassifications of similar events).

                  Applicable Percentage means (i) 100%, if the conversion date
is within forty (40) days after the first closing, (ii) 90%, if conversion date
is within eighty (80) days, but more than forty (40) days after the first
closing and (iii) 85% if the conversion date is more than eighty (80) days after
the first closing. The holder of the Debenture has the right to convert, at any
time or from time to time, all or any part of the outstanding and unpaid
principal amount of a Debenture into (i) Common Stock at the Conversion Price
and (ii) if such conversion takes place after the 80th day following the first
closing, warrants (the "Warrants") to acquire a number of shares of Common Stock
equal to 50% of the number of shares of Common Stock issuable upon such
conversion. The Warrants expire five years from the date of issuance and are
exercisable at a price of $4.00 per share.



<PAGE>

- ------------------                                            -----------------
CUSIP No.45253H101                                            Page 5 of 7 Pages
- ------------------                                            -----------------


                  Simultaneously, with the execution of the Securities Purchase
Agreement, the Company and CVI entered into a Registration Rights Agreement
pursuant to which the Company agreed to register the resale by CVI of the Common
Stock underlying the Debentures and the Warrants under the Securities Act of
1933, as amended.

                  CVI utilized its own funds to consummate the purchase of the
First Debenture and will utilize its own funds to consummate the purchase of the
Second Debenture.


Item 4. Purpose of Transaction.

                  CVI is acquiring the Debentures for investment purposes for
its own account. CVI does not currently have any plan or intention to acquire
additional securities of the Company other than in connection with its
obligation to acquire the Second Debenture at the second closing under the
Securities Purchase Agreement and its intention to acquire the shares of Common
Stock and Warrants issuable upon conversion of the Debentures and the shares of
Common Stock issuable upon exercise of the Warrants.

                  Except as stated above, CVI has no plans or proposals that
relate or would result in any of the transactions referred to in sub-items (a)
through (j) of Item 4 of Schedule 13D.


Item 5. Interest in Securities of the Issuer.

                  Based on a Conversion Price equal to the closing bid price of
the Company's Common Stock on The NASDAQ National Market on March 22, 1996,
($2.50), the total of number of shares that CVI beneficially owns through the
ownership of the First Debenture would be 1,000,000 shares of Common Stock or
6.1% of the outstanding Common Stock. No Warrants would be issuable if such
Debenture were converted as of such date. As described above under Item 3,
Warrants are issuable only if the conversion takes place after the 80th day
following the first closing or on or about June 13, 1996. However, assuming the
issuance of Warrants in connection with a conversion of the First Debenture at a
Conversion Price equal to $2.50, CVI would beneficially own 1,500,000 shares of
Common Stock (including 500,000 shares issuable upon exercise of Warrants) or
approximately 8.8% of the outstanding Common Stock.

                  Pursuant to the terms of the Debentures and the Warrants, the
holder thereof may never convert such Debentures or exercise such Warrants to
the extent that upon such conversion or exercise such holder and its affiliates
would hold in excess of 9.9% of the outstanding shares of Common Stock. In
addition, the disposition of the Debentures and the Warrants is restricted so
that, except in limited circumstances, the holder may not dispose of during any
90-day period Debentures and Warrants (or portions thereof) which, if converted
into Common Stock, would represent, at the time of the transfer, in the
aggregate, beneficial ownership by the transferee(s) of more than 9.9% of the
Common Stock then outstanding. As a result, beneficial ownership may never
exceed 9.9% of the outstanding shares of Common Stock solely on account of the
Debentures and the Warrants.(1)

                  The number of shares of Common Stock beneficially owned by CVI
will fluctuate depending on the Conversion Price from time to time in effect;
provided, however, except to the extent that shares are disposed of, the
beneficial ownership will never be less than the number of shares beneficially
owned based on the Fixed Conversion Price ($2.50).

- --------
     1 The closing of the purchase and sale of the Second Debenture is subject
to the approval of a Company's shareholders of an increase in the Company's
authorized number of shares of Common Stock from 20 million shares to at least
30 million shares. If such approval takes place, subject to certain conditions,
CVI will be bound to acquire an additional $2,500,000 principal amount
Debenture. In no event may CVI convert the Debentures or exercise the Warrants
to the extent it would beneficially own more than 9.9% of the Company's Common
Stock. Based on the 15,526,357 shares of Common Stock outstanding on March 22,
1996, the maximum number of shares into which CVI could convert the Debentures
and exercise the Warrants is 1,706,003.



<PAGE>

- ------------------                                            -----------------
CUSIP No.45253H101                                            Page 6 of 7 Pages
- ------------------                                            -----------------

To the extent the Conversion Price is less than the Fixed Conversion
Price, the number of shares beneficially owned by CVI will increase without any
action on the part of CVI, but such beneficial ownership can never exceed 9.9%
of the outstanding shares of Common Stock. The number of shares beneficially
owned by CVI can be determined by dividing the sum of the principal amount of
Debentures held by CVI plus accrued interest thereon by the Conversion Price
then in effect.

                  To the best knowledge of CVI, none of the individuals listed
in Item 2 hereof beneficially owns any Common Stock other than through their
ownership interest in CVI. Neither CVI, nor to the best of CVI's knowledge, any
of the individuals listed in Item 2 hereof has effected any transactions in the
Common Stock during the past 60 days other than CVI's purchase of the First
Debenture.

Item 6.   Contracts, Arrangements, Understandings or Relationships with
          Respect to Securities of the Issuer.

          None.

Item 7.   Material to be Filed as Exhibits.

          Exhibit A - Securities Purchase Agreement (together with Side Letter
          thereto and form of the Debentures, Warrants and Registration Rights
          Agreement).

          Exhibit B - Limited Power of Attorney.



<PAGE>

- ------------------                                            -----------------
CUSIP No.45253H101                                            Page 7 of 7 Pages
- ------------------                                            -----------------

                                    SIGNATURE

                 After reasonable inquiry and to the best of my knowledge and
belief, I certify that the information set forth in this statement is true,
complete and correct.

                        CAPITAL VENTURES INTERNATIONAL

                        By:  Arbit, Inc. (pursuant to a Limited Power
                             of Attorney, a copy of which is filed as
                             an exhibit hereto)

                        By: /s/ Arthur Dantchik
                            --------------------------------------------  
                            Arthur Dantchik, President






<PAGE>

                                    EXHIBIT A

                          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



                                      



<PAGE>

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
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




                                       2

<PAGE>

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 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




                                       3

<PAGE>



         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 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.




                                       4

<PAGE>

         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 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





                                       5

<PAGE>

("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 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




                                       6

<PAGE>

"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 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.




                                       7

<PAGE>

         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 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




                                       8

<PAGE>

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).

         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).




                                       9

<PAGE>

         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 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




                                       10

<PAGE>

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.

                  (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:




                                       11

<PAGE>

                  (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.




                                       12

<PAGE>

         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




                                       13

<PAGE>

         With copy to:

         Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
         One Financial Center
         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.




                                       14

<PAGE>

         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.

         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.




                                       15

<PAGE>

         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.
         ------------------------------------




                                       16

<PAGE>

                                                                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.

<PAGE>

               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 $5.00 per share, at anytime following March 25,
1998, the Borrower shall have the right, exercisable on not less 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




                                       2

<PAGE>

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 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




                                       3

<PAGE>

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 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) $2.50 per share (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 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




                                       4

<PAGE>

Stock which are available to effect such conversion including, with the Holder's
written consent, any shares underlying Warrants issued or then or thereafter
issuable ("Borrowed Shares"). 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 (including Borrowed Shares if
any) 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 Default
Amount (as defined below) on the first day of the Conversion Default (the
"Conversion Default Date"), where (i) N = the number of days 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 and (ii) the
Default Amount means the Excess Amount plus the principal amount of this
Debenture that would not be convertible as a result of this Section 2.3 but for
the Borrowed Shares (the "Borrowed Amount"); provided, however that the Borrowed
Amount shall not be included in the Default Amount for any period of time after
the Conversion Default Date that the Market Price (as defined in the Warrants)
of the Common Stock is below the exercise price of the Warrants then in effect.
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,
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




                                       5

<PAGE>

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. 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




                                       6

<PAGE>

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 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




                                       7

<PAGE>

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 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




                                       8

<PAGE>

                 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 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.




                                       9

<PAGE>

                 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 2.1, 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 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:




                                       10

<PAGE>

                                                                    Exhibit A

                              NOTICE OF CONVERSION
                            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](1) 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

- --------
1 References to Warrants should be included only after the 80th day following
the Closing Date in respect of the First Closing.




                                       11

<PAGE>

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.)




                                       12

<PAGE>

                                                                     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)

                                       



<PAGE>




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 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 deliv ered, 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

                                       -2-



<PAGE>





         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.

                  (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, mer ger, 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 sub stantially 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.


                                       -3-



<PAGE>




         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 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

                                       -4-



<PAGE>




                  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 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.


                                       -5-



<PAGE>





                           (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.

                                       -6-



<PAGE>





                  (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

                                       -7-



<PAGE>




         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.

                  (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;

                                       -8-



<PAGE>





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.

                  (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

                                       -9-



<PAGE>





                  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 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").


                                      -10-



<PAGE>





                  (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.

                  (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

                                      -11-



<PAGE>






         $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 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.


                                      -12-



<PAGE>




                  (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___



                                      -13-



<PAGE>







                           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.






<PAGE>




                               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.





<PAGE>




                                                                     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.



<PAGE>

                           (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").

                           (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




                                       2


<PAGE>

         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 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.




                                       3



<PAGE>

                  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 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.



                                       4



<PAGE>

         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 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




                                       5


<PAGE>

         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.

                  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




                                       6


<PAGE>

         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.

                  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




                                       7


<PAGE>

         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 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




                                       8


<PAGE>

         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:

                  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




                                       9


<PAGE>

         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 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)




                                       10


<PAGE>

         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 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



                                       11


<PAGE>

         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 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



                                       12


<PAGE>

         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.

         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.




                                       13

<PAGE>

         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 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,




                                       14

<PAGE>

         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

         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




                                       15


<PAGE>

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




                                       16

<PAGE>

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

IMMUNOGEN, INC.


By:   _____________________________________
Name: _____________________________________
Its:  _____________________________________


CAPITAL VENTURES INTERNATIONAL


By:   _____________________________________
Name: _____________________________________
Its:  _____________________________________




                                       17


<PAGE>


                         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



<PAGE>

ImmunoGen, Inc.
March 21, 1996
Page 2

          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 (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:____________________________



<PAGE>
                                    EXHIBIT B

                            LIMITED POWER OF ATTORNEY

THIS LIMITED POWER OF ATTORNEY given on the 11th day of October, 1993 by CAPITAL
VENTURES INTERNATIONAL, (hereinafter called "the Company") whose Registered
Office is situated at Third Floor, One Regis Place, PO Box 1787, Grand Cayman,
Cayman Islands, B.W.I.

WHEREAS by agreement dated August 28, 1989 by and between the Company and Arbit
Inc., the Company expressly authorized Arbit Inc. to enter into transactions in
certain designated areas as defined in the Agreement attached hereto marked
"Appendix 1".

NOW THIS DEED WITNESSETH that Ian A.N. Wight (Director) and Woodbourne
Associates (Cayman) Limited (Secretary) of the Company, hereby appoint on behalf
of the Company the firm of ARBIT INC. which through its officers, directors and
employees is hereby formally granted limited power of attorney for the purpose
of entering into transactions on behalf of and for the account of the Company
and to take any actions on behalf of the Company as may be necessary to
consummate such transactions, including but not limited to instructing the
transfer of funds where necessary and executing required documentation.

IN WITNESS WHEREOF the Company has caused its common seal to be hereunto affixed
the day and year above written.

THE COMMON SEAL OF                                   /s/
CAPITAL VENTURES INTERNATIONAL                      ---------------------------
was hereunto affixed in the                         Ian A.N. Wight 
presence of:                                        (Director)     
                                        

/s/
- --------------------------------------
Witness

                                                    /s/
                                                    ---------------------------
                                                    Woodbourne Associates
                                                    (Cayman) Limited
                                                    (Secretary)



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