<PAGE>   1
 
                            SCHEDULE 14A INFORMATION
 
          PROXY STATEMENT PURSUANT TO SECTION 14(A) OF THE SECURITIES
                              EXCHANGE ACT OF 1934
 
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 sec.240.14a-11(c) or sec.240.14a-12
 
                                ImmunoGen, Inc.
                (Name of Registrant as Specified In Its Charter)
 
                   (Name of Person(s) Filing Proxy Statement)
 
PAYMENT OF FILING FEE (CHECK THE APPROPRIATE BOX):
[ ] No fee required.
[ ] Fee computed on table below per Exchange Act Rules 14a-6(i)(1) 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:
 
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<PAGE>   2
 
                             [ImmunoGen Letterhead]
 
                                                                    July 3, 1997
 
Dear Shareholder:
 
     You are cordially invited to attend a Special Meeting of Shareholders of
ImmunoGen, Inc. to be held at 10:00 a.m. on Monday, August 11, 1997 at the
offices of the Company, 333 Providence Highway, Norwood, Massachusetts.
 
     At the Special Meeting the Company will seek shareholder approval of a
proposal to amend the Company's Restated Articles of Organization to increase
the number of authorized shares of the Company's common stock. The Board of
Directors recommends approval of this proposal. Such other business will be
transacted as may properly come before the Special Meeting.
 
     Whether you plan to attend the Special Meeting or not, it is important that
you promptly complete, sign, date and return the enclosed proxy card in
accordance with the instructions pertaining to the card. This will ensure your
proper representation at the Special Meeting.
 
                                            Sincerely,
 
                                            /s/ Mitchel Sayare
                                            MITCHEL SAYARE
 
           YOUR VOTE IS IMPORTANT. PLEASE RETURN YOUR PROXY PROMPTLY.

<PAGE>   3
 
                                IMMUNOGEN, INC.
                             333 PROVIDENCE HIGHWAY
                          NORWOOD, MASSACHUSETTS 02062
                   NOTICE OF SPECIAL MEETING OF SHAREHOLDERS
                         TO BE HELD ON AUGUST 11, 1997
 
To the Shareholders of
  ImmunoGen, Inc.:
 
     Notice is hereby given that a Special Meeting of Shareholders of ImmunoGen,
Inc. (the "Company") will be held at the offices of the Company, 333 Providence
Highway, Norwood, Massachusetts, on Monday, August 11, 1997 at 10:00 a.m. Boston
time for the following purposes:
 
     1.  To consider and act upon a proposal to amend the Company's Restated
         Articles of Organization to increase the number of authorized shares of
         Common Stock from 30 million to 50 million shares.
 
     2.  To transact such other business as may properly come before the Meeting
         or any adjournment(s) thereof.
 
     Only shareholders of record at the close of business on June 25, 1997 will
receive notice of the Meeting and be entitled to vote at the Meeting or any
adjournment(s) thereof. The transfer books will not be closed.
 
     You are cordially invited to attend the Meeting in person, if possible.
WHETHER YOU PLAN TO ATTEND THE MEETING OR NOT, PLEASE COMPLETE, SIGN AND DATE
THE ENCLOSED PROXY AND RETURN IT IN THE ENVELOPE ENCLOSED FOR THIS PURPOSE. The
Proxy is revocable by the person giving it 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 Meeting and voting in
person.
 
                                            By order of the Board of Directors
 
                                            LOGO
                                            JONATHAN L. KRAVETZ, ESQ.
                                            Clerk
 
July 3, 1997

<PAGE>   4
 
                                IMMUNOGEN, INC.
                             333 PROVIDENCE HIGHWAY
                          NORWOOD, MASSACHUSETTS 02062
                        -------------------------------
 
                                PROXY STATEMENT
                        -------------------------------
 
          ------------------------------------------------------------
 
                        SPECIAL MEETING OF SHAREHOLDERS
          ------------------------------------------------------------
 
                         TO BE HELD ON AUGUST 11, 1997
 
                              GENERAL INFORMATION
 
     Introduction.  This Proxy Statement is furnished in connection with the
solicitation by and on behalf of the Board of Directors of ImmunoGen, Inc. (the
"Company") of Proxies for use at a Special Meeting of Shareholders of the
Company to be held at the offices of the Company, 333 Providence Highway,
Norwood, Massachusetts on Monday, August 11, 1997 at 10:00 a.m. and at any
adjournments thereof (the "Meeting"), and, together with the enclosed Form of
Proxy, is being mailed to the shareholders on or about July 3, 1997.
 
     Revocability of Proxies.  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, or by attending the Meeting and voting in person.
 
     Cost of Solicitation.  The entire cost of this solicitation will be paid by
the Company. In addition, the Company may reimburse brokerage firms and other
persons representing beneficial owners of shares for their reasonable expenses
in forwarding solicitation material to such beneficial owners. Solicitation of
proxies by mail may be supplemented by telephone, telegram, telex, telecopy and
personal solicitation by directors, officers or employees of the Company. No
additional compensation will be paid for such solicitation. The Company does not
presently intend to engage a proxy solicitation agent to assist in the
solicitation of proxies for the Meeting.
 
     Quorum and Voting.  Only shareholders of record of the
Company's            shares of Common Stock, $.01 par value (the "Common Stock")
outstanding as of the close of business on June 25, 1997, will be entitled to
vote. Each share of Common Stock is entitled to one vote at the Meeting. The
presence, in person or by Proxy, of the holders of a majority of the outstanding
shares of Common Stock entitled to vote is necessary to constitute a quorum at
the Meeting. No appraisal rights exist for any action to be taken at the
Meeting.
 
                                        1

<PAGE>   5
 
            PROPOSAL: INCREASE IN AUTHORIZED SHARES OF COMMON STOCK
 
     The Company's Board of Directors recommends that the shareholders consider
and approve a proposal to amend the Corporation's Restated Articles of
Organization to increase the authorized shares of Common Stock, $.01 par value
per share, from 30,000,000 to 50,000,000 shares. The additional shares of Common
Stock for which authorization is sought would be identical to the shares of
Common Stock of the Company now authorized. Holders of the Common Stock do not
have preemptive rights to subscribe to additional securities which may be issued
by the Company.
 
     In June 1996, the Company's shareholders voted to increase the authorized
shares of the Company's Common Stock from 20,000,000 to 30,000,000. Based on the
market price of the Common Stock at that time, the Company believed that this
increase would be sufficient to cover its requirements for the foreseeable
future. However, because of the substantial decline in the market price of the
Common Stock over the past year, the Company's reserve of authorized but
unissued and unreserved Common Stock has been depleted as a result of
conversions of the Company's convertible debentures and convertible preferred
stock and the issuance of warrants in connection therewith. Financing agreements
entered into in March 1996 and October 1996 have resulted, as of June 11, 1997,
in the issuance of 5,356,360 shares of Common Stock and warrants to purchase an
additional 3,321,126 shares of Common Stock. After giving effect to additional
amounts the Company has agreed to reserve in connection with these financing
agreements, the Company currently does not have any unreserved shares available
for issuance.
 
     In March 1996, the Company entered into an agreement with an institutional
investor, whereby the investor agreed to purchase $5.0 million of convertible
securities in a private placement. This investment was effected in two
installments: $2.5 million of subordinated convertible debentures were issued in
March 1996 (the "March 1996 Debentures") and an additional $2.5 million of such
debentures were issued in June 1996 (the "June 1996 Debentures"). In October
1996, the June 1996 Debentures were converted into 2,500 shares of the Company's
Series A Convertible Preferred Stock (the "Series A Stock"). The $2.5 million
March 1996 Debentures plus accrued interest thereon were converted into
1,018,000 shares of Common Stock in June 1996. The Series A Stock is convertible
into shares of the Common Stock at 85% of the market price on the date of
conversion. As of June 11, 1997, 1,400 shares of the Series A Stock plus accrued
dividends thereon had been converted into 1,328,744 shares of Common Stock.
 
     In connection with the March 1996 Debenture, warrants to purchase 1,009,000
shares of the Common Stock were issued. Of these warrants, 509,000 have an
exercise price of $4.00 per share and 500,000 have an exercise price of $6.00
per share. All of the warrants expire in 2001. In connection with the issuance
of the Series A Stock, warrants to purchase 664,372 shares of Common Stock had
been issued as of June 11, 1997. These warrants have an exercise price of $4.00
per share and expire in 2002. Conversion of the remaining Series A Stock will
entitle the holder to receive a number of warrants equal to 50% of the number of
shares issued on the conversion of the Series A Stock. These warrants will have
an exercise price of $4.00 per share and will be exercisable for five years from
their issue dates.
 
     In October 1996 and January 1996, the Company issued 3,000 shares each of
its Series B and Series C Convertible Preferred Stock (the "Series B Stock" and
the "Series C Stock") to an institutional investor in a private placement, for a
total investment of $6.0 million. Both series are convertible at 85% of the
market price of the Common Stock on the date of conversion. As of June 11, 1996,
all of the Series B Stock plus accrued dividends thereon had been converted into
1,384,823 shares of Common Stock, and 1,825 shares of the Series C Stock plus
accrued dividends thereon had been converted into 1,624,793 shares of Common
Stock.
 
     In connection with the issuance of the Series B Stock and Series C Stock,
the Company issued warrants to purchase 1,647,754 shares of Common Stock. These
warrants were issued at prices ranging from $2.31 per share to $5.49 per share
and expire five years from their issue dates.
 
                                        2

<PAGE>   6
 
     As of June 11, 1997, the Company has 21,343,981 shares of Common Stock
issued and outstanding. As of that date, a total of 2,177,676 shares of Common
Stock would be required to consummate the conversion of the remaining Series A
Stock and Series C Stock and related warrants. The Company has agreed to reserve
a sufficient number of shares of its Common Stock to allow such conversion stock
to issue. The number of shares required to be issued would increase if the
market price for the Common Stock decreases below $1.63 per share and a
conversion occurs at or below that price. In addition, as of June 11, 1997, the
Company has options to purchase 1,456,266 shares of Common Stock outstanding
under the Company's stock option plans, and additional warrants to purchase
276,738 shares of Common Stock.
 
     The Company's Board of Directors believes that the increase in the
authorized Common Stock is in the best interest of the Company and its
shareholders. Because of its continuing losses from operations and working
capital deficit, the Company will be required to obtain additional capital to
satisfy its ongoing capital needs and to continue its operations. The proposed
amendment will give the Company a sufficient number of unreserved and unissued
shares to allow the Company to pursue equity financing transactions and other
transactions which management believes may enhance shareholder value. Any
remaining authorized shares may be issued in the future by the Board of
Directors, without further shareholder approval (unless required by applicable
laws, regulations or rules), for such corporate purposes as the Board may deem
in the best interest of the Company. In addition to raising capital through
equity financings, the additional shares of Common Stock for which approval is
sought may also be used to purchase assets or services and compensate employees
or consultants. Other than in connection with the conversion of the Series A
Stock, the Series C Stock, and the related warrants, the Company has no current
commitment to issue additional shares of Common Stock.
 
     The increase in the authorized shares proposed by the Board is substantial
and is designed to provide flexibility to the Company's management. The issuance
of a significant amount of additional authorized Common Stock, however, will
result in significant dilution of the beneficial ownership interests and/or
voting power of the Company's shareholders.
 
     The additional shares of Common Stock for which authorization is sought
could be used for purposes that might be deemed to be in defense of a potential
takeover threat. For example, shares of Common Stock could be issued to
purchasers favoring the Board of Directors, thereby making removal of the
incumbent Board more difficult and making acquisition of a sufficient number of
shares to accomplish a takeover more costly. The Company does not presently
contemplate using any of the authorized shares of Common Stock for such
purposes.
 
     THE AFFIRMATIVE VOTE OF THE HOLDERS OF A MAJORITY OF THE OUTSTANDING SHARES
OF COMMON STOCK ENTITLED TO VOTE AT THE MEETING IS REQUIRED TO AUTHORIZE THE
PROPOSED INCREASE IN THE AUTHORIZED NUMBER OF SHARES OF COMMON STOCK, WITH
ABSTENTIONS AND BROKER NON-VOTES BEING TREATED AS VOTES AGAINST THE PROPOSAL.
 
     THE BOARD OF DIRECTORS RECOMMENDS A VOTE "FOR" THIS PROPOSAL.
 
                                        3

<PAGE>   7
 
                             PRINCIPAL SHAREHOLDERS
 
     The following table sets forth certain information regarding the beneficial
ownership of shares of Common Stock as of June 6, 1997 by (i) each person or
entity known by the Company to be a beneficial owner of more than 5% of the
outstanding shares of Common Stock, (ii) each Director of the Company, (iii) the
Company's Chief Executive Officer and each executive officer of the Company
whose total annual salary and bonus exceeded $100,000 for the fiscal year ended
June 30, 1997, and (iv) 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
                   NAMES AND ADDRESS OF                       BENEFICIALLY         BENEFICIALLY
                    BENEFICIAL OWNER**                           OWNED                OWNED
- ----------------------------------------------------------  ----------------       ------------
<S>                                                         <C>                    <C>
Aeneas Venture Corporation(2).............................      1,181,159(1)            5.7%(1)
600 Atlantic Avenue
Boston, Massachusetts 02210

Capital Ventures International(3).........................      2,277,061               9.9%
One Capitol Place
P.O. Box 1787 GT
Grand Cayman
Cayman Islands, BWI

Southbrook International Investments, Ltd.(4).............      3,299,264               4.9%
c/o Icaza, Gonzales-Ruiz & Aleman (BVI) Limited
Vanterpool Plaza, 2nd floor
Wickhams Cay 1
P.O. Box 873
Roadtown, Tortola
British Virgin Islands

Mitchel Sayare(5).........................................        511,834(1)            2.4%(1)
David W. Carter(6)........................................          --                   --
Michael R. Eisenson(7)....................................          --                   --
Stuart F. Feiner(8).......................................          2,100(1)             *
Donald E. O'Neill(9)......................................         12,500(1)             *
Walter A. Blattler(10)....................................        265,061(1)            1.3%(1)
Dixie-Lee W. Esseltine, M.D.(11)..........................         90,034(1)             *
John M. Lambert(12).......................................        107,175(1)             *
Frank J. Pocher(13).......................................        221,791(1)            1.1%(1)
All current executive officers and Directors as a group
  (9 persons)(14).........................................      1,012,071(1)            4.7%(1)
</TABLE>

 
- ---------------
 
   * Represents beneficial ownership of less than 1% of the Common Stock.
 
  ** Addresses are given for beneficial owners of more than 5% of the
     outstanding Common Stock only.

 (1) Share ownership includes shares of Common Stock issuable upon exercise of
     certain outstanding options as described in the footnotes below.

 (2) Includes 12,500 shares of Common Stock issuable upon the exercise of
     Director options within 60 days after June 6, 1997.

 (3) Includes 2,246,162 shares of Common Stock which Capital Ventures
     International ("CVI") may acquire upon the exercise of warrants (the "CVI
     Warrants") and/or conversion of Series A Stock. The Articles of
     Organization of the Company and the CVI Warrants limit the right of CVI to
     convert Series A Stock and exercise CVI Warrants such that the maximum
     number of shares of the Company's Common Stock which may at any time be
     deemed to be beneficially owned by CVI upon conversion of the Series A
     Stock and exercise of the CVI Warrants may not, together with any other
     shares of the Company's Common Stock then owned by CVI, exceed 9.9% of the
     then issued and outstanding shares of the Company's Common Stock.
 
                                        4

<PAGE>   8
 
 (4) Includes 2,780,349 shares which Southbrook International Investments, Ltd.
     may acquire upon the exercise of warrants and/or conversion of Series C
     Stock. The Convertible Preferred Stock Purchase Agreement between the
     Company and Southbrook International Investors, Ltd. ("Southbrook") limits
     the conversion right of Southbrook such that the maximum number of shares
     of the Company's Common Stock issued upon conversion of the preferred stock
     and warrants may not exceed 4.9% of the then issued and outstanding shares
     of the Company's Common Stock following such conversion or exercise.
 
 (5) Includes 373,834 shares of Common Stock which Mr. Sayare may acquire upon
     the exercise of options within 60 days after June 6, 1997.
 
 (6) David W. Carter was elected a Director on June 17, 1997. Mr. Carter owns no
     shares of Common Stock.
 
 (7) Michael R. Eisenson, a Director of the Company, is President and Chief
     Executive Officer of Harvard Private Capital Group, Inc., the investment
     advisor for Aeneas Venture Corporation. Mr. Eisenson owns no shares of
     Common Stock and disclaims beneficial ownership of the shares owned by
     Aeneas Venture Corporation. Pursuant to an agreement among the Company,
     Aeneas Venture Corporation and Mr. Eisenson, grants of stock options in
     respect of Mr. Eisenson's service as a Director are issued directly to
     Aeneas Venture Corporation.
 
(8) Stuart F. Feiner, a Director of the Company, is the Executive Vice
    President, General Counsel and Secretary of Inco Limited, which owns 11,856
    shares of Common Stock. He is also President of Inco Securities Corp., a
    subsidiary of Inco Limited, which owns 258,947 shares of Common Stock, and
    Chairman of the general partner of North American Partners Limited
    Partnership II, which owns 19 shares of Common Stock. Mr. Feiner disclaims
    beneficial ownership of the shares of Common Stock held by each of such
    shareholders. Mr. Feiner individually owned 2,100 shares as of June 6, 1997.
    He is also named as direct owner of non-qualified options to acquire 12,500
    shares of Common Stock granted by the Company in July 1992 as to 10,000
    shares and July 1996 as to 2,500 shares, which options are exercisable
    within 60 days after June 6, 1997; however, Mr. Feiner disclaims all
    beneficial interest in the derivative securities and underlying shares
    pursuant to an arrangement made between Mr. Feiner and Inco Limited,
    assigning all benefit to that entity.
 
 (9) Consists of 12,500 shares of Common Stock which Mr. O'Neill may acquire
     upon the exercise of options within 60 days after June 6, 1997.
 
(10) Includes 202,061 shares of Common Stock which Dr. Blattler may acquire upon
     the exercise of options within 60 days after June 6, 1997.
 
(11) Includes 90,034 shares of Common Stock which Dr. Esseltine may acquire upon
     the exercise of options within 60 days after June 6, 1997.
 
(12) Includes 107,175 shares of Common Stock which Dr. Lambert may acquire upon
     the exercise of options within 60 days after June 6, 1997.
 
(13) Mr. Pocher passed away on April 12, 1997. Includes 211,791 shares of Common
     Stock which Mr. Pocher's estate may acquire upon the exercise of options
     within 60 days after June 6, 1997.
 
(14) See also footnotes (5), (7), (8), (9), (10), (11) and (12).
 
                                        5

<PAGE>   9
 
                    SHAREHOLDER PROPOSALS AND OTHER MATTERS
 
     In order to be considered for inclusion in the proxy statement distributed
to shareholders prior to the annual meeting in 1997, a shareholder proposal must
be received by the Company no later than June 11, 1997. Proposals should be
delivered in writing to ImmunoGen, Inc., 333 Providence Highway, Norwood,
Massachusetts 02062.
 
     The Board of Directors 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 by the persons voting the Proxies 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.
 
                                            By order of the Board of Directors
 
                                            JONATHAN L. KRAVETZ, ESQ.
                                            Clerk
 
July 3, 1997
 
                                        6

<PAGE>   10
PROXY                           IMMUNOGEN, INC.                            PROXY

                   PROXY SOLICITED BY THE BOARD OF DIRECTORS
                            OF IMMUNOGEN, INC. FOR A
                        SPECIAL MEETING OF SHAREHOLDERS
                          TO BE HELD ON AUGUST 11, 1997

The undersigned hereby acknowledges receipt of the Notice of Special Meeting of
Shareholders and Proxy Statement dated July 3, 1997, and does hereby appoint
Mitchel Sayare and Kathleen A. Carroll, 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
Special Meeting of Shareholders to be held at the offices of the Company, 333
Providence Highway, Norwood, Massachusetts on August 11, 1997, at 10:00 a.m. 
and at any adjournments thereof. 

             FILL IN REVERSE SIDE AND MAIL IN THE ENCLOSED ENVELOPE
- --------------------------------------------------------------------------------
The shares represented hereby will be voted as directed herein.  If no
direction is indicated, such shares will be voted FOR Item 1.

                                                        ----------------        
                                                        I plan to attend
                                                           the meeting.
                                                        ----------------

Item 1. To approve the proposal to               
amend the Corporation's Restated 
Articles of Organization to increase 
the authorized shares of Common 
Stock from 30 million to 50 million 
shares.                               
                                                
    FOR   AGAINST  ABSTAIN
    / /    / /      / /

     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:
                                  ---------------------------------------
                        Signature:
                                  ---------------------------------------
                        Date:
                              --------------------------------------------
                        Date:
                              --------------------------------------------


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"PLEASE MARK INSIDE BLUE BOXES SO THAT
DATA PROCESSING EQUIPMENT WILL RECORD
YOUR VOTES"
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