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                         CAPITAL VENTURES INTERNATIONAL


March 21, 1996

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

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

Dear Frank:

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

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

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

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

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



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