<Page>
                            SCHEDULE 14A INFORMATION
 
                  Proxy Statement Pursuant to Section 14(a) of
                      the Securities Exchange Act of 1934
 

<Table>
      <S>        <C>
      Filed by the Registrant /X/
 
      Filed by a Party other than the Registrant / /
 
      Check the appropriate box:
      /X/        Preliminary Proxy Statement
      / /        CONFIDENTIAL, FOR USE OF THE COMMISSION ONLY (AS PERMITTED
                 BY RULE 14a-6(e)(2))
      / /        Definitive Proxy Statement
      / /        Definitive Additional Materials
      / /        Soliciting Material Pursuant to Section 240.14a-12
 
                                  IMMUNOGEN, INC.
      -----------------------------------------------------------------------
                 (Name of Registrant as Specified In Its Charter)
 
      -----------------------------------------------------------------------
           (Name of Person(s) Filing Proxy Statement, if other than the
                                    Registrant)
</Table>

 
Payment of Filing Fee (Check the appropriate box):
 

<Table>
<S>        <C>  <C>
/X/        No fee required.
/ /        Fee computed on table below per Exchange Act Rules 14a-6(i)(4)
           and 0-11.
           (1)  Title of each class of securities to which transaction
                applies:
                ----------------------------------------------------------
           (2)  Aggregate number of securities to which transaction
                applies:
                ----------------------------------------------------------
           (3)  Per unit price or other underlying value of transaction
                computed pursuant to Exchange Act Rule 0-11 (set forth the
                amount on which the filing fee is calculated and state how
                it was determined):
                ----------------------------------------------------------
           (4)  Proposed maximum aggregate value of transaction:
                ----------------------------------------------------------
           (5)  Total fee paid:
                ----------------------------------------------------------
 
/ /        Fee paid previously with preliminary materials.
 
/ /        Check box if any part of the fee is offset as provided by
           Exchange Act Rule 0-11(a)(2) and identify the filing for which
           the offsetting fee was paid previously. Identify the previous
           filing by registration statement number, or the Form or
           Schedule and the date of its filing.
 
           (1)  Amount Previously Paid:
                ----------------------------------------------------------
           (2)  Form, Schedule or Registration Statement No.:
                ----------------------------------------------------------
           (3)  Filing Party:
                ----------------------------------------------------------
           (4)  Date Filed:
                ----------------------------------------------------------
</Table>

<Page>
IMMUNOGEN, INC.
------------------------------------------------------------
128 Sidney Street, Cambridge, MA 02139                 TEL: (617) 995-2500  FAX:
(617) 995-2510
 
                                                                          , 2001
 
Dear Shareholder:
 
    You are cordially invited to attend the 2001 Annual Meeting of Shareholders
of ImmunoGen, Inc. (the "Company"), to be held at 10:00 a.m., Boston time, on
Tuesday, November 13, 2001 at the offices of the Company, 128 Sidney Street,
Cambridge, Massachusetts.
 
    At the Annual Meeting, six members will be elected to the Board of
Directors. The Company will seek shareholder approval of a proposal to increase
the aggregate number of shares for which stock options may be granted under the
Company's Restated Stock Option Plan from 4,850,000 to 7,350,000. The Company
will also seek shareholder approval to adopt the Company's 2001 Non-Employee
Director Stock Plan (the "2001 Plan") and reserve 50,000 shares of its common
stock for stock or stock units which may be granted under the 2001 Plan. The
Company will also seek shareholder approval to amend the Company's Restated
Articles of Organization to increase the number of the Company's authorized
shares of common stock from 50,000,000 to 75,000,000. The Board of Directors
recommends the approval of these proposals, each of which is described in the
enclosed proxy statement, which we encourage you to read carefully.
 
    We hope that you will be able to attend the Annual Meeting. Whether you plan
to attend the Annual Meeting or not, it is important that your shares are
represented. Therefore, you are urged to complete, sign, date and return the
enclosed proxy card promptly in accordance with the instructions set forth on
the card. This will ensure your proper representation at the Annual Meeting.
 
                                          Sincerely,
                                          /s/ MITCHEL SAYARE
                                          MITCHEL SAYARE
                                          PRESIDENT, CHIEF EXECUTIVE OFFICER
                                          AND CHAIRMAN OF THE BOARD
 
           YOUR VOTE IS IMPORTANT. PLEASE RETURN YOUR PROXY PROMPTLY.
<Page>
                                IMMUNOGEN, INC.
 
                               128 SIDNEY STREET
                         CAMBRIDGE, MASSACHUSETTS 02139
 
                    NOTICE OF ANNUAL MEETING OF SHAREHOLDERS
                        TO BE HELD ON NOVEMBER 13, 2001
 
To the Shareholders
  of ImmunoGen, Inc.:
 
    Notice is hereby given that the Annual Meeting of Shareholders of
ImmunoGen, Inc., a Massachusetts corporation (the "Company"), will be held at
the offices of the Company, 128 Sidney Street, Cambridge, Massachusetts, on
Tuesday, November 13, 2001 at 10:00 a.m., Boston time, for the following
purposes:
 
    1.  To elect six members to the Board of Directors to hold office until the
       next annual meeting of shareholders and until their successors are duly
       elected and qualified;
 
    2.  To consider and act upon a proposal to increase from 4,850,000 shares to
       7,350,000 shares the aggregate number of shares of the Company's common
       stock, $.01 par value per share (the "Common Stock"), for which stock
       options may be granted under the Company's Restated Stock Option Plan;
 
    3.  To consider and act upon a proposal to adopt the Company's 2001
       Non-Employee Director Stock Plan (the "2001 Plan") and reserve 50,000
       shares of Common Stock for stock or stock units which may be granted
       under the 2001 Plan;
 
    4.  To consider and act upon a proposal to amend the Company's Restated
       Articles of Organization to increase from 50,000,000 shares to 75,000,000
       shares the aggregate number of shares of Common Stock authorized to be
       issued by the Company; and
 
    5.  To transact such other business as may properly come before the Annual
       Meeting or any adjournments thereof.
 
    The Board of Directors has fixed the close of business on September 14, 2001
as the record date for the determination of Shareholders entitled to notice of
and to vote at the Annual Meeting and at any adjournments thereof.
 
    You are cordially invited to attend the Annual Meeting in person, if
possible. WHETHER YOU PLAN TO ATTEND THE ANNUAL MEETING OR NOT, PLEASE COMPLETE,
SIGN AND DATE THE ENCLOSED PROXY AND RETURN IT IN THE ENVELOPE ENCLOSED FOR THIS
PURPOSE. Your Proxy is revocable at any time prior to the exercise thereof by
written notice received by the Company, by delivery of a duly executed proxy
bearing a later date, or by attending the Annual Meeting and voting in person.
 
                                          By order of the Board of Directors
                                          /s/ JONATHAN L. KRAVETZ, ESQ.
                                          JONATHAN L. KRAVETZ, ESQ.
                                          CLERK
 
            , 2001
<Page>
               PRELIMINARY COPIES FILED PURSUANT TO RULE 14a-6(a)
 
                                IMMUNOGEN, INC.
                               128 SIDNEY STREET
                         CAMBRIDGE, MASSACHUSETTS 02139
                                  617-995-2500
                            ------------------------
                                PROXY STATEMENT
                             ---------------------
 
                   ------------------------------------------
                         ANNUAL MEETING OF SHAREHOLDERS
                   ------------------------------------------
 
                        TO BE HELD ON NOVEMBER 13, 2001
 
                              GENERAL INFORMATION
 
    This Proxy Statement is furnished in connection with the solicitation by and
on behalf of the Board of Directors (the "Board of Directors" or "Board") of
ImmunoGen, Inc., a Massachusetts corporation (the "Company"), of proxies, in the
accompanying form, to be used at the Annual Meeting of Shareholders to be held
at the offices of the Company, 128 Sidney Street, Cambridge, Massachusetts on
Tuesday, November 13, 2001 at 10:00 a.m., Boston time, and at any adjournments
thereof (the "Meeting").
 
    Where the shareholder specifies a choice on the proxy as to how his or her
shares are to be voted on a particular matter, the shares will be voted
accordingly. If no choice is specified, the shares will be voted as follows:
 
    - FOR the election of the six nominees for Director named herein,
 
    - FOR the proposal to increase from 4,850,000 shares to 7,350,000 shares the
      aggregate number of shares of the Company's common stock, $.01 par value
      per share (the "Common Stock") for which stock options may be granted
      under the Company's Restated Stock Option Plan,
 
    - FOR the proposal to adopt the Company's 2001 Non-Employee Director Stock
      Plan (the "2001 Plan") and reserve 50,000 shares of Common Stock which may
      be granted under the 2001 Plan, and
 
    - FOR the proposal to amend the Company's Restated Articles of Organization
      to increase from 50,000,000 shares to 75,000,000 shares the aggregate
      number of shares of Common Stock authorized to be issued by the Company.
 
    Any proxy given pursuant to this solicitation may be revoked by the person
giving it at any time before its use by delivering to the Company a written
notice of revocation or a duly executed proxy bearing a later date. Any
shareholder who has executed a proxy but is present at the Meeting, and who
wishes to vote in person, may do so by revoking his or her proxy as described in
the preceding sentence. Shares represented by valid proxies in the form
enclosed, received in time for use at the Meeting and not revoked at or prior to
the Meeting, will be voted at the Meeting. The presence, in person or by proxy,
of the holders of a majority
 
                                       1
<Page>
of the outstanding shares of the Company's Common Stock is necessary to
constitute a quorum at the Meeting. Votes of shareholders of record who are
present at the meeting in person or by proxy, abstentions, and broker non-votes
(as defined below) are counted as present or represented at the meeting for
purposes of determining whether a quorum exists. No appraisal rights exist for
any action to be taken at the Meeting.
 
    Nominees for election as Directors at the meeting will be elected by a
plurality of the votes of the shares present in person or represented by proxy
at the meeting. Withholding authority to vote for a nominee for Director will
have no effect on the outcome of the vote. For the proposals to increase from
4,850,000 shares to 7,350,000 shares the aggregate number of shares of the
Company's Common Stock for which stock options may be granted under the
Company's Restated Stock Option Plan and to adopt the Company's 2001 Plan and
reserve 50,000 shares of Common Stock which may be granted under the 2001 Plan,
the affirmative vote of the majority of shares of Common Stock present or
represented by proxy and entitled to vote on the matter is necessary for
approval. For the proposal to amend the Company's Restated Articles of
Organization, the affirmative vote of a majority of the shares of Common Stock
outstanding and entitled to vote at the Meeting is necessary for approval.
Because abstentions are treated as shares present or represented and entitled to
vote, abstentions with respect to these proposals have the same effect as a vote
against the proposal.
 
    If you hold your shares of Common Stock through a broker, bank or other
representative, generally the broker or your representative may only vote the
Common Stock that it holds for you in accordance with your instructions.
However, if it has not timely received your instructions, the broker or your
representative may vote on certain matters for which it has discretionary voting
authority. If a broker or your representative cannot vote on a particular matter
because it does not have discretionary voting authority, this is a "broker
non-vote" on that matter. As to the election of Directors and the proposals
relating to the stock plans, broker non-votes are not deemed to be present and
represented and are not entitled to vote, and therefore will have no effect on
the outcome of the vote. As to the proposal to amend the Company's Restated
Articles of Organization, which requires the affirmative vote of a majority of
the Common Stock outstanding and entitled to vote at the Meeting, broker
non-votes have the same effect as negative votes.
 
    The close of business on September 14, 2001 has been fixed as the record
date for determining the shareholders entitled to notice of and to vote at the
Meeting. As of the close of business on September 14, 2001, the Company had
[      ] shares of Common Stock outstanding and entitled to vote. Holders of
Common Stock are entitled to one vote per share on all matters to be voted on by
shareholders.
 
    The cost of soliciting proxies, including expenses in connection with
preparing and mailing this Proxy Statement, will be borne by the Company. In
addition, the Company will reimburse brokerage firms and other persons
representing beneficial owners of Common Stock of the Company for their expenses
in forwarding proxy material to such beneficial owners. Solicitation of proxies
by mail may be supplemented by telephone, telegram, telex and personal
solicitation by the Directors, officers or employees of the Company. No
additional compensation will be paid for such solicitation.
 
    This Proxy Statement and the accompanying proxy are being mailed on or about
October 12, 2001 to all Shareholders entitled to notice of and to vote at the
Meeting.
 
    The Annual Report to Shareholders for the fiscal year ended June 30, 2001 is
being mailed to the Shareholders with this Proxy Statement, but does not
constitute a part hereof.
 
                                       2
<Page>
         SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT
 
    The following table sets forth certain information as of August 31, 2001
concerning the beneficial ownership of the Common Stock by each shareholder
known by the Company to be the beneficial owner of more than 5.0% of the
outstanding shares of Common Stock, each current member of the Board of
Directors, each executive officer of the Company named in the Summary
Compensation Table below, and all current executive officers and Directors of
the Company as a group. Except as otherwise indicated, each shareholder has sole
voting and investment power with respect to the shares beneficially owned.
 

<Table>
<Caption>
                                                                                  PERCENTAGE
                                                              NUMBER OF SHARES    OF SHARES
                                                                BENEFICIALLY     BENEFICIALLY
NAME AND ADDRESS OF BENEFICIAL OWNER*                            OWNED (1)        OWNED (1)
-------------------------------------                         ----------------   ------------
<S>                                                           <C>                <C>
Capital Ventures International (2)..........................     2,815,185            7.3%
  One Capitol Place, P.O. Box 1787 GT
  Grand Cayman, Cayman Islands, BWI
Mitchel Sayare (3)..........................................       950,430            2.5%
Walter A. Blattler (4)......................................       550,145            1.4%
David W. Carter (5).........................................        85,000             **
Michael R. Eisenson (6).....................................            --             **
Mark B. Skaletsky (7).......................................        19,167             **
Stuart F. Feiner (8)........................................        44,166             **
Pauline Jen Ryan (9)........................................        28,334             **
John M. Lambert (10)........................................       336,492             **
All current executive officers and Directors as a group
  (8 persons) (11)..........................................     2,013,734            5.2%
</Table>

 
------------------------
 
*   Addresses are given for beneficial owners of more than 5% of the outstanding
    Common Stock only.
 
**  Represents beneficial ownership of less than 1% of the Common Stock.
 
(1) The number of shares of Common Stock issued and outstanding on August 31,
    2001 was 38,552,902. Share ownership includes shares of Common Stock
    issuable upon exercise of certain outstanding options and warrants within
    60 days after August 31, 2001, as described in the footnotes below.
 
(2) Includes 2,475,185 shares of Common Stock which Capital Ventures
    International ("CVI") may acquire upon the exercise of warrants to purchase
    Common Stock. The Company's Restated Articles of Organization, as amended,
    and the warrants held by CVI (the "CVI Warrants") limit the right of CVI to
    exercise the CVI Warrants such that the maximum number of shares of Common
    Stock which may at any time be deemed to be beneficially owned by CVI upon
    the exercise of the CVI Warrants may not, together with any other shares of
    Common Stock then owned by CVI, exceed 9.9% of the then issued and
    outstanding shares of Common Stock. Also includes 340,000 shares of Common
    Stock which CVI may acquire upon the exercise of a warrant to purchase
    Common Stock which was issued in November 2000 (the "November 2000
    Warrant"). The November 2000 Warrant limits CVI's right to exercise the
    warrant such that the maximum number of shares which may at any time be
    deemed to be beneficially owned by CVI upon the exercise of the
    November 2000 Warrant may not, together with any other shares of Common
    Stock then owned by CVI, exceed 4.9% of the then issued and outstanding
    shares of Common Stock.
 
                                       3
<Page>
(3) Includes 715,334 shares of Common Stock which Mr. Sayare may acquire upon
    the exercise of options within 60 days after August 31, 2001.
 
(4) Includes 467,084 shares of Common Stock which Dr. Blattler may acquire upon
    the exercise of options within 60 days after August 31, 2001.
 
(5) Consists of 85,000 shares of Common Stock which Mr. Carter may acquire upon
    the exercise of options within 60 days after August 31, 2001.
 
(6) Mr. Eisenson, a Director of the Company, is President and Chief Executive
    Officer of Charlesbank Capital Partners, LLC, the successor to Harvard
    Private Capital Group, Inc. and the investment advisor to Aeneas Venture
    Corporation ("Aeneas"). Mr. Eisenson owns no shares of Common Stock and
    disclaims beneficial ownership of the shares owned by Aeneas. Pursuant to an
    agreement among the Company, Aeneas and Mr. Eisenson, grants of stock
    options in connection with Mr. Eisenson's service as a Director are granted
    directly to Aeneas. Pursuant to such grants, Aeneas may acquire 97,500
    shares of Common Stock within 60 days after August 31, 2001.
 
(7) Consists of 19,167 shares of Common Stock which Mr. Skaletsky may acquire
    upon the exercise of options within 60 days after August 31, 2001.
 
(8) Mr. Feiner, a Director of the Company, is Executive Vice President, General
    Counsel and Secretary of Inco Limited. He is also Chairman of the trustee of
    NAVF II Liquidating Trust, which owns 19 shares of Common Stock. Mr. Feiner
    disclaims beneficial ownership of the shares of Common Stock held by such
    shareholder. He was also granted non-qualified options to acquire an
    aggregate of 105,000 shares of Common Stock in each of July 1992,
    July 1996, December 1997, July 1998 and July 2000. Pursuant to such option
    grants, Mr. Feiner may directly acquire 97,500 shares of Common Stock within
    60 days after August 31, 2001. However, Mr. Feiner disclaims beneficial
    ownership of a portion of the options pursuant to an arrangement made
    between Mr. Feiner and Inco Limited, whereby Mr. Feiner assigned options to
    acquire a total of 53,334 shares of Common Stock to that entity.
 
(9) Consists of 28,334 shares of Common Stock which Ms. Ryan may acquire upon
    the exercise of options within 60 days after August 31, 2001.
 
(10) Includes 292,201 shares of Common Stock which Dr. Lambert may acquire upon
    the exercise of options within 60 days after August 31, 2001.
 
(11) See footnotes (3)-(10).
 
    INFORMATION ABOUT THE COMPANY'S DIRECTORS AND CURRENT EXECUTIVE OFFICERS
 
    Under the Company's By-Laws, the number of members of the Company's Board of
Directors is fixed from time to time by the shareholders, and Directors serve in
office until the next annual meeting of shareholders and until their successors
have been elected and qualified. The number of members of the Company's Board of
Directors is currently fixed at six. The nominees for the Board of Directors to
be elected at the Meeting are Mitchel Sayare, Ph.D., Walter A. Blattler, Ph.D.,
David W. Carter, Michael R. Eisenson, Stuart F. Feiner and Mark B. Skaletsky.
 
                                       4
<Page>
    Set forth below are the names of the persons nominated as Directors, their
ages, their offices in the Company, if any, their principal occupations or
employment for the past five years, the length of their tenure as Directors and
the names of other public companies in which such persons hold directorships.
 

<Table>
<Caption>
NAME OF DIRECTOR                AGE       PRINCIPAL OCCUPATIONS DURING AT LEAST THE LAST FIVE YEARS
----------------              --------    ---------------------------------------------------------
<S>                           <C>        <C>
Mitchel Sayare..............     52      Mitchel Sayare, Ph.D., Chief Executive Officer, a Director
                                         since 1986 and Chairman of the Board since 1989, joined the
                                         Company in 1986. From 1986 until 1992, and currently since
                                         1994, Mr. Sayare has served as President of the Company.
                                         From 1982 to 1985, Mr. Sayare was Vice President for
                                         Development at Xenogen, Inc., a biotechnology company
                                         specializing in monoclonal antibody-based diagnostic systems
                                         for cancer. From 1977 to 1982, Mr. Sayare was Assistant
                                         Professor of Biophysics and Biochemistry at the University
                                         of Connecticut. He holds a Ph.D. in Biochemistry from Temple
                                         University School of Medicine. Mr. Sayare serves on the
                                         Board of Directors of ImmuCell Corporation, in addition to a
                                         number of private companies.
 
Walter A. Blattler..........     52      Walter A. Blattler, Ph.D., elected a Director in
                                         September 1995, served as Vice President, Research and
                                         Development of the Company from 1987 to October 1994 and as
                                         the Company's Senior Vice President, Research and
                                         Development from October 1994 to October 1996. Since 1996,
                                         Dr. Blattler has served as the Company's Executive Vice
                                         President, Science and Technology. Dr. Blattler joined the
                                         Company in October 1987. From 1981 to 1987, Dr. Blattler was
                                         chief scientist for the ImmunoGen-supported research program
                                         at the Dana-Farber Cancer Institute. Dr. Blattler received
                                         his Ph.D. from the Swiss Federal Institute of Technology in
                                         Zurich in 1978.
 
David W. Carter.............     63      David W. Carter, a Director since June 1997, is Co-Chief
                                         Executive Officer and a Director of Xenogen, Inc., which he
                                         joined in 1997. From 1991 to 1997, Mr. Carter was the
                                         President and Chief Executive Officer of Somatix Therapy
                                         Corporation. Mr. Carter also serves on the Board of
                                         Directors of Cell Genesys, Inc.
 
Michael R. Eisenson.........     45      Michael R. Eisenson, a Director since 1986, is President and
                                         Chief Executive Officer of Charlesbank Capital Partners, LLC
                                         (the successor to Harvard Private Capital Group, Inc., which
                                         he joined in 1986). Between 1981 and 1986, Mr. Eisenson held
                                         the position of Manager, Boston Consulting Group.
                                         Mr. Eisenson serves on the Boards of Directors of CCC
                                         Information Services Group, Inc., Playtex Products, Inc.,
                                         and United Auto Group, Inc.
</Table>

 
                                       5
<Page>
 

<Table>
<Caption>
NAME OF DIRECTOR                AGE       PRINCIPAL OCCUPATIONS DURING AT LEAST THE LAST FIVE YEARS
----------------              --------    ---------------------------------------------------------
<S>                           <C>        <C>
Stuart F. Feiner............     53      Stuart F. Feiner, a Director since 1984, has been Executive
                                         Vice President, General Counsel and Secretary of Inco
                                         Limited since August 1993, after having served as Vice
                                         President, General Counsel and Secretary of Inco Limited
                                         from April 1992 to August 1993. From January 1984 until
                                         April 1992, Mr. Feiner was President of Inco Venture Capital
                                         Management, the venture capital unit of Inco Limited.
                                         Mr. Feiner serves on the Boards of Directors of certain
                                         private companies funded by Inco Venture Capital Management.
 
Mark B. Skaletsky...........     53      Mark B. Skaletsky, a Director since March 2000, has served
                                         as the Chairman of the Board and Chief Executive Officer of
                                         The Althexis Company since March 2001. Upon the completion
                                         of a merger between The Althexis Company and Microcide
                                         Pharmaceuticals, Inc., Mr. Skaletsky will be appointed
                                         President, Chairman of the Board and Chief Executive Officer
                                         of Microcide Pharmaceuticals, Inc. Prior to The Althexis
                                         Company, Mr. Skaletsky served as President, Chief Executive
                                         Officer, and a Director of GelTex Pharmaceuticals, Inc.
                                         from 1993 until its acquisition by Genzyme Corporation in
                                         2000. From 1988 to 1993, he was Chairman and Chief Executive
                                         Officer of Enzytech, Inc., a biotechnology company, and from
                                         1983 to 1988 he was President and Chief Operating Officer of
                                         Biogen, Inc., also a biotechnology company. He is a Director
                                         of Isis Pharmaceuticals, Inc.
</Table>

 
COMMITTEES OF THE BOARD OF DIRECTORS AND MEETINGS
 
    AUDIT COMMITTEE.  David W. Carter, Michael R. Eisenson, Stuart F. Feiner and
Mark B. Skaletsky currently serve on the Company's Audit Committee. The Audit
Committee reviews the engagement of the Company's independent accountants,
reviews quarterly and annual financial statements, considers matters relating to
accounting policy and internal controls and reviews the scope of annual audits.
 
    COMPENSATION COMMITTEE.  David W. Carter, Michael R. Eisenson, Stuart F.
Feiner, and Mark B. Skaletsky currently comprise the Compensation Committee. The
Compensation Committee reviews, approves and makes recommendations concerning
the Company's compensation policies, practices and procedures to ensure that the
legal and fiduciary responsibilities of the Board are carried out and that such
policies, practices and procedures contribute to the success of the Company. The
Compensation Committee also administers the Company's Restated Stock Option Plan
and will administer the 2001 Plan.
 
    NOMINATING COMMITTEE.  The Company does not have a standing Nominating
Committee.
 
    MEETING ATTENDANCE.  During the fiscal year ended June 30, 2001, there were
three meetings of the Board, three meetings of the Compensation Committee and
four meetings of the Audit Committee. The Compensation Committee met on the same
dates as the meetings of the Board of Directors. Further, from time to time, the
members of the Board and its Committees acted by unanimous written consent or
actions by the Directors without a meeting pursuant to Massachusetts law.
Messrs. Sayare, Eisenson, Feiner and Dr. Blattler attended all meetings of the
Board. Messrs. Eisenson and Feiner attended all of the meetings of the
Compensation Committee. Mr. Skaletsky attended two meetings each of the Board,
the Audit Committee and the Compensation Committee. Mr. Carter attended one
meeting each of the Board and
 
                                       6
<Page>
the Compensation Committee. Mr. Feiner attended all of the meetings of the Audit
Committee. Messrs. Eisenson and Carter attended three meetings of the Audit
Committee.
 
    COMPENSATION COMMITTEE INTERLOCKS AND INSIDER PARTICIPATION.  The
Compensation Committee members during fiscal year 2001 were Messrs. David W.
Carter, Michael R. Eisenson, Stuart F. Feiner, and Mark B. Skaletsky. None of
these Directors is or has been an officer or employee of the Company.
Mr. Sayare, though not a member of the Committee, assists the Committee in
determining any compensation to be awarded to executive officers other than
himself. Mr. Sayare provides supplemental information regarding performance
evaluations of executive officers other than himself.
 
COMPENSATION OF DIRECTORS
 
    Non-employee Directors are entitled to receive cash compensation of $1,500
per Board meeting attended, and, commencing in the quarter ended March 31, 2001,
a quarterly retainer of $3,000. Messrs. Feiner and Eisenson were paid $4,500 for
attendance at Board meetings; Mr. Skaletsky was paid $3,000 for attendance at
Board meetings and Mr. Carter was paid $1,500 for attendance at Board meetings
during 2001. In July 2001, the Company paid each non-employee Director $6,000,
in arrears, representing the third and fourth fiscal quarter 2001 retainers.
Directors are also reimbursed for travel expenses incurred with respect to
attending Board meetings. No compensation is paid for attendance at, or
activities related to, Audit or Compensation Committee meetings.
 
    Under the Company's Restated Stock Option Plan, each non-employee Director,
upon first being elected or appointed to the Board after July 9, 1992, and on
every fourth anniversary thereof (assuming he remains a non-employee Director),
receives options to purchase 10,000 shares of Common Stock. Further, the Board
may also vote, at its discretion, to issue additional options as deemed
appropriate. All options granted under the Plan have ten year terms and exercise
prices which are equal to the fair market value of the Common Stock on the date
of grant.
 
CURRENT EXECUTIVE OFFICERS
 
    The names of, and certain other information as of the date hereof regarding,
each current executive officer of the Company who is not also a member of the
Board is set forth below. Executive officers serve at the pleasure of the Board.
 

<Table>
<Caption>
NAME OF EXECUTIVE OFFICER       AGE            PRINCIPAL OCCUPATIONS DURING THE LAST FIVE YEARS
-------------------------     --------         ------------------------------------------------
<S>                           <C>        <C>
Gregg Beloff................     33      Gregg Beloff, Vice President, Finance and Chief Financial
                                         Officer, joined the Company in March 2001. From 1998 to 2001
                                         he was employed at Adams, Harkness & Hill, Inc., most
                                         recently as a Vice President in Investment Banking. From
                                         1993 to 1996 Mr. Beloff was employed as an attorney at the
                                         law firm of Gaffin & Krattenmaker, P.C. Mr. Beloff holds a
                                         Juris Doctorate from the University of Pittsburgh and a
                                         Masters of Business Administration from Carnegie Mellon
                                         University.
</Table>

 
                                       7
<Page>
 

<Table>
<Caption>
NAME OF EXECUTIVE OFFICER       AGE            PRINCIPAL OCCUPATIONS DURING THE LAST FIVE YEARS
-------------------------     --------         ------------------------------------------------
<S>                           <C>        <C>
John M. Lambert, Ph.D.......     50      John M. Lambert, Ph.D., Senior Vice President,
                                         Pharmaceutical Development, joined the Company in 1987.
                                         Dr. Lambert served as the Company's Senior Director of
                                         Research from October 1994 to November 1996. Prior to
                                         joining ImmunoGen, Dr. Lambert was Assistant Professor of
                                         Pathology at the Dana-Farber Cancer Institute, where he
                                         worked on the research program supported by ImmunoGen.
                                         Dr. Lambert received his Ph.D. in Biochemistry from
                                         Cambridge University in England.
 
Pauline Jen Ryan............     34      Pauline Jen Ryan, Vice President, Business Development,
                                         rejoined the Company in May 1999. From 1998 to 1999,
                                         Ms. Ryan was a Vice President of Capital Management
                                         Consulting, Inc., a biomedical consulting firm. From 1994 to
                                         1997, she was Director of Business Development of
                                         Organogenesis, Inc., a biotechnology company. Ms. Ryan holds
                                         a Masters of Business Administration from Northwestern
                                         University's Kellogg School of Management.
 
Virginia A. Lavery..........     37      Virginia Lavery, Senior Corporate Controller and Treasurer,
                                         joined the Company in December 2000. During 2000, Ms. Lavery
                                         was self-employed as a financial consultant. From 1999 to
                                         2000, Ms. Lavery was interim Chief Financial Officer of
                                         Dynamics Research Corporation, a publicly-traded government
                                         contractor, after having served as Corporate Controller
                                         since 1998. From 1989 to 1998, Ms. Lavery was a Certified
                                         Public Accountant with Arthur Andersen, LLP.
</Table>

 
                             EXECUTIVE COMPENSATION
 
    The following table (the "Summary Compensation Table") sets forth certain
information regarding compensation paid during each of the Company's last three
fiscal years to the Company's Chief Executive Officer and to each of the
Company's executive officers, other than the Chief Executive Officer, whose
total annual salary and bonus exceeded $100,000 in fiscal year 2001.
 
                                       8
<Page>
                           SUMMARY COMPENSATION TABLE
 

<Table>
<Caption>
                                                                                  LONG-TERM
                                                                                 COMPENSATION
                                                                                    AWARDS
                                                                                 ------------
                                                        ANNUAL COMPENSATION       SECURITIES     ALL OTHER
                                                     -------------------------    UNDERLYING    COMPENSATION
NAME AND PRINCIPAL POSITION                 YEAR     SALARY ($)(1)   BONUS ($)   OPTIONS (#)       ($)(2)
---------------------------               --------   -------------   ---------   ------------   ------------
<S>                                       <C>        <C>             <C>         <C>            <C>
Mitchel Sayare, Ph.D....................    2001        342,346       130,000       165,500         3,428
Chairman of the Board, Chief                2000        330,385            --        85,000         3,522
Executive Officer and President             1999        273,000            --        80,000         3,068
 
Walter A. Blattler, Ph.D................    2001        257,500        80,000       150,000         3,040
Director and Executive Vice                 2000        262,353            --        80,000         3,156
President, Science and Technology           1999        201,539            --        75,000         1,783
 
John M. Lambert, Ph.D...................    2001        207,375        50,000        94,500         3,039
Senior Vice President,                      2000        182,464            --        54,000         2,763
Pharmaceutical Development                  1999        167,019            --       141,667         1,329
 
Pauline Jen Ryan........................    2001        167,692        40,000        67,500         2,497
Vice President,                             2000        140,308            --        25,000         2,266
Business Development                        1999         13,846            --        40,000           128
</Table>

 
------------------------
 
(1) Includes amounts, if any, deferred by each individual under the
    ImmunoGen, Inc. 401(k) Plan and Trust (the "401(k) Plan").
 
(2) Fiscal year 2001 amounts include term life insurance premiums of $1,189 for
    Mr. Sayare, $916 for Dr. Blattler, $687 for Dr. Lambert and $558 for
    Ms. Ryan. Also included are matching contributions under the 401(k) Plan of
    $2,239 for Mr. Sayare, $2,124 for Dr. Blattler, $2,352 for Dr. Lambert and
    $1,939 for Ms. Ryan.
 
                                       9
<Page>
                       OPTION GRANTS IN LAST FISCAL YEAR
 
    The following table sets forth information regarding each stock option
granted during fiscal year 2001 to each individual named in the Summary
Compensation Table.
 

<Table>
<Caption>
                                              PERCENTAGE                                POTENTIAL REALIZABLE
                                NUMBER OF      OF TOTAL                                   VALUE AT ASSUMED
                                SECURITIES     OPTIONS                                  ANNUAL RATES OF STOCK
                                UNDERLYING    GRANTED TO                                 PRICE APPRECIATION
                                 OPTIONS     EMPLOYEES IN                                FOR OPTION TERM (3)
                                 GRANTED     FISCAL YEAR      EXERCISE     EXPIRATION   ---------------------
NAME                              (#)(1)         (%)        PRICE ($)(2)      DATE       5% ($)      10% ($)
----                            ----------   ------------   ------------   ----------   ---------   ---------
<S>                             <C>          <C>            <C>            <C>          <C>         <C>
Mitchel Sayare, Ph.D..........   165,500         15.7          20.75        01/25/11    2,159,703   5,473,111
Walter A. Blattler, Ph.D......   150,000         14.3          20.75        01/25/11    1,957,435   4,960,523
John M. Lambert, Ph.D.........    94,500          9.0          20.75        01/25/11    1,233,184   3,125,130
Pauline Jen Ryan..............    67,500          6.4          20.75        01/25/11     880,846    2,232,236
</Table>

 
--------------------------
 
(1) All options were granted on January 25, 2001 and vest ratably over three
    years beginning on the date of grant. Under certain circumstances, vesting
    of options may be accelerated and options may become fully exercisable.
 
(2) The exercise price was equal to the fair market value of the Common Stock on
    the date of grant.
 
(3) Amounts represent hypothetical gains that could be achieved for the
    respective options if exercised at the end of the option term. These gains
    are based on assumed rates of stock price appreciation of 5% and 10%
    compounded annually from the date the respective options were granted to
    their expiration date. The gains shown are net of the option exercise price,
    but do not include deductions for taxes or other expenses associated with
    the exercise. Actual gains, if any, on stock option exercises will depend on
    the future performance of the Common Stock, the optionee's continued
    employment through the option period and the date on which the options are
    exercised. These rates of appreciation are mandated by the rules of the
    Securities and Exchange Commission (the "Commission") and do not represent
    the Company's estimate or projection of future Common Stock prices.
 
              AGGREGATED OPTION EXERCISES IN LAST FISCAL YEAR AND
                         FISCAL YEAR-END OPTION VALUES
 
    The following table provides information as to each individual named in the
Summary Compensation Table regarding the exercise of options during the 2001
fiscal year. In addition, this table includes the number of shares covered by
both exercisable and unexercisable options as of June 30, 2001 and the value of
"in the money" options, which values represent the positive spread between the
exercise price of any such option and the fiscal year-end value of the Common
Stock.
 

<Table>
<Caption>
                                                             NUMBER OF SECURITIES
                                     SHARES                 UNDERLYING UNEXERCISED         VALUE OF UNEXERCISED
                                    ACQUIRED                   OPTIONS AT FISCAL           IN-THE-MONEY OPTIONS
                                       ON       VALUE            YEAR-END (#)            AT FISCAL YEAR-END ($)(2)
                                    EXERCISE   REALIZED   ---------------------------   ---------------------------
NAME                                  (#)       ($)(1)    EXERCISABLE   UNEXERCISABLE   EXERCISABLE   UNEXERCISABLE
----                                --------   --------   -----------   -------------   -----------   -------------
<S>                                 <C>        <C>        <C>           <C>             <C>           <C>
Mitchel Sayare, Ph.D.............    8,889     113,895      715,334        248,832      12,128,471      1,215,723
Walter A. Blattler, Ph.D.........       --          --      467,084        228,333       8,246,649      1,148,759
John M. Lambert, Ph.D............       --          --      292,201        159,166       5,167,100        936,378
Pauline Jen Ryan.................       --          --       28,334        104,166         460,807        570,948
</Table>

 
------------------------
 
(1) Amounts shown in this column do not necessarily represent actual value
    realized from the sale of the shares acquired upon exercise of the option
    because in many cases the shares are not sold upon
 
                                       10
<Page>
    exercise but continue to be held by the executive officer exercising the
    option. The amounts shown represent the difference between the option
    exercise price and the market price on the date of exercise, which is the
    amount that would have been realized if the shares had been sold immediately
    upon exercise.
 
(2) Value is based on the last sale price per share ($20.00) on June 29, 2001,
    as reported on the Nasdaq National Market, less the applicable option
    exercise price. Each option has an exercise price equal to the fair market
    value of the Common Stock on the date of grant.
 
                EMPLOYMENT CONTRACTS, TERMINATION OF EMPLOYMENT
                        AND CHANGE IN CONTROL AGREEMENTS
 
    The Company entered into employment agreements with Dr. Blattler,
Dr. Lambert, Mr. Beloff, Ms. Ryan and Ms. Lavery on their respective hire dates.
These agreements currently provide for annual salaries of $260,000 for
Dr. Blattler, $210,000 for Dr. Lambert, $175,000 for Mr. Beloff, $180,000 for
Ms. Ryan and $120,000 for Ms. Lavery. The Company or the executive officer may
terminate these agreements upon 90 days' prior written notice. In addition, each
agreement provides that the Company may terminate the employment of the
executive officer at any time for cause (as defined in the respective
agreements). The agreements with Dr. Blattler and Dr. Lambert also provide that
these executive officers will not engage in any business competitive with the
business of the Company for a period of two years following termination of
employment.
 
    In making determinations of salary and benefits to be provided under these
employment agreements, the Compensation Committee of the Board of Directors took
into consideration, in addition to contractual commitments, Company personnel
policies.
 
    The stock option agreements between the Company and members of its senior
management group provide that all unvested options of the affected employee will
become immediately exercisable in instances where:
 
    (i) (A)  a person becomes the beneficial owner of fifty percent or more of
             the voting securities of the Company, or
 
       (B)  the Board approves a consolidation or merger of the Company whereby
            the shareholders of the Company would not retain fifty percent or
            more of the voting securities of the Company after the consolidation
            or merger, and
 
    (ii) (A)  a material change is made in the Restated Stock Option Plan or an
              option granted thereunder (except as provided in Section 16(b) of
              the Plan), or
 
       (B)  the affected employee is, within two years, terminated for any
            reason other than for cause.
 
                                       11
<Page>
                         STOCK PRICE PERFORMANCE GRAPH
 
    The graph and table below compare the annual percentage change in the
Company's cumulative total stockholder return on its Common Stock for the period
from June 30, 1996 through June 30, 2001 (as measured by dividing (i) the sum of
(A) the cumulative amount of dividends for the measurement period, assuming
dividend reinvestment, and (B) the difference between the Company's share price
at the end and the beginning of the measurement period; by (ii) the share price
at the beginning of the measurement period) with the total cumulative return of
the Nasdaq Stock Market Index (U.S.) and the Nasdaq Pharmaceutical Stocks Total
Return Index during such period. The Company has not paid any dividends on the
Common Stock, and no dividends are included in the representation of the
Company's performance. The stock price performance shown in the graph below is
not necessarily indicative of future price performance. This graph is not
"soliciting material", is not deemed filed with the Commission and is not to be
incorporated by reference in any filing of the Company under the Securities Act
of 1933 or the Securities Exchange Act of 1934 whether made before or after the
date hereof and irrespective of any general incorporation language in any such
filing. Information used on the graph for the Nasdaq Pharmaceutical Stocks Total
Return Index and the Nasdaq Stock Market Index (U.S.) was prepared by the Center
for Research in Security Prices, a source believed to be reliable, but the
Company is not responsible for any errors or omissions in such information.
 
EDGAR REPRESENTATION OF DATA POINTS USED IN PRINTED GRAPHIC
 
Dollars
 

<Table>
<Caption>
                                   1996     1997     1998     1999     2000     2001
<S>                               <C>      <C>      <C>      <C>      <C>      <C>
IMMUNOGEN, INC.                   $100.00   $39.08   $43.75   $57.83  $301.58  $500.00
NASDAQ STOCK MARKET INDEX (U.S.)  $100.00  $121.60  $160.06  $230.22  $340.37  $184.51
NASDAQ PHARMACEUTICAL             $100.00  $101.37  $103.59  $145.33  $333.54  $280.67
</Table>

 
    The above graph and table assume $100 invested on June 30, 1996 with all
dividends reinvested, in each of the Common Stock, the Nasdaq Stock Market Index
(U.S.) and the Nasdaq Pharmaceutical Stocks Total Return Index. Upon written
request by any shareholder, the Company will promptly provide a list of the
companies comprising the Nasdaq Pharmaceutical Stocks Total Return Index.
 
                                       12
<Page>
           COMPENSATION COMMITTEE REPORT ON EXECUTIVE COMPENSATION(1)
 
    The Compensation Committee of the Board (the "Committee") is comprised
entirely of non-employee Directors. The Committee determines the base salaries
of the Company's executive officers and the amount of annual bonus awards, if
any, to be paid to the executive officers. In addition, the Committee
administers the Company's Restated Stock Option Plan, as amended (the "Plan"),
under which stock options may be granted to executive officers and other
employees of the Company, as well as to non-employee Directors.
 
COMPENSATION POLICY AND COMPONENTS OF COMPENSATION
 
    The Committee's fundamental executive compensation philosophy is to enable
the Company to attract and retain key executives, and to motivate those
executives to achieve the Company's long-term objective of becoming a profitable
company. Attracting and retaining key executives is important to any
organization. This challenge is especially difficult in the biotechnology
industry where executives may be required to remain focused and committed
throughout many years of product development and financial instability.
 
    Each executive officer's compensation package is reviewed at least annually
and may be comprised of up to three components: base salary, incentive cash
bonuses and stock options. In addition, the Company's executive officers are
eligible to participate in all employee benefit programs generally available to
all other ImmunoGen employees.
 
    Progress toward the Company's broad strategic goal of becoming a profitable
biopharmaceutical company is measured by specific corporate objectives and
annual milestones. Personal objectives and milestones by which individual
executives of the Company are evaluated fit within the framework of the
Company's overall goals and objectives. Factors such as changes in business
conditions and other relevant external circumstances are also taken into
consideration.
 
BASE SALARIES OF EXECUTIVE OFFICERS
 
    The Committee sets the salaries of the Company's executive officers by
reviewing independently-prepared surveys of biotechnology industry compensation
as well as other available information on the base salaries of executive
officers in comparable positions in other biotechnology companies. There is
substantial overlap between the biotechnology companies whose compensation
practices are reflected in such surveys and the biotechnology companies which
are included in the Nasdaq Pharmaceutical Stocks Total Return Index (see "Stock
Price Performance Graph"). Comparative factors considered include, but are not
limited to, company size, stage of development of a company's products, and
geographic location. The Committee uses the collected data as well as the
experience of the members of the Committee in hiring and managing personnel to
set salaries. The Committee also takes into account, for both current and new
executive officers, competitive industry factors, breadth of experience, length
of service and recent individual performance. It is not the Company's intent to
establish fixed levels of compensation in general or for specific positions, but
rather to establish compensation on a case-by-case basis as recommended by
 
------------------------
 
(1) The report of the Compensation Committee of the Board shall not be deemed
    incorporated by reference by any general statement incorporating by
    reference this Proxy Statement into any filing under the Securities Act of
    1933, as amended, or under the Securities Exchange Act of 1934, as amended,
    except to the extent that the Company specifically incorporates this report
    by reference.
 
                                       13
<Page>
management and confirmed by the Committee. The Company's executive officer
salaries, as currently paid, are estimated to range from the 45th percentile to
the 95th percentile of comparable biotechnology companies. The salary of the
Company's Chairman of the Board, President and Chief Executive Officer is
estimated to be in the 85th percentile of such range.
 
    In certain cases initial annual base compensation was established pursuant
to employment agreements with executive officers (see "Employment Contracts,
Termination of Employment and Change in Control Agreements"). The terms of such
employment contracts were reviewed and authorized by the Board (including
members of the Committee but excluding any interested officer) and were
consistent with the Company's compensation policies then in place.
 
    In October 2000, Dr. Blattler's base salary was set at $260,000 annually.
Dr. Lambert's base salary was set at $210,000 during January 2001 and
Ms. Ryan's base salary was set at $180,000 in February 2001. Mr. Beloff's base
salary was set at $175,000 in March 2001. Ms. Lavery's base salary was set at
$120,000 in December 2000.
 
BONUS AWARDS
 
    The Company does not have formal incentive or bonus plans for executives. In
January 2001, the Committee approved bonuses for all employees of the Company in
recognition that a number of key business objectives and milestones had been
achieved over the past year. Accordingly, pursuant to this approval, on
January 25, 2001, Dr. Blattler was awarded a bonus of $80,000, Dr. Lambert was
awarded a bonus of $50,000, Ms. Ryan was awarded a bonus of $40,000 and
Ms. Lavery was awarded a bonus of $7,200. For the two fiscal years ended
June 30, 2000, no bonuses had been awarded to any executive officer of the
Company.
 
STOCK OPTION PLAN
 
    Subject to the provisions of the Plan, the Committee has the authority to
determine the terms under which options are granted and the individuals to whom
such options may be granted. The Committee believes that equity participation is
a key component of its executive compensation program. The stock option program
is the Company's major long-term incentive plan, designed to retain executive
officers and other employees and motivate them to enhance shareholder value by
aligning the long-term interests of the Company's employees with those of its
shareholders. Stock options provide an effective long-term incentive for all
employees to create shareholder value since the full benefit of the options
cannot be realized unless an appreciation in the price of the Company's Common
Stock occurs. The executive officers participate in the Plan in the same manner
as all of the Company's full-time employees. Initial stock option awards for new
employees, which are individually determined prior to employment, are derived
from the employee's anticipated contribution to the Company's growth and are
designed to be competitive with awards granted by other biotechnology companies.
Subsequent annual stock option awards are based on historical levels of prior
grants, position within the Company and individual performance. For fiscal years
1999 and 2000, stock options were awarded to all current executive officers in
January 1999 and January 2000, respectively. For fiscal year 2001, stock options
were awarded to all current executive officers in January 2001. All options are
issued having exercise prices equal to the fair market value of the Company's
Common Stock on the date of grant. The options granted in fiscal years 1999,
2000 and 2001 vest as to one-third of the shares on each anniversary date of the
date of grant. Vesting of options may be accelerated and options may become
fully exercisable upon the occurrence of
 
                                       14
<Page>
certain events such as a change in control of the Company (see "Employment
Contracts, Termination of Employment and Change in Control Agreements").
 
    In addition to incentive stock options, the Committee also has discretionary
authority under the Plan to grant non-qualified options to certain individuals,
including executive officers of the Company. Of the current executive officers,
Mr. Sayare, Dr. Blattler, Dr. Lambert, Mr. Beloff, Ms. Ryan and Ms. Lavery have
been granted non-qualified options. With the exception of Mr. Beloff and
Ms. Lavery, in each case, the options were granted with exercise prices equal to
the fair market value of the Common Stock on the date of grant and vested over
three years. Mr. Beloff's and Ms. Lavery's non-qualified options were also
granted at fair market value and vest over four years.
 
COMPENSATION OF THE CHIEF EXECUTIVE OFFICER
 
    Mr. Sayare's annual base salary has been determined in accordance with the
criteria outlined in other sections of this report, the Committee's evaluation
of the Company's overall performance and Mr. Sayare's individual performance.
Performance was measured by the achievement of certain goals over the last
several years, including the consummation of several significant financing
transactions and the continued development of the Company's key technology
platforms. Mr. Sayare's salary was increased to $360,000 on February 1, 2001.
 
    In conjunction with the bonuses approved by the Committee in January 2001,
Mr. Sayare was paid a cash bonus of $130,000 in January 2001. No cash bonus was
paid to Mr. Sayare during fiscal years 1999 and 2000. In fiscal 1999,
Mr. Sayare was awarded options to purchase 80,000 shares of Common Stock. In
fiscal 2000, Mr. Sayare was awarded options to purchase 85,000 shares of Common
Stock. In fiscal 2001, Mr. Sayare was awarded options to purchase 165,500 shares
of Common Stock. The options granted to Mr. Sayare in fiscal years 1999, 2000
and 2001 vest as to one-third of the shares each year after issuance beginning
one year from the date of grant. All options are subject to Mr. Sayare's
continued employment with the Company and have exercise prices equal to the fair
market value of the Common Stock on the date of grant.
 
CERTAIN AGREEMENTS
 
    The Company has entered into agreements with certain of its executive
officers relating to employment and separation. In making determinations of
salary and benefits to be provided under these employment and separation
agreements, the Committee took into consideration contractual commitments and
Company policies. (See "Employment Contracts, Termination of Employment and
Change in Control Agreements.")
 
Members of the ImmunoGen, Inc. Compensation Committee
David W. Carter
Michael R. Eisenson
Stuart F. Feiner
Mark B. Skaletsky
 
                           REPORT OF AUDIT COMMITTEE
 
    The Audit Committee of the Board of Directors, which consists entirely of
directors who meet the independence and experience requirements of the Nasdaq
National Market, has furnished the following report.
 
                                       15
<Page>
    The Audit Committee assists the Board in overseeing and monitoring the
integrity of the Company's financial reporting process, its compliance with
legal and regulatory requirements and the quality of its internal and external
audit processes. The role and responsibilities of the Audit Committee are set
forth in a written Charter adopted by the Board, most recently amended in
August 2001, which is attached as Appendix A to this Proxy Statement (as so
amended, the "Charter"). As set forth in the Charter, management of the Company
is responsible for the preparation, presentation and integrity of the Company's
financial statements, the Company's accounting and financial reporting
principles and internal accounting controls designed to assure compliance with
accounting standards and applicable laws and regulations. The Company's
independent auditors are responsible for auditing the Company's financial
statements and expressing an opinion as to their conformity with generally
accepted accounting principles. The Audit Committee reviews and reassesses the
Charter annually and recommends any changes to the Board for approval.
 
    The Audit Committee is responsible for overseeing the Company's overall
financial reporting process. In fulfilling its responsibilities for the
financial statements for the fiscal year ended June 30, 2001, the Audit
Committee took the following actions:
 
    - Reviewed and discussed the audit plans, audit scope, identification of
      audit risks and the audited financial statements for the year ended
      June 30, 2001 with management and PricewaterhouseCoopers LLP, the
      Company's independent auditors;
 
    - Discussed with PricewaterhouseCoopers LLP the matters required to be
      discussed by Statement on Auditing Standards No. 61 relating to the
      conduct of the audit; and
 
    - Received written disclosures and the letter from PricewaterhouseCoopers
      LLP regarding its independence as required by Independence Standards Board
      Standard No. 1. The Audit Committee further discussed with
      PricewaterhouseCoopers LLP their independence. The Audit Committee
      considered, with a view to maintaining the independence of its independent
      auditors, the nature and scope of the non-audit services supplied to the
      Company by its independent auditors.
 
    - The Audit Committee also considered the status of pending litigation,
      taxation matters and other areas of oversight relating to the financial
      reporting and audit process that the Committee determined appropriate.
 
    The members of the Audit Committee are not professionally engaged in the
practice of auditing or accounting and are not employed by the Company for
accounting or financial management or for any aspects of the Company's systems
of internal accounting control. Members of the Audit Committee rely, without
independent verification, on the information provided to them and on the
representations made by management and the Company's independent auditors.
Accordingly, the Audit Committee's oversight does not provide an independent
basis to determine that management has maintained procedures that are designed
to assure compliance with accounting standards and applicable laws and
regulations. In addition, the Audit Committee's considerations and discussions
referred to above do not assure that the audit of the Company's financial
statements has been carried out in accordance with generally accepted auditing
standards.
 
    Based upon the reports, discussions and reviews described in this Report,
and subject to the limitations on the role and responsibilities of the Committee
referred to above and in the Charter of the Audit Committee, the Committee has
recommended to the Board and the Board has approved, that the
 
                                       16
<Page>
audited financial statements for the year ended June 30, 2001 be included in the
Company's Annual Report to Shareholders for the fiscal year ended June 30, 2001
and the Company's Annual Report on Form 10-K for the fiscal year ended June 30,
2001 to be filed with the Securities and Exchange Commission.
 
Members of the ImmunoGen, Inc. Audit Committee
David W. Carter
Michael R. Eisenson
Stuart F. Feiner, Chairman
Mark B. Skaletsky
 
            SECTION 16(a) BENEFICIAL OWNERSHIP REPORTING COMPLIANCE
 
    Section 16(a) of the Securities Exchange Act of 1934, as amended, requires
the Company's Directors and executive officers, and persons holding more than
10% of the Common Stock, to file with the Securities and Exchange Commission
(the "SEC") initial reports of ownership and reports of changes in ownership of
Common Stock. Executive officers, Directors and greater than 10% shareholders
are required by SEC regulation to furnish the Company with copies of all
Section 16(a) forms they file.
 
    To the Company's knowledge, based solely on review of the copies of such
reports furnished to it and written representations that no other reports were
required, during the fiscal year ended June 30, 2001, the Company's executive
officers, Directors and greater than 10% beneficial owners of its Common Stock
complied with all applicable Section 16(a) filing except that Form 5 reports
were filed late on behalf of Walter A. Blattler (reporting two transactions),
Stuart F. Feiner (reporting one transaction), John M. Lambert (reporting two
transactions), Pauline Jen Ryan (reporting two transactions), and a Form 3
report was filed late on behalf of Virginia A. Lavery (reporting two
transactions).
 
                              CERTAIN TRANSACTIONS
 
    The holder of approximately 533,841 shares of Common Stock (the "Registrable
Securities") is entitled to certain rights to register such shares under the
Securities Act of 1933, as amended, for sale to the public pursuant to a
Registration Rights Agreement by and among the Company and the holders of
Registrable Securities, as amended (the "Registration Rights Agreement"). The
holder of Registrable Securities is Aeneas Venture Corporation ("Aeneas").
Michael R. Eisenson, a Director of the Company, is President and Chief Executive
Officer of Charlesbank Capital Partners, LLC, the successor to Harvard Private
Capital Group, Inc. and the investment advisor to Aeneas. Aeneas has the right
to require the Company, on not more than two occasions, whether or not the
Company proposes to register any of its Common Stock for sale, to register all
or part of their shares for sale to the public under the Securities Act, subject
to certain conditions and limitations. In addition, Aeneas may require the
Company to register all or part of its shares on Form S-3 (or a successor short
form of registration) if the Company then qualifies for use of such form,
subject to certain conditions and limitations.
 

                              INDEPENDENT AUDITORS
 
    PricewaterhouseCoopers LLP, independent accountants, audited the Company's
financial statements for the fiscal year ended June 30, 2001. The Company
expects that representatives of PricewaterhouseCoopers LLP will be present at
the Meeting, with the opportunity to make a statement if they so desire, and
will be available to respond to appropriate questions.
 
                                       17
<Page>
AUDIT FEES
 
    The Company paid PricewaterhouseCoopers LLP a total of $70,372 for their
audit of the Company's annual financial statements for the fiscal year ended
June 30, 2001 and for their review of the Company's Quarterly Reports on
Form 10-Q filed during the last fiscal year.
 
FINANCIAL INFORMATION SYSTEMS DESIGN AND IMPLEMENTATION FEES
 
    PricewaterhouseCoopers LLP did not bill any fees for professional services
rendered to the Company for information technology services relating to
financial information systems design and implementation for the fiscal year
ended June 30, 2001.
 
ALL OTHER FEES
 
    During the Company's fiscal year ended June 30, 2001, the Company paid
PricewaterhouseCoopers LLP a total of $153,000 for their services provided in
connection with the Company's November 2000 public offering of Common Stock,
including reviews of the filing documents and issuance of their comfort letter
to the underwriters.
 
APPOINTMENT OF NEW AUDITORS
 
    On August 31, 2001, the Company engaged Ernst & Young as its independent
certifying accountants for the year ending June 30, 2002. In connection with
this engagement, the Company dismissed PricewaterhouseCoopers LLP, effective
upon completion of their audit of the Company's financial statements for the
year ended June 30, 2001. The engagement of Ernst & Young and the dismissal of
PricewaterhouseCoopers LLP was reviewed and recommended by the Audit Committee
and approved by the Board of Directors.
 
    The reports of PricewaterhouseCoopers LLP with respect to the Company for
fiscal years 1999 and 2000 contained no adverse opinion or disclaimer of opinion
and were not qualified or modified as to uncertainty, audit scope or accounting
principles. During fiscal years 1999 and 2000 and through August 31, 2001, there
were no disagreements between the Company and PricewaterhouseCoopers LLP on any
matter of accounting principles or practices, financial statement disclosure, or
auditing scope or procedure, which disagreements, if not resolved to the
satisfaction of PricewaterhouseCoopers LLP, would have caused that firm to make
reference to the subject matter of the disagreements in its report on the
Company's financial statements for such years.
 
    During fiscal years 1999 and 2000 and through August 31, 2001, the Company
did not consult with Ernst & Young regarding either the application of
accounting principles to a specified transaction, the type of audit opinion that
might be rendered on the Company's financial statements or any matter that was
the subject of a disagreement or reportable event with PricewaterhouseCoopers
LLP.
 
    Pursuant to Item 304(a)(3) the Company has requested that
PricewaterhouseCoopers LLP furnish it with a letter addressed to the SEC stating
whether or not it agrees with the above statements. A copy of such letter was
included as an exhibit to the Company's Current Report on Form 8-K filed on
September 7, 2001.

 
                                       18
<Page>
                             ELECTION OF DIRECTORS
                                (NOTICE ITEM 1)
 
    Under the Company's By-Laws, the number of Directors is fixed from time to
time by the shareholders, and Directors serve in office until the next Annual
Meeting and until their successors have been elected and qualified. At the
Meeting the number of Directors will be fixed at six and six Directors will be
elected.
 
    The enclosed Proxy, unless authority to vote is withheld, will be voted for
the election of the nominees named herein as Directors of the Company. The
nominees are Mitchel Sayare, Ph.D., Walter A. Blattler, Ph.D., David W. Carter,
Michael R. Eisenson, Stuart F. Feiner, and Mark B. Skaletsky. The Board has no
reason to believe that any nominee will become unavailable. However, in the
event that any one or more of such nominees shall unexpectedly become
unavailable for election, votes will be cast, pursuant to authority granted by
the enclosed Proxy, for such person or persons as may be designated by the
Board.
 
    A plurality of the votes cast at the Meeting is required to elect each
nominee as a Director.
 
 THE BOARD OF DIRECTORS RECOMMENDS THAT THE NUMBER OF DIRECTORS BE FIXED AT SIX
           AND RECOMMENDS THE ELECTION OF THE NOMINEES AS DIRECTORS.
 
AMENDMENT TO INCREASE THE AGGREGATE NUMBER OF SHARES FOR WHICH STOCK OPTIONS MAY
           BE GRANTED UNDER THE COMPANY'S RESTATED STOCK OPTION PLAN
                                (NOTICE ITEM 2)
 
    The Board recommends that the shareholders consider and approve a proposed
amendment to the Company's Restated Stock Option Plan (the "Plan") adopted by
the Board on April 26, 2001. For a description of the Plan, see "Summary of
Restated Stock Option Plan," below. The proposed amendment to the Plan would
increase the number of shares of Common Stock reserved for the grant of options
as described below.
 
    On April 26, 2001, the Board authorized, subject to shareholder approval, an
amendment to the Plan to increase the number of shares reserved for grants of
options under the Plan from 4,850,000 shares of Common Stock to 7,350,000 shares
of Common Stock. The Board also authorized on that date the grant of certain
options. Of the 7,350,000 shares authorized as of April 26, 2001, 4,918,747
shares were subject to outstanding options or had been exercised as of
August 22, 2001.
 
    The Board believes that having additional shares available for grants of
options under the Plan is both necessary and desirable in order to enable the
Company to continue to attract and retain qualified employees, consultants and
Directors.
 
    The following table shows the total number of stock option grants made under
the Plan to the identified individuals and groups as of August 22, 2001, which
grants are subject to the approval of the increase in the number of shares
reserved under the Plan by the shareholders.
 
                                       19
<Page>
                               NEW PLAN BENEFITS
                           RESTATED STOCK OPTION PLAN
 

<Table>
<Caption>
NAME AND POSITION                                 DOLLAR VALUE ($)   NUMBER OF UNITS
-----------------                                 ----------------   ---------------
<S>                                               <C>                <C>
Gregg Beloff, Chief Financial Officer and Vice
  President, Finance............................      1,050,000          75,000
Virginia A. Lavery, Senior Corporate Controller
  and Treasurer.................................        274,712          11,475
Current Executive Officers......................      1,324,712          86,475
Non-Executive Officer Employees.................        454,143          27,200
</Table>

 
                     SUMMARY OF RESTATED STOCK OPTION PLAN
 
    Under the Plan, incentive stock options, within the meaning of Section 422
of the Internal Revenue Code (the "Code"), may be granted to key employees of
the Company, and non-qualified options may be granted to key employees,
Directors and consultants of the Company. Approximately 85 persons are currently
eligible to participate in the Plan.
 
    The exercise price of incentive stock options granted under the Plan may not
be less than 100% of the fair market value of the Company's Common Stock on the
date of grant. However, in the case of incentive stock options granted under the
Plan to holders of more than 10% of the voting power of the Company, the
exercise price may not be less than 110% of the fair market value of the Common
Stock on the date of grant. The exercise price of non-qualified options must be
equal to at least the par value per share of Common Stock on the date of grant.
Options granted under the Plan may not be exercised later than 10 years from the
date of grant, except in the case of incentive stock options issued with an
exercise price of at least 110% of fair market value, which may not be exercised
later than five years from the date of grant. Options generally vest over
periods of up to four years, and are intended to create long-term incentive and
motivation for the Company's employees, Directors and consultants, as well as to
provide those persons with the perspective of the Company's shareholders in
assessing corporate results.
 
    The Plan provides that the aggregate fair market value of shares issuable
upon the exercise of incentive stock options exercisable for the first time
during any one calendar year may not exceed $100,000. Options granted under the
Plan become exercisable in installments over the option term. Options granted
under the Plan expire upon termination of an optionee's employment for cause.
Generally, options must be exercised within three months after termination of an
optionee's employment for any other reason (except death or disability), and
within one year after an optionee's death or disability, but in no event later
than the originally prescribed term of the option. The Plan terminates on
November 12, 2007, unless previously terminated by vote of the shareholders of
the Company. The Plan provides for termination of all options granted thereunder
in the event of dissolution or liquidation of the Company, subject to the right
of optionees (or optionees' survivors) to exercise all their options immediately
before such event, to the extent that the right to exercise such options has
accrued as of the date immediately prior to such dissolution or liquidation.
 
    The Plan also provides that, in the event of a consolidation of the Company
with, or acquisition of the Company by, another entity, or sale of all or
substantially all of the assets of the Company, the Board will make appropriate
provision for options granted pursuant to the Plan. The Directors will either
(i) continue options granted under the Plan (if necessary substituting rights to
shares of the successor entity), or (ii) require that all options granted
thereunder be exercised, or (iii) terminate all options in exchange for a fair
market value cash payment. If the Board should utilize either of the methods
described in (ii) or
 
                                       20
<Page>
(iii) above, all options granted pursuant to the Plan will be made fully
exercisable irrespective of original vesting schedules.
 
    The stock option agreements between the Company and members of its senior
management group provide that in instances where:
 
    (i) (A)  a person becomes the beneficial owner of fifty percent or more of
             the voting securities of the Company, or
 
       (B)  the Board approves a consolidation or merger of the Company whereby
            the shareholders of the Company would not retain fifty percent or
            more of the voting securities of the Company after the consolidation
            or merger, and
 
    (ii) (A)  a material change is made in the Plan or an option granted
              thereunder (except as provided in Section 16 (b) of the Plan), or
 
       (B)  the affected employee is, within two years, terminated for any
            reason other than for cause,
 
then all unvested options of the affected employee will become immediately
exercisable.
 
    The Plan is administered by the Committee. Subject to the provisions of the
Plan, the Committee has the authority to determine the terms under which options
are granted, including the individuals to whom such options may be granted, the
exercise price and number of shares subject to each option, the time or times
during which all or a portion of each option may be exercised and certain other
provisions. The Plan may be amended by the shareholders of the Company. The Plan
may also be amended by the Board or the Committee without approval by the
shareholders except to the extent that shareholder approval is required to
ensure favorable tax treatment under the Code for incentive stock options or in
order to ensure the qualification of the Plan under Rule 16b-3 promulgated under
the Securities Exchange Act of 1934. Any optionee who is adversely affected by
any such changes must consent thereto in writing.
 
    To date, all options have been granted with exercise prices equal to the
fair market value of the Company's Common Stock on the date of grant. As of
September 14, 2001, as reported by the Nasdaq National Market System, the
closing price for the Common Stock was $[PRICE].
 
                       FEDERAL INCOME TAX CONSIDERATIONS
 
    The following is a description of certain U.S. federal income tax
consequences of the issuance and exercise of options under the Plan:
 
    Options granted under the Plan may be either incentive stock options, or
ISOs, which satisfy the requirements of Section 422 of the Code or non-qualified
stock options, or NQSOs, which are not intended to meet such requirements. The
federal income tax treatment for the two types of options differs as follows:
 
    ISOS.  No taxable income is recognized by the optionee at the time of the
    option grant, and no taxable income is generally recognized at the time the
    option is exercised. The optionee will, however, recognize taxable income in
    the year in which the purchased shares are sold or otherwise made the
    subject of a taxable disposition. For federal tax purposes, dispositions are
    divided into two categories: qualifying and disqualifying. A qualifying
    disposition occurs if the sale or other disposition is made after the
    optionee has held the shares for more than two years after the option grant
    date and more than one year after the exercise date. If either of these two
    holding periods is not satisfied, then a disqualifying disposition will
    result.
 
                                       21
<Page>
    Upon a qualifying disposition of the shares, the optionee will recognize
    long-term capital gain in an amount equal to the excess of the amount
    realized upon the sale or other disposition of the purchased shares over the
    exercise price paid for those shares. If there is a disqualifying
    disposition of the shares, then the excess of the fair market value of the
    shares on the exercise date (or the amount realized on a disqualifying sale,
    if less) over the exercise price paid for those shares will be taxable as
    ordinary income to the optionee; any additional gain or loss recognized upon
    the disposition will be taxable as a capital gain or loss.
 
    NON-QUALIFIED OPTIONS.  No taxable income is recognized by an optionee upon
    the grant of a non-statutory option. The optionee will in general recognize
    ordinary income, in the year in which the option is exercised, equal to the
    excess of the fair market value of the purchased shares on the exercise date
    over the exercise price paid for the shares, and the optionee will be
    required to satisfy the tax withholding requirements applicable to such
    income.
 
    The Company will generally be entitled to an income tax deduction equal to
the amount of ordinary income recognized by the optionee with respect to the
exercise of a non-statutory option or the disqualifying disposition of an
incentive stock option. The deduction will in general be allowed for the taxable
year of the Company in which such ordinary income is recognized by the optionee.
 
    The affirmative vote of a majority of the shares present or represented and
entitled to vote at the Meeting is required to approve the increase in the
aggregate number of shares of Common Stock available under the Plan.
 
THE BOARD OF DIRECTORS RECOMMENDS A VOTE "FOR" APPROVAL OF THE AMENDMENT TO THE
                          RESTATED STOCK OPTION PLAN.
 
        ADOPTION OF THE COMPANY'S 2001 NON-EMPLOYEE DIRECTOR STOCK PLAN
       AND THE RESERVATION OF 50,000 SHARES OF COMMON STOCK FOR ISSUANCE
                              UNDER THE 2001 PLAN
                                (NOTICE ITEM 3)
 
                                    GENERAL
 
    The Company's 2001 Non-Employee Director Stock Plan (the "2001 Plan") was
adopted by the Company's Board on August 31, 2001 with 50,000 shares of Common
Stock reserved for issuance under the 2001 Plan. The 2001 Plan provides for the
grant of cash, stock and stock units to non-employee Directors of the Company as
compensation for their service as Directors of the Company. The purpose of the
2001 Plan is to encourage ownership of shares of common stock by non-employee
Directors in order to attract such Directors and induce them to work for the
benefit of the Company. Subject to the provisions of the 2001 Plan, the Board,
or its appointed administrator (the "Administrator"), may make determinations as
to which non-employee Directors are to receive stock and the terms of such
grants. The Board believes that the adoption of the 2001 Plan is advisable to
give the Company the flexibility needed to attract, retain and motivate
non-employee Directors. Only non-employee Directors at the time of the grant are
eligible to participate in the 2001 Plan.
 
    As of September 14, 2001, no shares of Common Stock had been granted under
the 2001 Plan. The number of shares issuable pursuant to the 2001 Plan may be
increased at any time and from time to time by the Board or Administrator with
the approval of the shareholders.
 
                                       22
<Page>
                       MATERIAL FEATURES OF THE 2001 PLAN
 
    The 2001 Plan provides that non-employee Directors may elect to receive all
or a portion of their compensation for their service as Directors in the form of
cash, shares of Common Stock or in the form of stock units. Stock units are
equivalent in value to a share of Common Stock, and are credited by means of a
bookkeeping entry in the books of the Company.
 
    During the course of a year, the number of stock units to be credited in the
books of the Company to the account of a participant in the 2001 Plan will be
obtained by dividing the total dollar amount that the participant elects to be
paid in stock units under the 2001 Plan by the fair market value of the Common
Stock on the last date of the Company's fiscal quarter in which the award is
made.
 
    Stock awards under the 2001 Plan are not transferable by the participant
other than by will or by the laws of descent and distribution, or as otherwise
determined by the Administrator and set forth in the agreement relating to an
award. In the event of the termination of a participant's service as a
non-employee Director, the participant shall receive a lump sum payment in cash
equal to the number of stock units recorded in the participant's account on date
of termination of service as a director multiplied by the fair market value of
the shares at the time of termination.
 
    Appropriate adjustments will be made to the terms of the awards given under
the 2001 Plan in the event of a stock dividend, stock split, merger or
consolidation or other similar significant corporate event involving the
Company.
 
                       FEDERAL INCOME TAX CONSIDERATIONS
 
    The following is a description of certain U.S. federal income tax
consequences of the issuance of stock and stock units under the 2001 Plan.
 
    RECEIPT OF STOCK.  The recipient of a stock award will generally be subject
to tax at ordinary income rates on the fair market value of any Common Stock
issued under the award, and the Company will generally be entitled to a
deduction equal to the amount of ordinary income recognized by the recipient.
The capital gain or loss holding period for any Common Stock distributed under a
stock award will begin on the date when the recipient recognizes ordinary income
in respect of that distribution.
 
    RECEIPT OF STOCK UNITS.  The recipient of a stock award who elects to
receive a share unit under the 2001 Plan will generally be subject to tax at
ordinary income rates at the time that cash is actually or constructively
received by them under the 2001 Plan. The Company generally will be entitled to
a deduction in the same year that the income is recognized by the participant
equal to the amount that is taxable as ordinary income to the director.
 
    The affirmative vote of a majority of the shares present or represented and
entitled to vote at the Meeting is required to approve the adoption of the
Company's 2001 Plan and the reservation of 50,000 shares of Common Stock which
may be granted under the 2001 Plan.
 
     THE BOARD OF DIRECTORS RECOMMENDS APPROVAL OF THE ADOPTION OF THE 2001
NON-EMPLOYEE DIRECTOR STOCK PLAN AND THE RESERVATION OF 50,000 SHARES OF COMMON
                STOCK WHICH MAY BE GRANTED UNDER THE 2001 PLAN.
 
                                       23
<Page>
AMENDMENT OF THE COMPANY'S ARTICLES OF ORGANIZATION TO INCREASE FROM 50,000,000
   SHARES TO 75,000,000 SHARES THE AGGREGATE NUMBER OF SHARES OF COMMON STOCK
                     AUTHORIZED TO BE ISSUED BY THE COMPANY
                                (NOTICE ITEM 4)
 
    The Board of Directors has determined that it is advisable to increase the
Company's authorized Common Stock from 50,000,000 shares to 75,000,000 shares,
and has voted to recommend that the Shareholders adopt an amendment to the
Company's Articles of Organization effecting the proposed increase.
 
    As of August 22, 2001, approximately 38,600,000 shares of Common Stock were
issued and outstanding (excluding treasury shares) and approximately an
additional 7,200,000 shares of Common Stock were reserved for issuance upon the
conversion of existing securities and exercise of rights granted under the
Company's stock and stock option plans. With 50,000,000 shares of Common Stock
currently authorized, a total of approximately 4,200,000 shares of Common Stock
are available for future issuance. In the event that Notice Item 2 and Notice
Item 3 are approved by the shareholders, an additional 2,550,000 shares of
Common Stock will be reserved for the exercise of options or stock grants under
the Company's stock option and Director Stock Plans, thereby reducing the shares
available for future issuance to 1,600,000.
 
    The Board of Directors believes it continues to be in the best interest of
the Company to have sufficient additional authorized but unissued shares of
Common Stock available in order to provide flexibility for corporate action in
the future. Management believes that the availability of additional authorized
shares for issuance from time to time in the Board of Directors' discretion in
connection with possible acquisitions of other companies, future financings,
investment opportunities, stock splits or dividends or for other corporate
purposes is desirable in order to avoid repeated separate amendments to the
Company's Articles of Organization and the delay and expense incurred in holding
special meetings of the shareholders to approve such amendments. There are at
present no specific understandings, arrangements or agreements with respect to
any future acquisitions that would require the Company to issue a material
amount of new shares of its Common Stock. The Board of Directors believes that
the currently available unissued shares do not provide sufficient flexibility
for corporate action in the future.
 
    No further authorization by vote of the shareholders will be solicited for
the issuance of the additional shares of Common Stock proposed to be authorized,
except as might be required by law, regulatory authorities or rules of the
Nasdaq National Market or any other stock exchange on which the Company's shares
may then be listed. The issuance of additional shares of Common Stock could have
the effect of diluting existing shareholder earnings per share, book value per
share and voting power. The shareholders of the Company do not have any
preemptive right to purchase or subscribe for any part of any new or additional
issuance of the Company's securities.
 
    The affirmative vote of a majority of the outstanding shares of Common Stock
entitled to vote at the Meeting is required to approve the amendment to the
Company's Articles of Organization to effect the proposed increase in the
Company's authorized shares.
 
       THE BOARD OF DIRECTORS RECOMMENDS A VOTE TO APPROVE THE AMENDMENT
      TO THE COMPANY'S ARTICLES OF ORGANIZATION, AND PROXIES SOLICITED BY
         THE BOARD WILL BE VOTED IN FAVOR THEREOF UNLESS A STOCKHOLDER
                     HAS INDICATED OTHERWISE ON THE PROXY.
 
                                       24
<Page>
                    SHAREHOLDER PROPOSALS AND OTHER MATTERS
 
    In order to be considered for inclusion in the proxy statement distributed
to shareholders prior to the Company's annual meeting in 2002, the Company must
receive a shareholder proposal no later than June 14, 2002. Proposals should be
delivered in writing to ImmunoGen, Inc., 128 Sidney Street, Cambridge,
Massachusetts 02139. One or more shareholders who hold at least a one-tenth part
in interest of the capital stock entitled to vote at the meeting and who do not
wish to include their proposal in the Company's proxy statement but who wish to
present a proposal at the Company's annual meeting of shareholders in 2002, must
notify the Company in writing at the above-referenced address no later than
October 24, 2002. All other shareholders who wish to present a proposal at such
annual meeting, but who do not wish to include their proposals in the Company's
proxy statement, must notify the Company in writing at the above-referenced
address no later than August 28, 2002 in order for their proposal to be
considered timely for purposes of Rule 14a-4 under the Securities Exchange Act
of 1934, as amended. Pursuant to Rule 14a-4, management proxies may confer
discretionary authority to vote on any matters presented by a shareholder at the
Meeting if the proposals are received by the Company after August 28, 2002.
 
    The Board does not know of any other matters which will be brought before
the Meeting. If other business is properly presented for consideration at the
Meeting, it is intended that the shares represented by the enclosed Proxy will
be voted in accordance with their judgment on such matters.
 
    In order that your shares may be represented if you do not plan to attend
the Meeting, and in order to assure the required quorum, please complete, sign,
date and return your Proxy promptly.
 
    The Company's Annual Report on Form 10-K for the fiscal year ended June 30,
2001 (other than exhibits attached thereto) filed by the Company with the
Commission, which provides additional information about the Company, is
available to beneficial owners of the Company's Common Stock without charge upon
written or oral request to the Company's Investor Relations Department at 128
Sidney Street, Cambridge, Massachusetts 02139 (tel: 617-995-2500).
 
                                          By order of the Board of Directors
                                          JONATHAN L. KRAVETZ, ESQ.
                                          Clerk
 
            , 2001
 
                                       25
<Page>
                                IMMUNOGEN, INC.
     CHARTER OF THE AUDIT COMMITTEE OF THE BOARD OF DIRECTORS, AS AMENDED*
 
I.  PURPOSE OF THE AUDIT COMMITTEE
 
    The purpose of the Audit Committee is to provide assistance to the Board of
Directors (the "Board") of ImmunoGen, Inc. (the "Company") in fulfilling its
oversight responsibilities relating to (i) corporate accounting principles,
policies and procedures, (ii) financial reporting practices and procedures,
(iii) the adequacy of the systems of internal accounting control, and (iv) the
quality and integrity of the financial information and reports of the Company.
To accomplish this purpose, the Audit Committee's primary duties and
responsibilities are to:
 
    1.  assist the Board in overseeing the Company's accounting and financial
       reporting principles, policies and procedures and internal accounting
       controls and procedures, all of which are designed to assure compliance
       with accounting standards and applicable laws and regulations;
 
    2.  assist the Board in overseeing the Company's financial statements and
       the independent audit thereof;
 
    3.  assist the Board in selecting the outside auditors to be retained (or
       nominate the outside auditors to be proposed for shareholder approval in
       any proxy statement), evaluate and, when deemed appropriate, report to
       the Board for their approval the replacement of the outside auditors; and
 
    4.  evaluate the independence of the outside auditors.
 
    The function of the Audit Committee is oversight. In fulfilling its role, it
is recognized that the members of the Audit Committee are not full-time
employees of the Company and are not, and do not represent themselves to be,
accountants or auditors by profession or experts in the fields of accounting or
auditing, including, but not limited to, in respect of the independence of
independent auditors. As such, it is not the duty or the responsibility of the
Audit Committee or any of its members to conduct any type of audit or accounting
review or procedure or set auditor independence standards. Each member of the
Audit Committee shall be entitled to reasonably rely on (i) the integrity of
those persons and organizations within and outside the Company from whom it
receives information, (ii) the accuracy of the financial and other information
provided to the Audit Committee by such persons or organizations absent actual
knowledge to the contrary (in which event such occurrence shall be promptly
reported to the Board) and (iii) representations made by the management of the
Company as to any information technology, internal audit and other non-audit
services provided by the outside auditors of the Company.
 
    The management of the Company is responsible for the preparation,
presentation and integrity of the Company's financial statements. Management is
also responsible for establishing and maintaining the appropriate accounting and
financial reporting principles and policies and internal controls and procedures
designed to assure compliance with accounting and independence standards and all
applicable laws and regulations. The outside auditors are responsible for
planning and carrying out a proper audit and reviews, including reviews of the
Company's quarterly financial statements prior to the filing of each quarterly
report on Form 10-Q with regulatory bodies and stock exchanges or similar bodies
and other procedures and in connection with the Company's annual financial
statements.
 
    The outside auditors for the Company are ultimately accountable to the Board
and to the Audit Committee.
 
                                      A-1
<Page>
II.  COMPOSITION OF THE AUDIT COMMITTEE
 
    The Audit Committee shall be comprised of at least three Directors, as
determined by the Board. Each member must be independent of the Company and its
subsidiaries; that is, no member of the Audit Committee may be an officer or
employee of the Company or its subsidiaries, and each member shall be free of
any relationship with the Company or its subsidiaries which, in the opinion of
the Board, would interfere with the exercise of independent judgment in carrying
out the responsibilities of a Director. In addition, each member must otherwise
satisfy the applicable independence and financial and other requisite expertise
requirements under the applicable rules of the National Association of
Securities Dealers, Inc. or any other applicable securities exchange or other
regulatory body.
 
    The members of the Audit Committee shall be elected by the Board annually.
The Chairperson shall be selected by the Board from among its members, and
either the Board or the Chairperson shall periodically review the performance of
each member of the Audit Committee to assess his or her effectiveness as an
Audit Committee member.
 
III.  MEETINGS OF THE AUDIT COMMITTEE
 
    The Audit Committee shall meet at least four times annually, or more
frequently as circumstances dictate, to meet its oversight responsibility,
including discussing with management the annual audited financial statements,
the quarterly financial statements or any other matters which the Audit
Committee deems necessary to carry out its duties and responsibilities. The
Audit Committee may request any officer or employee of the Company or the
Company's outside counsel or outside auditors to attend a meeting of the Audit
Committee or to meet with any members of, or consultants to, the Audit
Committee. In addition, as part of its job to foster open communication, the
Audit Committee should meet at least annually with management and the outside
auditors separately to discuss any matters that the Audit Committee or any of
these groups believe should be discussed privately.
 
    Members of the Audit Committee may participate in a meeting of the Audit
Committee by means of a conference call or other communications equipment by
means of which all persons participating in the meeting can hear each other.
 
IV.  DUTIES AND POWERS OF THE AUDIT COMMITTEE
 
    To carry out its purposes, the Audit Committee shall have the following
duties and powers:
 
    1.  WITH RESPECT TO THE OUTSIDE AUDITORS
 
       (a) to review the performance of the outside auditors and to make
           recommendations to the Board regarding the appointment or termination
           of the outside auditors;
 
       (b) to review the nature and scope of all audit and non-audit services of
           the outside auditors and the fees charged by the outside auditors for
           audit and non-audit services;
 
       (c) to ensure that the outside auditors prepare and deliver annually a
           formal, written statement (the "Statement") delineating all
           relationships between such auditors and the Company consistent with
           Independence Standards Board Standard No. 1 (it being understood that
           the outside auditors are responsible for the accuracy and
           completeness of this Statement), addressing at least the matters set
           forth in such Standard No. 1 and any non-audit services
 
                                      A-2
<Page>
           provided to the Company and any similar standards or other
           requirements applicable to such auditors;
 
       (d) to review and discuss with the outside auditors any relationships or
           services disclosed in the Statement that may impact the objectivity
           and independence of the outside auditors and to recommend that the
           Board take appropriate action in response to this Statement to
           satisfy itself of the outside auditors' independence and objectivity;
           in connection herewith, the outside auditors shall also submit to the
           Company annually a formal written statement of the fees billed for
           each of the following categories of services rendered by the outside
           auditors: (a) the audit of the Company's annual financial statements
           for the most recent fiscal year and the reviews of the financial
           statements included in the Company's quarterly reports on Form 10-Q
           for that fiscal year; (b) information technology consulting services
           for the most recent fiscal year, in the aggregate and by each
           service, separately identifying fees for such services relating to
           financial information systems design and implementation; and (c) all
           other services rendered by the outside auditors for the most recent
           fiscal year, in the aggregate and by each service; the Committee
           shall, if applicable, also consider whether the outside auditors'
           provision of the services referred to in (b) and/or (c) above or
           other non-audit services to the Company is compatible with
           maintaining the independence of the outside auditors; and
 
       (e) to instruct the outside auditors that the outside auditors are
           ultimately accountable to the Board and the Audit Committee.
 
    2.  WITH RESPECT TO DOCUMENTS/REPORTS REVIEW
 
       (a) to review and assess, at least annually, the adequacy of this
           Charter, and to make recommendations to the Board, as conditions
           dictate, to amend or update this Charter;
 
       (b) to review with management and the outside auditors the Company's
           annual financial statements, including a discussion with the outside
           auditors of the matters required to be discussed by Statement of
           Auditing Standards No. 61, COMMUNICATION WITH AUDIT COMMITTEES ("SAS
           No. 61"), and any similar standards or other requirements applicable
           to such auditors; and
 
       (c) to require the outside auditors to conduct an interim financial
           review, in accordance with Statement of Auditing Standards No. 71,
           INTERIM FINANCIAL INFORMATION, prior to filing by the Company of its
           quarterly report on Form 10-Q, and to discuss with the outside
           auditors, when needed as determined by SAS No. 61, matters relevant
           to the Form 10-Q prior to the Company's filing of such document or
           prior to the release of earnings. The Chairperson of the Audit
           Committee, or any individual member selected by the Audit Committee,
           may represent the entire Audit Committee for purposes of this
           paragraph.
 
    3.  WITH RESPECT TO THE FINANCIAL REPORTING PROCESS AND INTERNAL ACCOUNTING
       CONTROLS AND PROCEDURES
 
       (a) to advise management and the outside auditors that they are expected
           to provide to the Audit Committee a timely analysis of significant
           financial reporting issues and practices;
 
       (b) to consider any reports or communications (and management's responses
           thereto) submitted to the Audit Committee by the outside auditors
           required by or referred to in SAS 61, as may be modified or
           supplemented;
 
                                      A-3
<Page>
       (c) to review, in consultation with management and the outside auditors,
           the integrity of the Company's financial reporting processes, both
           internal and external;
 
       (d) to establish a regular system of reporting to the Audit Committee by
           management and the outside auditors regarding any significant
           judgments made in management's preparation of the financial
           statements and any significant difficulties encountered during the
           course of the review or audit, including any restrictions on the
           scope of work or access to required information;
 
       (e) to review any significant disagreement between management and the
           outside auditors in connection with the preparation of the financial
           statements;
 
       (f) during its regularly scheduled meetings, to meet separately with the
           outside auditors;
 
       (g) prior to the commencement thereof, to review the planned scope of the
           annual examination to be undertaken by the outside auditors as well
           as the results of such examination and the recommendations of the
           outside auditors;
 
       (h) prior to presentation to the Board, to review the annual financial
           statements (including the notes thereto) and the outside auditors'
           opinion to be included in the Company's Annual Report to
           Shareholders, Annual Report on Form 10-K and any other filings with
           regulatory bodies or stock exchanges; and
 
       (i) to obtain the outside auditors' assurance that the audit was
           conducted in a manner consistent with Section 10A of the Securities
           Exchange Act of 1934, as amended, and all applicable rules and
           regulations thereunder, which sets forth certain procedures to be
           followed in any audit of financial statements required under such
           rules and regulations.
 
    4.  WITH RESPECT TO LEGAL COMPLIANCE/GENERAL
 
       (a) to review, with the Company's counsel, any legal matter that could
           have a significant impact on the Company's financial statements;
 
       (b) to report through the Chairperson or other Audit Committee member to
           the Board following meetings of the Audit Committee; and
 
       (c) to maintain minutes or other records of meetings and activities of
           the Audit Committee.
 
V.  RESOURCES AND AUTHORITY OF THE AUDIT COMMITTEE
 
    The Audit Committee shall have the resources and authority appropriate to
fully discharge its functions, duties and responsibilities, including the
authority to engage outside auditors for special audits, reviews and other
procedures and to retain special counsel and other experts or consultants.
 
                                      A-4

<PAGE>

                                 IMMUNOGEN, INC.
                           RESTATED STOCK OPTION PLAN
                    (as amended through November 13, 2001)


<PAGE>

                                 IMMUNOGEN, INC.
                           RESTATED STOCK OPTION PLAN
                    (as amended through November 13, 2001)

1.   DEFINITIONS AND PURPOSES.

     A.  Definitions

     Unless otherwise specified or unless the context otherwise requires, the
following terms, as used in this Restated Stock Option Plan, have the following
meanings:

         1.   Administrator means the Board of Directors, unless it has
              delegated power to act on its behalf to a committee. (See Article
              3)

         2.   Affiliate means a corporation which, for purposes of Section 424
              of the Code, is a parent or subsidiary of the Company, direct or
              indirect.

         3.   Board of Directors means the Board of Directors of the Company.

         4.   Code means the United States Internal Revenue Code of 1986, as
              amended.

         5.   Committee means the Committee to which the Board of Directors has
              delegated power to act under or pursuant to the provisions of the
              Plan.

         6.   Company means ImmunoGen, Inc., a Massachusetts corporation.

         7.   Disability or Disabled means permanent and total disability as
              defined in Section 22(e)(3) of the Code.

         8.   Fair Market Value of a Share of Common Stock means:

              a.   If such Shares are then listed on any national securities
                   exchange, the fair market value shall be the mean between the
                   high and low sales prices, if any, on the largest such
                   exchange on the date of the grant of the Option, or, if none,
                   on the most recent trade date thirty (30) days or less prior
                   to the date of the grant of the Option;


                                       1


<PAGE>


              b.   If the Shares are not then listed on any such exchange, the
                   fair market value of such Shares shall be the last sale
                   price, if any, as reported in the National Association of
                   Securities Dealers Automated Quotation System (NASDAQ) for
                   the date of the grant of the Options, or if none, for the
                   most recent trade date thirty (30) days or less prior to the
                   date of the grant of the Option for which such last sale
                   price is reported;

              c.   If the Shares are not then either listed on any such exchange
                   or quoted in NASDAQ, the fair market value shall be the mean
                   between the average of the "Bid" and the average of the "Ask"
                   prices, if any, as reported in the National Daily Quotation
                   Service for the date of the grant of the option, or, if none,
                   for the most recent trade date thirty (30) days or less prior
                   to the date of the grant of the Option for which such
                   quotations are reported; and

              d.   If the market value cannot be determined under the preceding
                   three paragraphs, it shall be determined in good faith by the
                   Board of Directors.

         9.   ISO means an option meant to qualify as an incentive stock option
              under Code Section 422.

         10.  Key Employee means an employee of the Company or of an Affiliate
              (including, without limitation, an employee who is also serving as
              an officer or director of the Company or of an Affiliate),
              designated by the Administrator to be eligible to be granted one
              or more Options under the Plan.

         11.  Non-Qualified Option means an option which is not intended to
              qualify as an ISO.

         12.  Option means an ISO or Non-Qualified Option granted under the
              Plan.


                                       2


<PAGE>

         13.  Option Agreement means an agreement between the Company and a
              Participant executed and delivered pursuant to the Plan, in such
              form as the Administrator shall approve.

         14.  Participant means a Key Employee, director or consultant to whom
              one or more Options are granted under the Plan.

         15.  Participant's Survivors means a deceased Participant's legal
              representatives and/or any person or persons who acquired the
              Participant's rights to an Option by will or by the laws of
              descent and distribution.

         16.  Plan means this Restated Stock Option Plan.

         17.  Shares means shares of the common stock, $.01 par value, of the
              Company ("Common Stock") as to which Options have been or may be
              granted under the Plan or any shares of capital stock into which
              the Shares are changed or for which they are exchanged within the
              provisions of Article 2 of the Plan. The shares issued upon
              exercise of Options granted under the Plan may be authorized and
              unissued shares or shares held by the Company in its treasury, or
              both.

     B.  Purposes of the Plan

     The Plan is intended to encourage ownership of Shares by Key Employees,
non-employee directors and certain consultants of the Company in order to
attract such people, to induce them to work for the benefit of the Company or of
an Affiliate and to provide additional incentive for them to promote the success
of the Company or of an Affiliate. The Plan provides for the issuance of ISOs
and Non-Qualified Options. The Plan shall be treated as an amendment to and
restatement of the Company's 1986 Incentive Stock Option Plan. As amended and
restated the Plan shall apply to ISOs issued by the Company on or after the date
of such amendment of the Plan, but the Plan as so amended shall apply to any ISO
issued prior to such amendment if and only to the extent that the Incentive
Stock Option Agreement pursuant to which such ISO was granted is amended in
writing to adopt the amended terms of the Plan.


                                       3


<PAGE>

2.   SHARES SUBJECT TO THE PLAN.

     The number of Shares subject to this Plan as to which Options may be
granted from time to time shall be 7,350,000, or the equivalent of such number
of Shares after the Administrator, in its sole discretion, has interpreted the
effect of any stock split, stock dividend, combination, recapitalization or
similar transaction effected after such date.

     If an Option ceases to be "outstanding," in whole or in part, the Shares
which were subject to such Option shall be available for the granting of other
Options under the Plan. Any Option shall be treated as "outstanding" until such
Option is exercised in full, or terminates or expires under the provisions of
the Plan, or by agreement of the parties to the pertinent Option Agreement.

3.   ADMINISTRATION OF THE PLAN.

     The Administrator of the Plan will be the Board of Directors, except to the
extent the Board of Directors delegates its authority to a Committee of the
Board of Directors. The Plan is intended to comply with Rule 16b-3 or its
successors, promulgated pursuant to Section 16 of the Securities Exchange Act of
1934, as amended (the "1934 Act"), with respect to Participants who are subject
to Section 16 of the 1934 Act, and any provision in this Plan with respect to
such persons contrary to Rule 16b-3 shall be deemed null and void to the extent
permissible by law and deemed appropriate by the Administrator. Subject to the
provisions of the Plan, the Administrator is authorized to:

              a.   interpret the provisions of the Plan or of any Option or
                   Option Agreement and to make all rules and determinations
                   which it deems necessary or advisable for the administration
                   of the Plan;

              b.   determine which employees of the Company or of an Affiliate
                   shall be designated as Key Employees and which of the Key
                   Employees, directors and consultants shall be granted
                   Options;

              c.   determine the number of Shares for which an Option or Options
                   shall be granted; and


                                       4


<PAGE>

              d.   specify the terms and conditions upon which an Option or
                   Options may be granted; provided, however, that all
                   such interpretations, rules, determinations, terms and
                   conditions shall be made and prescribed in the context of
                   preserving the tax status under Code Section 422 of those
                   Options which are designated as ISOs. Subject to the
                   foregoing, the interpretation and construction by the
                   Administrator of any provisions of the Plan or of any Option
                   granted under it shall be final, unless otherwise determined
                   by the Board of Directors, if the Administrator is other than
                   the Board of Directors.

4.   ELIGIBILITY FOR PARTICIPATION.

     The Administrator will, in its sole discretion, name the Participants in
the Plan, provided, however, that each Participant must be a Key Employee,
director or consultant of the Company or of an Affiliate at the time an Option
is granted. Notwithstanding any of the foregoing provisions, the Administrator
may authorize the grant of an Option to a person not then an employee, director
or consultant of the Company or of an Affiliate. The actual grant of such
Option, however, shall be conditioned upon such person becoming eligible to
become a Participant at or prior to the time of the execution of the Option
Agreement evidencing such Option. ISOs may be granted only to Key Employees.
Non-Qualified Options may be granted to any Key Employee, director or consultant
of the Company or an Affiliate. Granting of any Option to any individual shall
neither entitle that individual to, nor disqualify him or her from,
participation in any other grant of Options.

5.   TERMS AND CONDITIONS OF OPTIONS.

     Each Option shall be set forth in an Option Agreement, duly executed by the
Company and by the Participant. The Option Agreements, which may be changed in
the Administrator's discretion for any particular Participant (provided that any
change in the Incentive Stock Option Agreement is not inconsistent with Code
Section 422), shall be subject to the following terms and conditions:

     Non-Qualified Options: Each Option intended to be a Non-Qualified Option
shall be subject to the terms and conditions which the Administrator determines
to be appropriate and in the best interest of the Company, subject to the
following minimum


                                       5


<PAGE>

standards for any such Non-Qualified Option:

              a.   The Option Agreement shall be in writing in the form approved
                   by the Administrator, with such modifications to such form as
                   the Administrator shall approve;

              b.   Option Price: The option price (per share) of the Shares
                   covered by each Option shall be determined by the
                   Administrator but shall not be less than the par value per
                   share of the Shares on the date of the grant of the Option.

              c.   Each Option Agreement shall state the number of Shares to
                   which it pertains; and

              d.   Each Option Agreement shall state the date on which it first
                   is exercisable and the date after which it may no longer be
                   exercised. Except as otherwise determined by the
                   Administrator, each Option granted hereunder shall become
                   cumulatively exercisable in four (4) equal annual
                   installments of twenty-five percent (25%) each, commencing on
                   the first anniversary date of the Option Agreement executed
                   by the Company and the Participant with respect to such
                   Option, and continuing on each of the next three (3)
                   anniversary dates.

              e.   Each Option shall terminate not more than 10 (ten) years from
                   the date of grant thereof or at such earlier time as the
                   Option Agreement may provide.

              f.   Directors' Options: Each director of the Company who is not
                   an employee of the Company or any Affiliate, upon first being
                   elected or appointed to the Board of Directors after July 9,
                   1992, and upon every fourth anniversary thereof provided that
                   on such dates such director has been in the continued and
                   uninterrupted service of the Company as a director since his
                   or her election or appointment and is a director and is not
                   an employee of the Company at such times, shall be granted a
                   Non-Qualified Option to purchase 10,000 Shares. Any
                   non-employee director serving in office on July 9, 1992, who
                   has been a member of 


                                       6


<PAGE>

                   the Board of Directors prior to such date shall be granted on
                   such date and upon every fourth anniversary thereof, a
                   Non-Qualified Option to purchase 10,000 Shares, provided that
                   on such date such director has been in the continued and
                   uninterrupted service of the Company as a director since July
                   9, 1992, and is a director and is not an employee of the
                   Company at such time. Each such Option shall (i) have an
                   exercise price equal to the Fair Market Value (per share) of
                   the Shares on the date of grant of the Option, (ii) have a
                   term of ten (10) years, and (iii) shall become cumulatively
                   exercisable in four (4) equal annual installments of
                   twenty-five percent (25%) each, upon completion of one full
                   year of service on the Board of Directors after the date of
                   grant, and continuing on each of the next three (3) full
                   years of service thereafter. Each director of the Company who
                   is not an employee of the Company or any Affiliate, upon
                   first being elected or appointed to the Board of Directors
                   after July 29, 1998, shall be granted, in addition to the
                   Non-Qualified Option to purchase 10,000 Shares as previously
                   described in this subparagraph (f), a Non-Qualified Option to
                   purchase 50,000 Shares. Such Option shall (i) have an
                   exercise price equal to the Fair Market Value (per share) of
                   the Shares on the date of grant of the Option, (ii) have a
                   term of ten (10) years, and (iii) shall become cumulatively
                   exercisable in three (3) equal annual installments of
                   one-third each, upon completion of one full year of service
                   on the Board of Directors after the date of grant, and
                   continuing on each of the next two (2) full years of service
                   thereafter. Any director entitled to receive an Option grant
                   under this subparagraph (f) may elect to decline the Option.
                   Notwithstanding the provisions of Paragraph 23 concerning
                   amendment of the Plan, the provisions of this subparagraph
                   (f) shall not be amended more than once every six months,
                   other than to comport with changes in the Code, the Employee
                   Retirement Income Security Act, or the rules thereunder. The
                   provisions of Articles 9, 10, 11 and 12 below shall not apply
                   to Options granted pursuant to this subparagraph (f).


                                       7


<PAGE>

     ISOs: Each Option intended to be an ISO shall be issued only to a Key
Employee and be subject to at least the following terms and conditions, with
such additional restrictions or changes as the Administrator determines are
appropriate but not in conflict with Code Section 422 and relevant regulations
and rulings of the Internal Revenue Service:

              a.   Minimum standards: The ISO shall meet the minimum standards
                   for Non-Qualified Options, as described above, except clauses
                   (a), (b) and (e) thereunder.

              b.   Option Agreement: The Option Agreement for an ISO shall be in
                   writing in substantially the form as approved by the
                   Administrator, with such changes to such form as the
                   Administrator shall approve, provided any changes are not
                   inconsistent with Code Section 422.

              c.   Option Price: Immediately before the Option is granted, if
                   the Participant owns, directly or by reason of the applicable
                   attribution rules in Code Section 424(d):

                   i.   Ten percent (10%) or less of the total combined voting
                        power of all classes of share capital of the Company or
                        an Affiliate, the Option price per share of the Shares
                        covered by each Option shall not be less than one
                        hundred percent (100%) of the Fair Market Value per
                        share of the Shares on the date of the grant of the
                        Option;

                   ii.  More than ten percent (10%) of the total combined voting
                        power of all classes of share capital of the Company or
                        an Affiliate, the Option price per share of the Shares
                        covered by each Option shall be not less than one
                        hundred ten percent (110%) of the said Fair Market Value
                        on the date of grant.


                                       8


<PAGE>

              d.   Term of Option: For Participants who own:

                   i.   Ten percent (10%) or less of the total combined voting
                        power of all classes of share capital of the Company or
                        an Affiliate, each Option shall terminate not more than
                        ten (10) years from the date of the grant or at such
                        earlier time as the Option Agreement may provide;

                   ii.  More than ten percent (10%) of the total combined voting
                        power of all classes of share capital of the Company or
                        an Affiliate, each Option shall terminate not more than
                        five (5) years from the date of the grant or at such
                        earlier time as the Option Agreement may provide.

              e.   Limitation on Yearly Exercise: The Option Agreements shall
                   restrict the amount of Options which may be exercisable in
                   any calendar year (under this or any other ISO plan of the
                   Company or an Affiliate) so that the aggregate Fair Market
                   Value (determined at the time each ISO is granted) of the
                   stock with respect to which ISOs are exercisable for the
                   first time by the Participant in any calendar year does not
                   exceed one hundred thousand dollars ($100,000), provided that
                   this subparagraph (e) shall have no force or effect if its
                   inclusion in the Plan is not necessary for Options issued as
                   ISOs to qualify as ISOs pursuant to Section 422(d) of the
                   Code.

              f.   Limitation on Grant of ISOs: No ISOs shall be granted after
                   the expiration of the earlier of ten (10) years from the date
                   of the adoption of the Plan by the Company or the approval of
                   the Plan by the shareholders of the Company.

6.   EXERCISE OF OPTION AND ISSUANCE OF SHARES.

     An Option (or any part or installment thereof) shall be exercised by giving
written notice to the Company at its principal office address, together with
provision for payment of the full purchase price in accordance with this
paragraph for the shares as to which such Option is being exercised, and upon
compliance with any other condition(s) set forth in the Option Agreement. Such
written notice shall be signed by the person


                                       9


<PAGE>

exercising the Option, shall state the number of Shares with respect to which
the Option is being exercised and shall contain any representation required by
the Plan or the Option Agreement. Payment of the purchase price for the shares
as to which such Option is being exercised shall be made (a) in United States
dollars in cash or by check, or (b) at the discretion of the Administrator,
through delivery of shares of Common Stock having a fair market value equal as
of the date of the exercise to the cash exercise price of the Option, determined
in good faith by the Board of Directors of the Company, (c) at the discretion of
the Administrator, by delivery of the grantee's personal recourse note bearing
interest payable not less than annually at no less than 100% of the applicable
Federal rate, as defined in Section 1274(d) of the Code, (d) at the discretion
of the Administrator, in accordance with a cashless exercise program established
with a securities brokerage firm, and approved by the Administrator, or (e) at
the discretion of the Administrator, by any combination of (a), (b), (c) and (d)
above. Notwithstanding the foregoing, the Administrator shall accept only such
payment on exercise of an ISO as is permitted by Section 422 of the Code.

     The Company shall then reasonably promptly deliver the Shares as to which
such Option was exercised to the Participant (or to the Participant's Survivors,
as the case may be). In determining what constitutes "reasonably promptly," it
is expressly understood that the delivery of the Shares may be delayed by the
Company in order to comply with any law or regulation which requires the Company
to take any action with respect to the Shares prior to their issuance. The
Shares shall, upon delivery, be evidenced by an appropriate certificate or
certificates for paid-up, non-assessable Shares.

     The Administrator shall have the right to accelerate the date of exercise
of any installment of any Option, provided that the Administrator shall not
accelerate the exercise date of any installment of any Option granted to any Key
Employee as an ISO (and not previously converted into a Non-Qualified Option
pursuant to Article 18) if such acceleration would violate the annual vesting
limitation contained in Section 422(d) of the Code, as described in paragraph
6(e).

7.   RIGHTS AS A SHAREHOLDER.

     No Participant to whom an Option has been granted shall have rights as a
shareholder with respect to any Shares covered by such Option, except after due
exercise of the Option and


                                       10


<PAGE>

provision for payment of the full purchase price for the Shares being purchased
pursuant to such exercise.

8.   ASSIGNABILITY AND TRANSFERABILITY OF OPTIONS.

     By its terms, an Option granted to a Participant shall not be transferable
by the Participant other than by will or by the laws of descent and distribution
or pursuant to a qualified domestic relations order as defined by the Code or
Title I of the Employee Retirement Income Security Act or the rules thereunder,
and shall be exercisable, during the Participant's lifetime, only by such
Participant (or by his or her legal representative). Such Option shall not be
assigned, pledged or hypothecated in any way (whether by operation of law or
otherwise) and shall not be subject to execution, attachment or similar process.
Any attempted transfer, assignment, pledge, hypothecation or other disposition
of any Option or of any rights granted thereunder contrary to the provisions of
this Plan, or the levy of any attachment or similar process upon an Option,
shall be null and void.

9.  EFFECT OF TERMINATION OF SERVICE OTHER THAN "FOR CAUSE".

     Except as otherwise provided in the pertinent Option Agreement, in the
event of a termination of service (whether as an employee or consultant) before
the Participant has exercised all Options, the following rules apply:

              a.   A Participant who ceases to be an employee or consultant of
                   the Company or of an Affiliate (for any reason other than
                   termination "for cause," Disability, or death for which
                   events there are special rules in Articles 10, 11, and 12,
                   respectively), may exercise any Option granted to him or her
                   to the extent that the right to purchase Shares has accrued
                   on the date of such termination of service, but only within
                   such term as the Administrator has designated in the
                   pertinent Option Agreement.

              b.   In no event may an Option Agreement provide, if the Option is
                   intended to be an ISO, that the time for exercise be later
                   than three (3) months after the Participant's termination of
                   employment.


                                       11


<PAGE>

              c.   The provisions of this paragraph, and not the provisions of
                   Article 11 or 12, shall apply to a Participant who
                   subsequently becomes disabled or dies after the termination
                   of employment or consultancy, provided, however, in the case
                   of a Participant's death, the Participant's survivors may
                   exercise the Option within six (6) months after the date of
                   the Participant's death, but in no event after the date of
                   expiration of the term of the Option.

              d.   Notwithstanding anything herein to the contrary, if
                   subsequent to a Participant's termination of employment or
                   consultancy, but prior to the exercise of an Option, the
                   Board of Directors determines that, either prior or
                   subsequent to the Participant's termination, the Participant
                   engaged in conduct which would constitute "cause," then such
                   Participant shall forthwith cease to have any right to
                   exercise any Option.

              e.   A Participant to whom an Option has been granted under the
                   Plan who is absent from work with the Company or with an
                   Affiliate because of temporary disability (any disability
                   other than a permanent and total Disability as defined in
                   Article 1 hereof), or who is on leave of absence for any
                   purpose, shall not, during the period of any such absence, be
                   deemed, by virtue of such absence alone, to have terminated
                   such Participant's employment or consultancy with the Company
                   or with an Affiliate, except as the Administrator may
                   otherwise expressly provide.

              f.   Options granted under the Plan shall not be affected by any
                   change of employment or other service within or among the
                   Company and any Affiliates so long as the Participant
                   continues to be an employee or consultant of the Company or
                   any Affiliate, provided, however, if a Participant's
                   employment by either the Company or an Affiliate should cease
                   (other than to become an employee of an Affiliate or the
                   Company), such termination shall affect the Participant's
                   rights under any Option granted to such Participant in
                   accordance with the terms of the Plan and the pertinent
                   Option Agreement.


                                       12


<PAGE>

10.  EFFECT OF TERMINATION OF SERVICE "FOR CAUSE".

     Except as otherwise provided in the pertinent Option Agreement, the
following rules apply if the Participant's service (whether as an employee or
consultant) is terminated "for cause" prior to the time that all of his or her
outstanding Options have been exercised:

              a.   All outstanding and unexercised Options as of the date the
                   Participant is notified his or her service is terminated "for
                   cause" will immediately be forfeited, unless the Option
                   Agreement provides otherwise.

              b.   For purposes of this Article, "cause" shall include (and is
                   not limited to) dishonesty with respect to the employer,
                   insubordination, substantial malfeasance or nonfeasance of
                   duty, unauthorized disclosure of confidential information,
                   and conduct substantially prejudicial to the business of the
                   Company or any Affiliate. The determination of the
                   Administrator as to the existence of cause will be conclusive
                   on the Participant and the Company.

              c.   "Cause" is not limited to events which have occurred prior to
                   a Participant's termination of service, nor is it necessary
                   that the Administrator's finding of "cause" occur prior to
                   termination. If the Administrator determines, subsequent to a
                   Participant's termination of service but prior to the
                   exercise of an Option, that either prior or subsequent to the
                   Participant's termination the Participant engaged in conduct
                   which would constitute "cause," then the right to exercise
                   any Option is forfeited.

              d.   Any definition in an agreement between the Participant and
                   the Company or an Affiliate which contains a conflicting
                   definition of "cause" for termination and which is in effect
                   at the time of such termination shall supersede the
                   definition in this Plan with respect to such Participant.


                                       13


<PAGE>

11.  EFFECT OF TERMINATION OF SERVICE FOR DISABILITY.

     Except as otherwise provided in the pertinent Option Agreement, a 
Participant who ceases to be an employee of or consultant to the Company or 
of an Affiliate by reason of Disability may exercise any Option granted to 
such Participant:

              a.   to the extent that the right to purchase Shares has accrued
                   on the date of his Disability; and

              b.   in the event rights to exercise the Option accrue
                   periodically, to the extent of a pro rata portion of any
                   additional rights as would have accrued had the Participant
                   not become Disabled prior to the end of the accrual period
                   which next ends following the date of Disability. The
                   proration shall be based upon the number of days of such
                   accrual period prior to the date of Disability.

     A Disabled Participant may exercise such rights only within a period of not
more than one (1) year after the date that the Participant became Disabled or,
if earlier, within the originally prescribed term of the Option.

     The Administrator shall make the determination both of whether Disability
has occurred and the date of its occurrence (unless a procedure for such
determination is set forth in another agreement between the Company and such
Participant, in which case such procedure shall be used for such determination).
If requested, the Participant shall be examined by a physician selected or
approved by the Administrator, the cost of which examination shall be paid for
by the Company.

12.  EFFECT OF DEATH WHILE AN EMPLOYEE OR CONSULTANT.

     Except as otherwise provided in the pertinent Option Agreement, in the
event of the death of a Participant to whom an Option has been granted while the
Participant is an employee or consultant of the Company or of an Affiliate, such
Option may be exercised by the Participant's Survivors:

              a.   to the extent exercisable but not exercised on the date of
                   death; and

              b.   in the case of an Option, in the event rights to exercise the
                   Option accrue periodically, to the extent of a pro rata
                   portion of such rights based upon the number of days prior to
                   the Participant's death and during the accrual period which
                   next ends following the date of death.


                                       14


<PAGE>

     If the Participant's Survivors wish to exercise the Option, they must take
all necessary steps to exercise the Option within one (1) year after the date of
death of such Participant, notwithstanding that the decedent might have been
able to exercise the Option as to some or all of the Shares on a later date if
he or she had not died and had continued to be an employee or consultant or, if
earlier, within the originally prescribed term of the Option.

13.  TERMINATION OF DIRECTORS' OPTION RIGHTS.

     Except as otherwise provided in the pertinent Option Agreement, if a
director who receives Options pursuant to Article 5, subparagraph (f):

              a.   ceases to be a member of the Board of Directors of the
                   Company for any reason other than death or Disability, any
                   then unexercised Options granted to such Director may be
                   exercised by the director within a period of ninety (90) days
                   after the date the director ceases to be a member of the
                   Board of Directors, but only to the extent of the number of
                   shares with respect to which the Options are exercisable on
                   the date the director ceases to be a member of the Board of
                   Directors, and in no event later than the expiration date of
                   the Option; or

              b.   ceases to be a member of the Board of Directors of the
                   Company by reason of his or her death or Disability, any then
                   unexercised Options granted to such Director may be exercised
                   by the director (or by the director's personal
                   representative, heir or legatee, in the event of death)
                   within a period of one hundred eighty (180) days after the
                   date the director ceases to be a member of the Board of
                   Directors, but only to the extent of the number of Shares
                   with respect to which the Options are exercisable on the date
                   the director ceases to be a member of the Board of Directors,
                   and in no event later than the expiration date of the Option.


                                       15


<PAGE>

14.  PURCHASE FOR INVESTMENT.

     Unless the offering and sale of the Shares to be issued upon the 
particular exercise of an Option shall have been effectively registered under 
the Securities Act of 1933, as now in force or hereafter amended (the "Act"), 
the Company shall be under no obligation to issue the Shares covered by such 
exercise unless and until the following conditions have been fulfilled:

              a.   The person(s) who exercise such Option shall warrant to the
                   Company, prior to receipt of the Shares, that such person(s)
                   are acquiring such Shares for their own respective accounts,
                   for investment, and not with a view to, or for sale in
                   connection with, the distribution of any such Shares, in
                   which event the person(s) acquiring such Shares shall be
                   bound by the provisions of the following legend which shall
                   be endorsed upon the certificate(s) evidencing their Shares
                   issued pursuant to such exercise or such grant:

                   "The shares represented by this certificate have been taken 
                   for investment and they may not be sold or otherwise 
                   transferred by any person, including a pledgee, in the 
                   absence of an effective registration statement of the shares
                   under the Securities Act of 1933 or an opinion of counsel 
                   satisfactory to the Company that an exemption from 
                   registration is then available."

              b.   The Company shall have received an opinion of its counsel
                   that the Shares may be issued upon such particular exercise
                   in compliance with the Act without registration thereunder.

     The Company may delay issuance of the Shares until completion of any action
or obtaining of any consent which the Company deems necessary under any
applicable law (including, without limitation, state securities or "blue
sky"laws).

15.  DISSOLUTION OR LIQUIDATION OF THE COMPANY.

     Upon the dissolution or liquidation of the Company, all Options granted 
under this Plan which as of such date shall not have been exercised will 
terminate and become null and void; provided, however, that if the rights of 
a Participant or a Participant's Survivors have not otherwise terminated and 
expired, the Participant or the Participant's Survivors will have the right 
immediately prior to such dissolution or liquidation to exercise any Option 
to the extent that the right to purchase Shares has accrued under the Plan as 
of the date immediately prior to such dissolution or liquidation.

                                       16


<PAGE>


16.  ADJUSTMENTS.

     Upon the occurrence of any of the following events, a Participant's rights
with respect to any Option granted to him or her hereunder which have not
previously been exercised in full shall be adjusted as hereinafter provided,
unless otherwise specifically provided in the written agreement between the
optionee and the Company relating to such Option:

              a.   Stock Dividends and Stock Splits. If the shares of Common
                   Stock shall be subdivided or combined into a greater or
                   smaller number of shares or if the Company shall issue any
                   shares of Common Stock as a stock dividend on its outstanding
                   Common Stock, the number of shares of Common Stock
                   deliverable upon the exercise of such Option shall be
                   appropriately increased or decreased proportionately, and
                   appropriate adjustments shall be made in the purchase price
                   per share to reflect such subdivision, combination or stock
                   dividend.

              b.   Consolidations or Mergers. If the Company is to be
                   consolidated with or acquired by another entity in a merger,
                   sale of all or substantially all of the Company's assets or
                   any similar transaction or transaction having the same effect
                   (an "Acquisition"), the Compensation Committee of the Board
                   of Directors of the Company, prior to consummation of such
                   Acquisition shall, as to outstanding Options, either (i) make
                   appropriate provisions, if necessary, for the continuation of
                   such Options by substituting on an equitable basis for the
                   shares then subject to such Options (A) the securities of any
                   successor or acquiring entity, or (B) other consideration
                   payable with respect to the outstanding shares of Common
                   Stock in connection with the Acquisition; or (ii) upon
                   written notice to the optionees, provide that all Options
                   must be exercised (all Options being made fully exercisable
                   for purposes of this subsection) within one year of the date
                   of such notice, at the end of which period the Options shall
                   terminate; or (iii) terminate all Options 


                                       17


<PAGE>

                   in exchange for a cash payment equal to the excess of the
                   fair market value of the shares subject to such Options (all
                   Options being made fully exercisable for purposes of this
                   subsection) over the exercise price thereof.

              c.   Recapitalization or Reorganization. In the event of a
                   recapitalization or reorganization of the Company (other than
                   a transaction described in subparagraph B above) pursuant to
                   which securities of the Company or of another corporation are
                   issued with respect to the outstanding shares of Common
                   Stock, an optionee upon exercising an Option shall be
                   entitled to receive for the purchase price paid upon such
                   exercise the securities he or she would have received if he
                   or she had exercised such Option prior to such
                   recapitalization or reorganization.

              d.   Modification of ISOs. Notwithstanding the foregoing, any
                   adjustments made pursuant to subparagraphs A, B or C with
                   respect to ISOs shall be made only after the Administrator,
                   after consulting with counsel for the Company, determines
                   whether such adjustment would constitute a "modification" of
                   such ISOs (as that term is defined in Section 424(h) of the
                   Code) or would cause any adverse tax consequences for the
                   holders of such ISOs. If the Administrator determines that
                   such adjustments made with respect to ISOs would constitute a
                   modification of such ISOs, it may refrain from making such
                   adjustments, unless the holder of an ISO specifically
                   requests in writing that such adjustment be made and such
                   writing indicates that the holder has full knowledge of the
                   consequences of such "modification" on his or her income tax
                   treatment with respect to the ISO.

17.  ISSUANCES OF SECURITIES.

     Except as expressly provided herein, no issuance by the Company of 
shares of stock of any class, or securities convertible into shares of stock 
of any class, shall affect, and no adjustment by reason thereof shall be made 
with respect to, the number or price of shares subject to Options. Except as 
expressly provided herein, no adjustments shall be made for dividends paid in 
cash or in property (including without limitation, securities) of the Company.

                                       18


<PAGE>


18.  FRACTIONAL SHARES.

     No fractional share shall be issued under the Plan and the person
exercising such right shall receive from the Company cash in lieu of such
fractional share equal to the Fair Market Value thereof.

19.  CONVERSION OF ISOS INTO NON-QUALIFIED OPTIONS:
     TERMINATION OF ISOS.

     The Administrator, at the written request of any optionee, may in its
discretion take such actions as may be necessary to convert such optionee's ISOs
(or any installments or portions of installments thereof) that have not been
exercised on the date of conversion into Non-Qualified Options at any time prior
to the expiration of such ISOs, regardless of whether the optionee is an
employee of the Company or an Affiliate at the time of such conversion. Such
actions may include, but not be limited to, extending the exercise period or
reducing the exercise price of the appropriate installments of such Options. At
the time of such conversion, the Administrator (with the consent of the
optionee) may impose such conditions on the exercise of the resulting
Non-Qualified Options as the Administrator in its discretion may determine,
provided that such conditions shall not be inconsistent with this Plan. Nothing
in the Plan shall be deemed to give any optionee the right to have such
optionee's ISO's converted into Non-Qualified Options, and no such conversion
shall occur until and unless the Administrator takes appropriate action. The
Administrator, with the consent of the optionee, may also terminate any portion
of any ISO that has not been exercised at the time of such termination.

20.  WITHHOLDING.

     Upon the exercise of a Non-Qualified Option for less than its fair market
value, the making of a Disqualifying Disposition (as defined in paragraph 21) or
the vesting of restricted Common Stock acquired on the exercise of an Option
hereunder, the Company may withhold from the optionee's wages, if any, or other
remuneration, or may require the optionee to pay additional federal, state, and
local income tax withholding and employee contributions to employment taxes in
respect of the amount that is considered compensation includible in such
person's gross income. The Administrator in its discretion may condition


                                       19


<PAGE>

the exercise of an Option for less than its fair market value or the vesting of
restricted Common Stock acquired by exercising an Option on the grantee's
payment of such additional income tax withholding and employee contributions to
employment taxes.

21.  NOTICE TO COMPANY OF DISQUALIFYING DISPOSITION.

     Each Key Employee who receives an ISO must agree to notify the Company in
writing immediately after the Key Employee makes a Disqualifying Disposition of
any shares acquired pursuant to the exercise of an ISO. A Disqualifying
Disposition is any disposition (including any sale) of such shares before the
later of (a) two years after the date the Key Employee was granted the ISO, or
(b) one year after the date the Key Employee acquired shares by exercising the
ISO. If the Key Employee has died before such stock is sold, these holding
period requirements do not apply and no Disqualifying Disposition can occur
thereafter.

22.  TERMINATION OF THE PLAN.

     Except as provided in the following sentence, the Plan will terminate on
November 12, 2007. The Plan may be terminated at an earlier date by vote of the
stockholders of the Company provided, however, that any such earlier termination
will not affect any Options granted or Option Agreements executed prior to the
effective date of such termination.

23.  AMENDMENT OF THE PLAN.

     The Plan may be amended by the stockholders of the Company. The Plan may
also be amended by the Administrator, including, without limitation, to the
extent necessary to qualify any or all outstanding ISOs granted under the Plan
or ISOs to be granted under the Plan for favorable federal income tax treatment
(including deferral of taxation upon exercise) as may be afforded incentive
stock options under Section 422 of the Code, to the extent necessary to ensure
the compliance of the Plan with Rule 16b-3 under the 1934 Act, and to the extent
necessary to qualify the shares issuable upon exercise of any outstanding
options granted, or options to be granted, under the Plan for listing on any
national securities exchange or quotation in any national automated quotation
system of securities dealers. Any amendment approved by the Administrator which
is of a scope that requires stockholder approval in order to ensure favorable
federal income tax treatment for any incentive stock options or requires
stockholder approval in order to ensure the qualification of the Plan under Rule
16b-3


                                       20


<PAGE>

shall be subject to obtaining such stockholder approval. Any modification or
amendment of the Plan shall not, without the consent of an optionee, adversely
affect his or her rights under an option previously granted to him or her. With
the consent of the optionee affected, the Administrator may amend outstanding
option agreements in a manner not inconsistent with the Plan.

24.  EMPLOYMENT OR OTHER RELATIONSHIP.

     Nothing in this plan or any Option Agreement shall be deemed to prevent the
Company or an Affiliate from terminating the employment, consultancy or director
status of a Participant, nor to prevent a Participant from terminating his or
her own employment, consultancy or director status or to give any Participant a
right to be retained in employment or other service by the Company or any
Affiliate for any period of time.

25.  GOVERNING LAW.

     This Agreement shall be construed and enforced in accordance with the laws
of The Commonwealth of Massachusetts.



                                       21


<PAGE>

                                 IMMUNOGEN, INC.

                      2001 NON-EMPLOYEE DIRECTOR STOCK PLAN

1.       DEFINITIONS.

         Unless otherwise specified or unless the context otherwise requires,
         the following terms, as used in this ImmunoGen, Inc. 2001 Non-Employee
         Director Stock Plan, have the following meanings:

                  ADMINISTRATOR means the Board of Directors, unless it has
                  delegated power to act on its behalf to the Committee, in
                  which case the Administrator means the Committee.

                  AFFILIATE means a corporation which, for purposes of Section
                  424 of the Code, is a parent or subsidiary of the Company,
                  direct or indirect.

                  ANNUAL AWARD means a grant under the Plan representing the
                  annual retainer payable to any Participant for serving on the
                  Board of Directors, payable in (i) Stock or (ii) Stock units,
                  based upon a Participant's election.

                  AWARD means an Annual Award or Meeting Award, or both, as the
                  context requires.

                  AWARD AGREEMENT means an agreement between the Company and a
                  Participant for the issuance of Stock delivered pursuant to
                  the Plan, in such form as approved by the Administrator, which
                  shall contain such terms and conditions as the Administrator
                  determines to be appropriate and in the best interest of the
                  Company.

                  BOARD OF DIRECTORS means the Board of Directors of the
                  Company.

                  CODE means the United States Internal Revenue Code of 1986, as
                  amended.

                  COMMITTEE means the committee of the Board of Directors to
                  which the Board of Directors has delegated power to act under
                  or pursuant to the provisions of the Plan.


<PAGE>

                  COMMON STOCK means shares of the Company's common stock, $.01
                  par value per share.

                  COMPANY means ImmunoGen, Inc., a Massachusetts corporation.

                  ELECTION FORM means the form on which a Participant elects the
                  form of Award he or she will receive pursuant to Section 6
                  hereof.

                  FAIR MARKET VALUE of a share of Common Stock means:

                  (1) If the Common Stock is listed on a national securities
                  exchange or traded in the over-the-counter market and sales
                  prices are regularly reported for the Common Stock, the
                  closing or last price of the Common Stock on the Composite
                  Tape or other comparable reporting system for the trading day
                  on the date of the grant of the Stock Right;

                  (2) If the Common Stock is not traded on a national securities
                  exchange but is traded on the over-the-counter market, if
                  sales prices are not regularly reported for the Common Stock
                  for the trading day referred to in clause (1), and if bid and
                  asked prices for the Common Stock are regularly reported, the
                  mean between the bid and the asked price for the Common Stock
                  at the close of trading in the over-the-counter market for the
                  trading day on which Common Stock was traded on the date of
                  the grant of the Stock Right; and

                  (3) If the Common Stock is neither listed on a national
                  securities exchange nor traded in the over-the-counter market,
                  such value as the Administrator, in good faith, shall
                  determine.

                  MEETING AWARD means a grant under the Plan representing the
                  meeting and committee attendance and chairperson fees payable
                  to any Participant for serving on the Board of Directors,
                  payable in (i) cash, (ii) Stock or (iii) Stock units, based
                  upon a Participant's election.

                  NON-EMPLOYEE DIRECTOR means each member of the Board of
                  Directors who is not an employee of the Company or of any
                  Affiliate.

                  PARTICIPANT means a Non-Employee Director to whom one or more
                  Stock Rights are granted under the Plan. As used herein,
                  "Participant" shall include "Participant's Survivors" where
                  the context requires.

                  PLAN means this ImmunoGen, Inc. 2001 Non-Employee Director
                  Stock Plan.

                  STOCK means shares of the Common Stock as to which Awards have
                  been or may be granted under the Plan or any shares of capital
                  stock into which the Stock are changed or for which they are
                  exchanged within the provisions of Section 3 of the Plan.

                                       2


<PAGE>

                  STOCK RIGHT means a right to Stock of the Company granted
                  pursuant to the Plan, in the form of an Award.

                  STOCK UNIT means a unit, equivalent in value to a share of
                  Common Stock, credited by means of a bookkeeping entry in the
                  books of the Company to a Participant's account pursuant to
                  the terms and conditions of the Plan.

                  SURVIVORS means a deceased Participant's legal representatives
                  and/or any person or persons who acquired the Participant's
                  rights to a Stock Right by will or by the laws of descent and
                  distribution.

                  TERMINATION DATE means the date upon which a Participant
                  ceases to be a member of the Board of Directors for any reason
                  whatsoever, including death or disability.

2.       PURPOSES OF THE PLAN.

         The purpose of the Plan is to provide a means by which the Company can
compensate Non-Employee Directors for their service on the Board of Directors
and to encourage continued ownership of Stock by the Non-Employee Directors, in
order to attract such people to serve on the Board of Directors and to induce
them to work for the benefit of the Company. The Plan provides for (i) the
granting of Awards to Non-Employee Directors and (ii) subject to Section 6
hereof, the election by Non-Employee Directors to have all or a portion of their
Awards credited in the form of Stock units.

3.       STOCK SUBJECT TO THE PLAN.

         The number of shares of Stock which may be the subject of Stock Rights
granted pursuant to this Plan shall be 50,000, or the equivalent of such number
of shares of Stock after the Administrator, in its sole discretion, has
interpreted the effect of any stock split, stock dividend, combination,
recapitalization or similar transaction in accordance with Section 17 of the
Plan.

4.       ADMINISTRATION OF THE PLAN.

         Subject to the provisions of the Plan, the Administrator is authorized
to:

         a.  Interpret the provisions of the Plan or of any Award Agreement and
             to make all rules and determinations which it deems necessary or
             advisable for the administration of the Plan;

         b.  Determine which Non-Employee Directors shall be granted Stock
             Rights; and

                                       3

<PAGE>

         c.  Determine the number of shares of Stock for which a Stock Right or
             Stock Rights shall be granted.

Subject to the foregoing, the interpretation and construction by the
Administrator of any provisions of the Plan or of any Stock Right granted under
it shall be final, unless otherwise determined by the Board of Directors, if the
Administrator is the Committee.

5.       ELIGIBILITY FOR PARTICIPATION.

         The Administrator will, in its sole discretion, name the Participants
in the Plan, provided, however, that each Participant must be a Non-Employee
Director at the time a Stock Right is granted. Notwithstanding the foregoing,
the Administrator may authorize the grant of a Stock Right to a person not then
a Non-Employee Director; provided, however, that the actual grant of such Stock
Right shall be conditioned upon such person becoming eligible to become a
Participant at or prior to the time of the delivery of the Award Agreement
evidencing such Stock Right. The granting of any Stock Right to any individual
shall neither entitle that individual to, nor disqualify him or her from,
participation in any other grant of Stock Rights.

6.       CERTAIN ELECTIONS.

         Each Participant shall, subject to the terms of this Section, have the
right to elect to receive all or a portion of (i) an Annual Award in the form of
Stock or Stock units and (ii) a Meeting Award in the form of cash, Stock or
Stock units, by completing, signing and delivering to the Treasurer of the
Company an Election Form (a) in respect of 2001 by September 30, 2001 and (b)
for each subsequent calendar year while this Plan is in effect by December 31 of
the immediately preceding year, provided that (1) any such election shall apply
only to Awards made after the date on which such election is made and (2) any
person who becomes a Non-Employee Director during the course of a calendar year
shall have the right to make his or her election in respect of Awards earned
after the date he or she becomes a Non-Employee Director by completing an
Election Form within 30 days after first becoming a Non-Employee Director. A
Participant shall elect to receive an Annual Award entirely in Stock or Stock
units but not in a combination thereof, and shall elect to receive annually all
Meeting Awards entirely in cash, Stock or Stock units but not in a combination
thereof.

         Any election made pursuant to the terms of this Plan shall remain
effective until it is changed by the completion, signature, and delivery to the
Treasurer of the Company of a new Election Form in accordance with the terms of
the Plan, which shall take effect for the calendar year(s) or other applicable
period commencing after the delivery of the Election Form.

         If no election is made, and no prior election remains effective, the
Participant shall be deemed to have elected to receive all Annual Awards in the
form of Stock and all Meeting Awards in the form of cash.

                                       4

<PAGE>

7.       NUMBER OF STOCK UNITS TO BE CREDITED.

         During a calendar year, the number of Stock units to be credited in the
books of the Company to the account of a Participant shall be such number of
Stock units (including fractional Stock units calculated to four decimal points
rounded down) as is obtained on a quarterly basis by dividing the total dollar
amount that the Participant has elected to be paid in Stock units under the Plan
by the Fair Market Value on the last date of the Company's fiscal quarter in
which the Awards for each calendar quarter are made.

         No fractional shares shall be issued under the Plan and the person
exercising a Stock Right shall receive from the Company cash in lieu of such
fractional shares equal to the Fair Market Value thereof. The receipt of Stock
units for a calendar year by a Participant shall be evidenced by an agreement in
writing between the Participant and the Company.

8.       CREDIT FOR DIVIDENDS ON STOCK UNITS.

         A Participant's account covering Stock units shall be credited with
dividend equivalents in the form of additional Stock units when cash dividends
are paid on the Common Stock. Such dividend equivalents shall be computed by
dividing: (a) the amount obtained by multiplying the amount of the dividend
declared and paid per share by the number of Stock units recorded in the
Participant's account on the record date for the payment of such dividend, by
(b) the Fair Market Value on the dividend payment date for such dividend, with
fractions computed to four decimal places (rounded down).

9.       REPORTING OF STOCK UNITS.

         Statements of Stock unit accounts will be provided to the Participants
on a quarterly basis.


10.      TERMS AND CONDITIONS OF AWARDS.

         Each Award to a Participant shall state the date prior to which the
Award must be accepted by the Participant, and the principal terms of each Award
shall be set forth in an Award Agreement, duly executed by the Company and, to
the extent required by law or requested by the Company, by the Participant. Such
Award Agreement shall state the number of shares of Stock or Stock units to
which the Award pertains.

                                       5

<PAGE>

11.      ACCEPTANCE OF AWARD AND ISSUE OF STOCK.

         An Award (or any part or installment thereof) shall be accepted by
executing the Award Agreement and delivering it to the Company at its principal
office address, upon compliance with any other conditions set forth in the Award
Agreement.

         The Company shall then reasonably promptly deliver the Stock as to
which such Award was accepted to the Participant (or to the Participant's
Survivors, as the case may be), subject to any escrow provision set forth in the
Award Agreement. In determining what constitutes "reasonably promptly," it is
expressly understood that the issuance and delivery of the Stock may be delayed
by the Company in order to comply with any law or regulation (including, without
limitation, state securities or "blue sky" laws) which requires the Company to
take any action with respect to the Stock prior to their issuance.

         The Administrator may, in its discretion, amend any term or condition
of an outstanding Award or Award Agreement provided (i) such term or condition
as amended is permitted by the Plan, and (ii) any such amendment shall be made
only with the consent of the Participant to whom the Award was made, if the
amendment is adverse to the Participant.

12.      RIGHTS AS A SHAREHOLDER.

         No Participant to whom a Stock Right has been granted shall have rights
as a shareholder with respect to any Stock covered by such Stock Right, except
after acceptance of the Award for the Stock and registration of the Stock in the
Company's stock register in the name of the Participant.

13.      ASSIGNABILITY AND TRANSFERABILITY OF STOCK RIGHTS.

         By its terms, a Stock Right granted to a Participant shall not be
transferable by the Participant other than (i) by will or by the laws of descent
and distribution, or (ii) as otherwise determined by the Administrator and set
forth in any applicable Award Agreement. The designation of a beneficiary of a
Stock Right by a Participant, with the prior approval of the Administrator and
in such form as the Administrator shall prescribe, shall not be deemed a
transfer prohibited by this Section. Except as provided above, a Stock Right
shall only be exercisable or may only be accepted, during the Participant's
lifetime, by such Participant (or by his or her legal representative) and shall
not be assigned, pledged or hypothecated in any way (whether by operation of law
or otherwise) and shall not be subject to execution, attachment or similar
process. Any attempted transfer, assignment, pledge, hypothecation or other
disposition of any Stock Right or of any rights granted thereunder contrary to
the provisions of this Plan, or the levy of any attachment or similar process
upon a Stock Right, shall be null and void.

                                       6

<PAGE>

14.      EFFECT OF TERMINATION OF DIRECTORSHIP ON STOCK UNITS.

         In the event of a termination of service as a Non-Employee Director of
the Company for any reason, a Participant shall receive, not later than thirty
days following the Participant's Termination Date, a lump sum payment in cash
equal to the number of Stock units recorded in the Participant's account on the
Termination Date multiplied by the Fair Market Value of the Stock on such date.
Upon payment in full of the Stock units, the Stock units shall be cancelled.

15.      OWNERSHIP OF STOCK RIGHTS.

         By accepting a Stock Right, a Participant acknowledges that one of the
Company's goals in implementing this Plan is to align the interests of
Participants with those of the Company's stockholders, and accordingly agrees to
use his or her best efforts to maintain ownership of the Stock Rights as a
long-term investment. As such, the person(s) who accept(s) such Stock Right
shall represent to the Company, prior to the receipt of Stock, that such
person(s) are acquiring such Stock for their own respective accounts, for
investment, and not with a view to, or for sale in connection with, the
distribution of any such Stock, in which event the person(s) acquiring such
Stock shall be bound by the provisions of the Plan.

         Unless the offering and sale of the Stock to be issued upon the
particular acceptance of a Stock Right shall have been effectively registered
under the Securities Act of 1933, as now in force or hereafter amended (the
"1933 Act"), the Company shall be under no obligation to issue such Stock unless
the following legend is endorsed upon the certificate(s) evidencing their Stock
issued pursuant to such exercise or such grant:

                  "The shares represented by this certificate have been taken
                  for investment and they may not be sold or otherwise
                  transferred by any person, including a pledgee, unless (1)
                  either (a) a Registration Statement with respect to such
                  shares shall be effective under the Securities Act of 1933, as
                  amended, or (b) the Company shall have received an opinion of
                  counsel satisfactory to it that an exemption from registration
                  under such Act is then available, and (2) there shall have
                  been compliance with all applicable state securities laws."

         At the discretion of the Administrator, the Company shall have received
an opinion of its counsel that the Stock may be issued upon such particular
acceptance in compliance with the 1933 Act without registration thereunder.

         Each Participant shall contiue to comply with the Company's then
applicable insider trading policy and rules with respect to blackout periods and
with all rules and regulations of the Securities and Exchange Commission
applicable to trading in securities by members of the Boards of Directors of
publicly traded companies at all times when making decisions with respect to the
purchase, ownership, and disposition of Stock under this Plan.

                                       7

<PAGE>

16.      DISSOLUTION OR LIQUIDATION OF THE COMPANY.

         Upon the dissolution or liquidation of the Company, all Stock Rights
which have not been accepted as of such date will terminate and become null and
void; provided, however, that if the rights of a Participant or a Participant's
Survivors have not otherwise terminated and expired, the Participant or the
Participant's Survivors will have the right immediately prior to such
dissolution or liquidation to exercise or accept any Stock Right to the extent
that the Stock Right is exercisable or subject to acceptance as of the date
immediately prior to such dissolution or liquidation.

17.      ADJUSTMENTS.

         Except as otherwise set forth in Section 8 hereof, upon the occurrence
of any of the following events, a Participant's rights with respect to any Stock
Right granted to him or her hereunder shall be adjusted as hereinafter provided,
unless otherwise specifically provided in any applicable Award Agreement:

         A. STOCK DIVIDENDS AND STOCK SPLITS. If (i) the shares of Common Stock
shall be subdivided or combined into a greater or smaller number of shares or if
the Company shall issue any shares of Common Stock as a stock dividend on its
outstanding Common Stock, or (ii) additional shares or new or different shares
or other securities of the Company or other non-cash assets are distributed with
respect to such shares of Common Stock, the number of shares of Common Stock
deliverable upon the exercise or acceptance of such Stock Right may be
appropriately increased or decreased proportionately to reflect such events. The
number of shares of Stock subject to the limitation in Section 4(c) shall also
be proportionately adjusted upon the occurrence of such events.

         B. CONSOLIDATIONS OR MERGERS. If the Company is to be consolidated with
or acquired by another entity in a merger, sale of all or substantially all of
the Company's assets or otherwise (an "Acquisition"), the Administrator or the
board of directors of any entity assuming the obligations of the Company
hereunder (the "Successor Board"), shall, with respect to outstanding Stock
Rights, either (i) make appropriate provisions for the continuation of such
Stock Rights by substituting on an equitable basis for the Stock then subject to
such Stock Rights either the consideration payable with respect to the
outstanding Stock in connection with the Acquisition or securities of any
successor or acquiring entity; or (ii) upon written notice to the Participants,
provide that all Stock Rights must be accepted (to the extent then subject to
acceptance) within a specified number of days of the date of such notice, at the
end of which period the offer of the Stock Rights shall terminate.

         C. RECAPITALIZATION OR REORGANIZATION. In the event of a
recapitalization or reorganization of the Company (other than a transaction
described in Subparagraph B above) pursuant to which securities of the Company
or of another corporation are issued with respect to the outstanding shares of
Common Stock, a Participant upon exercising or accepting a Stock Right shall be
entitled to receive for the purchase price, if any, paid upon such exercise or

                                       8

<PAGE>

acceptance the securities which would have been received if such Stock Right had
been exercised or accepted prior to such recapitalization or reorganization.

18.      ISSUANCES OF SECURITIES.

         Except as expressly provided herein, no issuance by the Company of
shares of stock of any class, or securities convertible into shares of stock of
any class, shall affect, and no adjustment by reason thereof shall be made with
respect to, the number or price of shares subject to Stock Rights. Except as
expressly provided herein, no adjustments shall be made for dividends paid in
cash or in property (including without limitation, securities) of the Company
prior to any issuance of Stock pursuant to a Stock Right.

19.      WITHHOLDING.

         In the event that any federal, state, or local income taxes, employment
taxes, Federal Insurance Contributions Act ("FICA") withholdings or other
amounts are required by applicable law or governmental regulation to be withheld
from the Participant's remuneration in connection with the exercise or
acceptance of a Stock Right, the Company may withhold from the Participant's
compensation, if any, or may require that the Participant advance in cash to the
Company, the statutory minimum amount of such withholdings unless a different
withholding arrangement, including the use of shares of the Company's Common
Stock or a promissory note, is authorized by the Administrator (and permitted by
law). For purposes hereof, the Fair Market Value of the Stock withheld for
purposes of payroll withholding shall be determined in the manner provided in
Section 1 above, as of the most recent practicable date prior to the date of
exercise. If the Fair Market Value of the Stock withheld is less than the amount
of payroll withholdings required, the Participant may be required to advance the
difference in cash to the Company.

20.      TERMINATION OF THE PLAN.

         The Plan will terminate 10 years after its approval by the shareholders
of the Company. The Plan may be terminated at an earlier date by vote of the
shareholders or the Board of Directors of the Company; provided, however, that
any such earlier termination shall not affect any Award Agreements executed
prior to the effective date of such termination.

21.      AMENDMENT OF THE PLAN AND AGREEMENTS.

         The Plan may be amended by the shareholders of the Company. The Plan
may also be amended by the Administrator, including, without limitation, to the
extent necessary to qualify the stock issuable upon exercise or acceptance of
any outstanding Stock Rights granted, or Stock Rights to be granted, under the
Plan for listing on any national securities exchange or quotation in any
national automated quotation system of securities dealers. Any amendment
approved by 

                                       9

<PAGE>

the Administrator which the Administrator determines is of a scope that requires
shareholder approval shall be subject to obtaining such shareholder approval.
Any modification or amendment of the Plan shall not, without the consent of a
Participant, adversely affect his or her rights under a Stock Right previously
granted to him or her. With the consent of the Participant affected, the
Administrator may amend outstanding Award Agreements in a manner which may be
adverse to the Participant but which is not inconsistent with the Plan. In the
discretion of the Administrator, outstanding Award Agreements may be amended by
the Administrator in a manner which is not adverse to the Participant.

22.      RELATIONSHIP WITH THE COMPANY.

         Nothing in this Plan or any Award Agreement shall be deemed to prevent
the Company from terminating the director status of a Participant, nor to
prevent a Participant from terminating his or her own director status or to give
any Participant a right to be retained in employment or other service by the
Company or any Affiliate for any period of time.

23.      GOVERNING LAW.

         This Plan shall be construed and enforced in accordance with the law of
The Commonwealth of Massachusetts.




                                     10
<Page>


PROXY                          IMMUNOGEN, INC.                             PROXY

                    PROXY SOLICITED BY THE BOARD OF DIRECTORS
                           OF IMMUNOGEN, INC. FOR THE
                         ANNUAL MEETING OF SHAREHOLDERS
                         TO BE HELD ON NOVEMBER 13, 2001

     The undersigned hereby acknowledges receipt of the Notice of Annual Meeting
of Shareholders and Proxy Statement dated ________, 2001, and does hereby
appoint Mitchel Sayare, Ph.D. and Walter A. Blattler, Ph.D. or either of them,
the undersigned's attorneys-in-fact and proxies, with full power of substitution
in each, for and in the name of the undersigned, with all the powers the
undersigned would possess if personally present, hereby revoking any proxy
heretofore given, to appear and represent and vote all shares of Common Stock of
ImmunoGen, Inc. which the undersigned would be entitled to vote if personally
present at the Annual Meeting of Shareholders to be held at the offices of the
Company, 128 Sidney St., Cambridge, Massachusetts on Tuesday, November 13, 2001,
at 10:00 a.m., Boston time, and at any adjournments thereof.

PLEASE FILL IN REVERSE SIDE AND MAIL IN THE ENCLOSED ENVELOPE

                             -FOLD AND DETACH HERE-



<PAGE>


THE SHARES REPRESENTED HEREBY WILL BE VOTED AS          Please mark your     [X]
DIRECTED HEREIN. IF NO DIRECTION IS INDICATED,          vote as indicated
SUCH SHARES WILL BE VOTED FOR ITEMS 1, 2, 3 and 4.      in this example

Item 1.   Election of Directors:

Mitchel Sayare, Walter A. Blattler, David W. Carter, Michael R. Eisenson, 
Stuart F. Feiner, and Mark B. Skaletsky

FOR ALL                             WITHHOLD
NOMINEES LISTED                     AUTHORITY
ABOVE                              to vote for
(except those crossed out)         all nominees
         [  ]                         [  ]


Item 2.   Increase in the aggregate number of shares for which stock options 
may be granted under the Company's Restated Stock Option Plan from 4,850,000 
to 7,350,000.

FOR INCREASE IN                      AGAINST                           ABSTAIN
NUMBER OF SHARES
         [  ]                         [  ]                                [  ]


Item 3.   Adoption of the Company's 2001 Non-Employee Director Stock Plan and 
the Reservation of 50,000 shares of Common Stock which may be granted under 
the 2001 Plan.

FOR ADOPTION OF                      AGAINST                           ABSTAIN
2001 PLAN
          [  ]                         [  ]                               [  ]

Item 4.   Amendment of the Company's Articles of Organization to Increase 
from 50,000,000 shares to 75,000,000 shares the aggregate number of shares of 
Common Stock authorized to be issued by the Company.

FOR AMENDMENT OF ARTICLES 
OF ORGANIZATION to increase 
number of shares authorized 
to be issued.                        AGAINST                           ABSTAIN 
          [  ]                         [  ]                               [  ]


I plan to attend the meeting. [  ]

     In their discretion the proxies are authorized to vote upon such other
business as may properly come before the meeting or any adjournments thereof.

     This proxy may be revoked in writing at any time prior to the voting
thereof.

     Please date and sign exactly as name appears on this card. Joint owners
should each sign. Please give full title when signing as executor,
administrator, trustee, attorney, guardian for a minor, etc. Signatures for
corporations and partnerships should be in the corporate or firm name by a duly
authorized person. Please return this proxy promptly in the enclosed envelope.


Signature:
          -------------------------------

Date:
     ------------------------------------

Signature:
          -------------------------------

Date:
     ------------------------------------