<PAGE>
 
                                 UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C.  20549


                                  SCHEDULE 13G/A


                   Under the Securities Exchange Act of 1934
                               (Amendment No. 5)*


                                 ImmunoGen, Inc.
        ________________________________________________________________
                                (Name of Issuer)


                                  Common Stock
        ________________________________________________________________
                         (Title of Class of Securities)


                                   45253H101
                         _____________________________
                                 (CUSIP Number)

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

*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter
disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).

                               Page 1 of 4 Pages

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- -------------------                         -----------------      
CUSIP No. 45253H101            13G          Page 2 of 4 Pages
- -------------------                         -----------------      
- --------------------------------------------------------------------------

 1.    NAME OF REPORTING PERSON
       S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
           Aenas Venture Corporation
- --------------------------------------------------------------------------
                                                           (a)        [_]
 2.    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
 
                                                           (b)        [_]
- --------------------------------------------------------------------------
 3.    SEC USE ONLY
- --------------------------------------------------------------------------
 4.    CITIZENSHIP OR PLACE OF ORGANIZATION
           Delaware
- --------------------------------------------------------------------------
                 5.    SOLE VOTING POWER    
  NUMBER OF                1,176,159 shares          
   SHARES        -------------------------------------------------------- 
BENEFICIALLY     6.    SHARED VOTING POWER                                  
  OWNED BY                 ---
    EACH         --------------------------------------------------------
  REPORTING      7.    SOLE DISPOSITIVE POWER 
   PERSON                  1,176,159 shares    
    WITH         --------------------------------------------------------
                 8.    SHARED DISPOSITIVE POWER 
                           --- 
- --------------------------------------------------------------------------
 9.    AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
       PERSON
                 1,176,159 shares
- --------------------------------------------------------------------------
 10.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES          [_]
       CERTAIN SHARES*
- --------------------------------------------------------------------------
 11.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)
                 9.3%
- --------------------------------------------------------------------------
 12.   TYPE OF REPORTING PERSON*
                 CO
- --------------------------------------------------------------------------

                     *SEE INSTRUCTIONS BEFORE FILLING OUT!

<PAGE>
 
                                 SCHEDULE 13G
                                 ------------

Item 1(a)   Name of Issuer:
               ImmunoGen, Inc.

     1(b)   Address of Issuer's Principal Executive Offices:
               148 Sidney Street
               Cambridge, MA  02139

Item 2(a)   Name of Person Filing:
               Aeneas Venture Corporation

     2(b)   Address of Principal Business Office or, if none,
               Residence:
                  c/o Harvard Management Company, Inc.
                  600 Atlantic Avenue
                  Boston, MA  02210

     2(c)   Citizenship:
                 Delaware

     2(d)   Title of Class of Securities:
                 Common Stock

     2(e)   CUSIP Number:
               45253H101

Item 3      If this statement is filed pursuant to Rules 13d-1(b), or
            13d-2(b):

                  The entity filing is a wholly-owned subsidiary of the
                  endowment fund of Harvard University.

Item 4(a)   Amount beneficially owned:
                  1,176,159 shares, 7,500 shares of which Aeneas Venture
                  Corporation has the right to acquire pursuant to options that
                  it holds.

     4(b)   Percent of Class:
                9.3%

     4(c)   Number of shares as to which such person has:

            (i) sole power to vote or to direct the vote:
                   1,176,159 shares


                               Page 3 of 4 Pages

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            (ii) shared power to vote or to direct the vote:
                          ______

            (iii) sole power to dispose or to direct the disposition of:
                     1,176,159 shares

            (iv) shared power to dispose or to direct the disposition of:
                          ______

Item 5      Ownership of Five Percent or less of a Class:
                  Not Applicable.

Item 6      Ownership of more than Five Percent on behalf of another person:
                  Not Applicable.

Item 7      Identification and Classification of the Subsidiary which Acquired
            the Security Being Reported on by the Parent Holding Company:
                  Not Applicable.

Item 8      Identification and Classification of Members of the Group:
                  Not Applicable.

Item 9      Notice of Dissolution of Group:
                  Not Applicable.

Item 10     Certification:

            By signing below the undersigned certifies that, to the best of its
            knowledge and belief, the securities referred to above were acquired
            in the ordinary course of business and were not acquired for the
            purpose of and do not have the effect of changing or influencing the
            control of the issuer of such securities and were not acquired in
            connection with or as a participant in any transaction having such
            purposes or effect.

            After reasonable inquiry and to the best of its knowledge and
            belief, the undersigned certifies that the information set forth in
            this statement is true, complete and correct.


                               AENEAS VENTURE CORPORATION


                               By:  /s/  Verne O. Sedlacek
                                  -----------------------------------------
                                 Name:    Verne O. Sedlacek
                                 Title:   Authorized Signatory

February 13, 1996

                               Page 4 of 4 Pages