<PAGE>   1
                                UNITED STATES
                     SECURITIES AND EXCHANGE COMMISSION
                           Washington, D.C.  20549

                                  FORM 10-Q

   [X]  QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
                       SECURITIES EXCHANGE ACT OF 1934

For the quarterly period ended   December 31, 1995
                                 ------------------

                                     OR

   [ ]  TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
                       SECURITIES EXCHANGE ACT OF 1934

For the transition period from                  to     
                               ----------------    ----------------

Commission file number  0-17999 
                       ---------

                               ImmunoGen, Inc.
           ------------------------------------------------------
           (Exact name of registrant as specified in its charter)

         Massachusetts                                     04-2726691
- - ----------------------------------          ------------------------------------
  (State or other jurisdiction of           (I.R.S. Employer Identification No.)
  incorporation or organization)

                              148 Sidney Street
                            Cambridge, MA  02139
        ------------------------------------------------------------
        (Address of principal executive offices, including zip code)
 
                               (617) 661-9312
            ----------------------------------------------------
            (Registrant's telephone number, including area code)


       ---------------------------------------------------------------
       (Former name, former address and former fiscal year, if changed
                             since last report.)

       Indicate by check mark whether the registrant (1) has filed all reports
 required to be filed by Section 13 or 15(d) of the Securities Exchange Act
 of 1934 during the preceding 12 months (or for such shorter period that the
 registrant was required to file such reports,) and (2) has been subject to
 such filing requirements for the past 90 days.

Yes  x   No    
    ---     ---

      Indicate the number of shares outstanding of each of the issuer's
 classes of common stock, as of the latest practicable date.
   
      At February 14, 1996 there were 15,526,357 shares of common stock, par 
value $.01 per share, of the registrant outstand/ing.

                         Total Number of Pages:  39
                         Exhibit Index at Page:  17


<PAGE>   2

<TABLE>
                               IMMUNOGEN, INC.

                              TABLE OF CONTENTS
                              -----------------      
<CAPTION>
                                                                                      Page
                                                                                      ----
<S>  <C>         <C>                                                                     <C>

PART I - FINANCIAL INFORMATION


     Item 1      Financial Statements

                 Conslidated Balance Sheets as of
                 June 30, 1995 and December 31, 1995...........................          3
                                                                                         
                 Consolidated Statements of Operations 
                 for the three months and the six months 
                 ended December 31, 1994 and 1995..............................          4     

                 Consolidated Statements of Stockholders'
                 Equity for the year ended June 30, 1995 and for
                 the six months ended December 31, 1995........................          5

                 Consolidated Statements of Cash Flows
                 for the six months ended December 31, 
                 1994 and 1995.................................................          6

                 Notes to Consolidated Financial Statements....................          7


     Item 2      Management's Discussion and Analysis of
                 Financial Condition and Results of Operations.................          9



PART II - OTHER INFORMATION


     Item 6      Exhibits and Reports on Form 8-K...............................        14

Signatures......................................................................        16
             
</TABLE>


<PAGE>   3


<TABLE>
IMMUNOGEN, INC.                                          
CONSOLIDATED BALANCE SHEETS                              
As of June 30, 1995 and December 31, 1995                
                                                         
                                                         
<CAPTION>
                                                                 June 30,                  December 31,
                                                                   1995                       1995
                                                              -------------               -------------
<S>                                                           <C>                         <C>
    ASSETS                                               
                                                                                  
Cash and cash equivalents                                     $   3,047,236               $     877,406
Other current assets (Note D)                                       293,852                   1,003,077
                                                              -------------               ------------- 
                                                                                  
      Total current assets                                        3,341,088                   1,880,483
                                                              -------------               ------------- 
                                                                                  
Property and equipment, net of accumulated                                        
   depreciation                                                  13,621,383                   5,084,142
Other assets (Note D)                                                83,700                   1,683,700
                                                              -------------               ------------- 
                                                                                  
           Total assets                                       $  17,046,171               $   8,648,325
                                                              =============               =============
                                                                                  
                                                                                  
LIABILITIES AND STOCKHOLDERS' EQUITY                                              
                                                                                  
Accounts payable                                                  2,229,003                   1,998,392
Accrued compensation                                                316,973                     432,811
Other accrued liabilities                                           978,253                     723,098
Subordinated  convertible debentures (Note C)                        -                          360,000
Current portion of capital lease obligations                        942,749                     136,340
                                                              -------------               ------------- 
                                                                                  
      Total current liabilities                                   4,466,978                   3,650,641
                                                              -------------               ------------- 
                                                                                  
Capital lease obligations                                         2,330,680                     109,157
Other non-current liabilities                                       125,354                     -
                                                                                  
Commitments                                                                       
                                                                                  
Redeemable convertible preferred stock,                                           
   $.01 par value; authorized 277,080 shares;                                     
   none issued                                                      -                           -
                                                                                   
Stockholders' equity (Note C):                                                    
   Common stock, $.01 par value; authorized                                       
      20,000,000 shares; issued and outstanding                                   
      12,578,606 and,15,074,226 shares as of                                     
      June 30, 1995 and December 31, 1995, respectively             125,786                     150,742
   Additional paid-in capital                                   118,988,736                 122,266,150
                                                              -------------               ------------- 
                                                                                  
                                                                119,114,522                 122,416,892
   Accumulated deficit                                         (108,991,363)               (117,528,365)
                                                              ------------                ------------- 
                                                                                  
      Total stockholders' equity                                 10,123,159                   4,888,527
                                                              -------------               ------------- 
                                                                                  
           Total liabilities and stockholders' equity         $  17,046,171               $   8,648,325
                                                              =============               =============


</TABLE>



   The accompanying notes are an integral part of the financial statements.



                                      3
                                      

<PAGE>   4


<TABLE>

IMMUNOGEN, INC.
CONSOLIDATED STATEMENTS OF OPERATIONS
For the three months and six months ended December 31, 1994 and 1995

<CAPTION>

                                   
                                                    Three Months Ended                               Six Months Ended
                                                        December 31,                                     December 31,
                                           --------------------------------------             ---------------------------------
                                                                                                             
                                               1994                      1995                     1994                 1995
                                           -----------               ------------             ------------          -----------
 <S>                                       <C>                       <C>                      <C>                   <C>            
 Revenues:                                                                                                          $   223,162
    Development fees                                                 $    86,834                             
    Interest                               $   142,956                    22,281              $    319,605               56,614  
    Licensing                                    -                         7,500                     -                    7,500
    Other                                       13,929                    13,929                    24,715               27,857
                                           -----------               -----------              ------------          -----------
                                                                                                             
       Total revenues                          156,885                   130,544                   344,320              315,133
                                           -----------               -----------              ------------          -----------
                                                                                                             
                                                                                                             
 Expenses:                                                                                                   
    Research and development                 5,164,704                 2,721,238                 9,718,052            5,645,838
    General and administrative                 921,371                   507,693                 1,917,612              961,264
    Interest                                   121,988                   228,166                   290,200              592,272
    Loss on disposal of assets                   
      (Note D)                                   -                     1,652,014                     -                1,652,014
                                           -----------               -----------              ------------          -----------
                                                                                                             
       Total expenses                        6,208,063                 5,109,111                11,925,864            8,851,388
                                           -----------               -----------              ------------          -----------
                                                                                                             
                                                                                                             
 Loss before income taxes                   (6,051,178)               (4,978,567)              (11,581,544)          (8,536,255)
                                                                                                             
 Income tax expense                              1,887                       294                     4,219                  747
                                           -----------               -----------              ------------          -----------
                                                                                                             
 Net loss                                  $(6,053,065)              $(4,978,861)             $(11,585,763)         $(8,537,002) 
                                           ===========               ===========              ============          ===========
                                                                                                             
                                                                                                              
 Loss per common share                     $     (0.48)              $     (0.36)             $      (0.92)         $     (0.65)
                                           ===========               ===========              ============          ===========
                                                                                                             
 Shares used in computing loss                                                                               
    per share amounts                       12,569,486                13,757,414                12,564,652           13,169,662
                                           ===========               ===========              ============          ===========
                                               

</TABLE>

   The accompanying notes are an integral part of the financial statements.


                                      4


<PAGE>   5


<TABLE>

IMMUNOGEN, INC.
CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY
For the six months ended December 31, 1995
<CAPTION>
                                                         Common Stock
                                              ------------------------------------
                                                                       Additional                      Total
                                                                        Paid-in       Accumulated   Stockholders'
                                                Shares      Amount      Capital         Deficit        Equity
                                              ----------   --------   ------------   -------------   -----------
<S>                                           <C>          <C>        <C>            <C>             <C>
                                                                                                                      
Balance at June 30, 1995                      12,578,606   $125,786   $118,988,736   $(108,991,363)  $10,123,159
                                              ----------   --------   ------------   -------------   -----------
                                                                                                                     
Stock options excercised                          11,000        110          7,041         -               7,151
Issuance and conversion of 7% subordinated                                                                           
   convertible debentures                      2,484,620     24,846      3,270,373         -           3,295,219
Net loss for the six  months ended                                                                                   
   December 31, 1995                              -            -           -            (8,537,002)   (8,537,002)
                                              ----------   --------   ------------   -------------   -----------
                                                                                                                     
Balance at  December 31, 1995                 15,074,226   $150,742   $122,266,150   $(117,528,365)  $ 4,888,527
                                              ==========   ========   ============   =============   ===========
                                                
</TABLE>
                                        
                                                


   The accompanying notes are an integral part of the financial statements.



                                      5


<PAGE>   6


<TABLE>
IMMUNOGEN, INC.
CONSOLIDATED STATEMENTS OF CASH FLOWS
For the six months ended December 31, 1994 and 1995

<CAPTION>                                                           
                                                                                  Six Months Ended
                                                                                    December 31,
                                                                  --------------------------------------------------
                                                                                             
                                                                         1994                            1995
                                                                  ------------------             -------------------

 <S>                                                                   <C>                                <C>
 Cash flows from operating activities:                                                       
    Net loss                                                           $(11,585,763)                      $(8,537,002)
    Adjustments to reconcile net loss to net                           
       cash used for operating activities:                                                   
         Depreciation and amortization                                    1,683,527                         1,590,150
         Loss  on disposal of facility                                        -                             1,652,014
         Other                                                                -                                34,199
         Changes in operating assets and liabilities:                                        
          Other current assets                                              319,306                            77,370
          Accounts payable                                                 (733,022)                         (230,611)
          Accrued compensation                                              114,299                           115,838
          Other accrued liabilities                                        (121,700)                          (35,659)
          Other non-current liabilities                                     (27,856)                          (27,856)
                                                                       ------------                       -----------
                                                                                             
         Net cash used for operating activities                         (10,351,209)                       (5,361,557)
                                                                       ------------                       -----------
                                                                                             
 Cash flows from investing activities:                                                       
    Capital expenditures                                                   (384,604)                          (18,251)
    Proceeds from sale of marketable securities                          18,660,193                             -
    Purchase of marketable securities                                    (5,466,562)                            -
                                                                       ------------                       -----------
                                                                                             
         Net cash provided by (used for)                                                     
           investing activities                                          12,809,027                           (18,251)
                                                                       ------------                       -----------
                                                                                             
 Cash flows from financing activities:                                                       
    Proceeds from  subordinated  convertible debentures                       -                             3,600,000
    Stock issuances, net                                                     14,545                             7,151
    Principal payments on capital lease obligations                        (471,241)                         (397,173)
                                                                       ------------                       -----------
                                                                                             
         Net cash provided by (used for)                                                     
          financing activities                                             (456,696)                        3,209,978
                                                                       ------------                       -----------
                                                                                             
 Net change in cash and cash equivalents                                  2,001,122                        (2,169,830)
                                                                       ------------                       -----------
                                                                                             
 Cash and cash equivalents, beginning balance                             1,572,389                         3,047,236
                                                                       ------------                       -----------
                                                                                            
 Cash and cash equivalents, ending balance                             $  3,573,511                       $   877,406
                                                                       ============                       ===========
                                                                                             
                                                                                             
 Supplemental disclosure of cash flow information:                                           
                                                                                             
    Cash paid for interest                                             $    163,949                       $   592,272
                                                                       ============                       ===========
                                                                                             
    Cash paid  for income taxes                                        $        456                       $     5,000
                                                                       ============                       ===========

<FN>

 Supplemental disclosure of noncash financing activities:

     Issuance of 2,484,620 shares of common stock for conversion of $3,295,219 of principal and interest on 7% subordinated
     convertible debentures to debenture holders.

</TABLE>


                                      6


<PAGE>   7
                               IMMUNOGEN, INC.

                 NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
                                

A. In the opinion of management, the accompanying financial statements 
include all adjustments, consisting of only normal recurring accruals, necessary
to present fairly the consolidated financial position, results of operations 
and cash flows of ImmunoGen, Inc. (the "Company"), which include those of its 
wholly-owned subsidiary, ImmunoGen Securities Corp., and its 72%-owned 
subsidiary, Apoptosis Technology, Inc. ("ATI").

The Company has been unprofitable since inception and expects to incur
net losses over the next several years, if it is able to raise sufficient
working capital to continue operations. The Company's cash resources at
December 31, 1995 were $877,000, and the Company has continued since that date
actively to seek additional capital by pursuing one or more financing
transactions and/or strategic partnering arrangements, as well as implementing
the lease assignments described in Note D. While the Company remains hopeful
that it will be able to consummate an additional financing transaction in the
near term, no assurance can be given that such financing will be available to
the Company on acceptable terms, if at all. If the Company is unable to obtain
financing on acceptable terms in order to maintain operations, it could be
forced to curtail or discontinue operations.

B. Effective September 1, 1995 the Company entered into an agreement to
sublease approximately 82% of one of its Cambridge, Massachusetts facilities and
to lease certain related equipment. The initial term of this sublease agreement
expires in February 1997, with two successive one-year renewal options, the
first of which has been exercised by the sublessee. Net receipts under this
agreement, which are credited to reduce operating expenses, are expected to
total approximately $1.7 million through February 1998, of which approximately
$550,000 is expected to be received by the Company in fiscal 1996.  
             
C. In August 1995 the Company issued $3.6 million of 7% subordinated
convertible debentures, due July 31, 1996, in a private placement to a small
number of overseas investors. As of December 13, 1995, debentures totalling
$3.24 million plus accrued interest thereon had been converted to shares of the
Company's Common Stock. On December 22, 1995 conversion notices for the
remaining $360,000 principal balance were received by the Company. However, 
because Nasdaq Stock Market regulations prohibit certain issuances of stock
without the prior approval of shareholders and such approval had not been
obtained, the Company was unable to convert the debentures at that time. 
Subsequent to December 31, 1995 the Company sought and received an exception to
the shareholder approval requirement from the Nasdaq Stock Market, the
shareholders were notified of the exception and in early 1996 the remaining
convertible debentures were converted into shares of the Company's Common Stock.
In total, 2,753,269 shares were issued to the holders of the $3.6 million 7%
subordinated convertible debentures for both principal and interest. 
                                
                                
                                      7

<PAGE>   8

D. In January 1996 the Company assigned its leases on its Canton,
Massachusetts production facility and equipment to another biotechnology
company. Under the terms of the agreements, the assignee has assumed all
payment obligations under the leases, which amount to approximately $116,000 per
month, and, in addition, will make cash payments to the Company totaling
approximately $2.4 million at various dates through July 1999. As of December
31, 1995 the Company's books reflect this January 1996 disposition of the
Company's Canton assets. Short-term and long-term amounts due the Company from
the assignee under these agreements are reflected in the Company's December 31,
1995 consolidated balance sheets in Other Current Assets and Other Assets,
respectively. Approximately $635,000 of this amount was received by the Company
in January 1996 and an additional $150,000 is expected to be received in
February 1996. In addition, the Company recognized a net loss on its equipment
lease at the Canton facility of approximately $1.7 million. The Company
estimates that the assignment of its Canton leases will save approximately
$140,000 per month in total monthly operating expenses.




                                      8

<PAGE>   9
 
                   MANAGEMENT'S DISCUSSION AND ANALYSIS
              OF FINANCIAL CONDITION AND RESULTS OF OPERATIONS
                                
                                
OVERVIEW

        The Company was formed to develop, produce and market commercial
cancer and other pharmaceuticals based on molecular immunology. The Company is
in a research and development phase and expects no revenues to be derived from
product sales in the near future.

        Since July 1, 1994 the Company has taken steps to reduce its operating
costs, including implementation of a restructuring plan in December 1994 and,
effective September 1, 1995, subleasing approximately 82% of one of its
Cambridge, Massachusetts facilities and leasing certain related equipment. The
initial term of this sublease agreement expires in February 1997, with two
one-year renewal options. During the quarter ended December 31, 1995, the
sublessee exercised the first option, extending the current term to February
1998.  In addition, effective January 1, 1996 the Company has assigned its
leases on its Canton, Massachusetts facility and equipment to another
biotechnology company. Under the terms of the agreements, the assignee has
assumed all payment obligations under the leases, which amount to approximately
$116,000 per month and, in addition, will make cash payments to the Company
totaling approximately $2.4 million at various dates to July 1999, of which
approximately $785,000 is expected in fiscal 1996. The Company estimates that
this transaction will save approximately $140,000 per month in total monthly
operating expenses.
 
        The Company has been unprofitable since inception and expects to incur
net losses over the next several years, if it is able to raise sufficient
working capital to continue operations. The Company's cash resources at
December 31, 1995 were $877,000, and the Company has continued since that date
actively to seek additional capital by pursuing one or more financing
transactions and/or strategic partnering arrangements, as well as implementing
the lease assignments discussed above. While the Company remains hopeful that
it will be able to consummate an additional financing transaction in the near
term, no assurance can be given that such financing will be available to the
Company on acceptable terms, if at all. If the Company is unable to obtain
financing on acceptable terms in order to maintain operations, it could be
forced to curtail or discontinue operations. 

RESULTS OF OPERATIONS

  Three Months Ended December 31, 1994 and 1995

        The Company's revenues decreased approximately 17% from approximately
$157,000 for the three months ended December 31, 1994 to approximately $131,000
in the same period in 1995. Interest income totalled approximately $143,000,
or 91%, of revenues for the three months 

                                      9

<PAGE>   10

ended December 31, 1994, compared to approximately $22,000, or 17%, of
revenues in the corresponding 1995 period. This 84% decrease in interest
income reflects the significant decrease in cash balances available for
investment in the 1995 period. Revenues for the three months ended December
31, 1995 include approximately $86,000 of development revenue, which represents
revenue earned under the Small Business Innovation Research Program of the
National Institutes of Health. Revenues in both periods also include a gain on
sale of assets which resulted from a sale/leaseback agreement for equipment at
the Canton facility executed in fiscal 1994 which had been deferred and
recorded as other income through December 1995.        

        Effective January 1, 1996, the Company assigned its facility and
equipment leases for its Canton, Massachusetts facility to another
biotechnology company. Under the terms of the agreements, the assignee has
assumed all payment obligations under the leases, which amount to approximately
$116,000 per month, and, in addition, will make cash payments to the Company
totaling approximately $2.4 million at various dates through July 1999. As of
December 31, 1995 the Company's books reflect this January 1996 disposition of
the Company's Canton assets. Short-term and long-term amounts due the Company
from the assignee under these agreements are reflected in the Company's
December 31, 1995 consolidated balance sheets in Other Current Assets and Other
Assets, respectively. Approximately $635,000 of this amount was received in
January 1996 and an additional $150,000 is expected to be received in February
1996. In addition, the Company recognized a net loss on its equipment lease at
the Canton facility of approximately $1.7 million. The Company estimates that
the assignment of its Canton leases will save approximately $140,000 per month
in total monthly operating expenses.

        The Company's total expenses, including the one-time charge to
recognize the disposal of the Canton assets, decreased approximately 18% from
approximately $6.2 million for the three months ended December 31, 1994 to
approximately $5.1 million in the same period in 1995. However, exclusive of
that charge, the Company's total expenses decreased approximately 44% to
approximately $3.5 million for the three months ended December 31, 1995. 

        Research and development costs constituted the primary component of the
Company's total ongoing expenses (83% and 79% for the three months ended
December 31, 1994 and 1995, respectively), decreasing from approximately $5.2
million for the three months ended December 31, 1994 to approximately $2.7
million for the three months ended December 31, 1995. This 47% decrease is
principally the result of the savings associated with the Company's
restructuring plan implemented in December 1994 and that portion of facilities
costs savings attributable to the facility sublease allocated to research and
development.

        General and administrative expenses decreased 45% from approximately
$921,000 for the three months ended December 31, 1994 to approximately $508,000
for the three months ended December 31, 1995. This decrease principally
represents savings associated with the restructuring plan and that portion of
facilities costs savings attributable to the facility sublease allocated to
general and administrative expenses.


                                     10


<PAGE>   11
        Interest expense increased 87% from approximately $122,000 for the
three months ended December 31, 1994 to approximately $228,000 for the three
months ended December 31, 1995. This increase is due to issuance costs
associated with, and accrued interest on, the Company's 7% subordinated
convertible debentures issued in an August 1995 private offering, offset
somewhat by lower interest costs associated with the decreasing principal
balances of the Company's capital lease agreements. Subsequent to December
1995 and the assignment of the Company's capital lease for equipment at the
Canton facility, only one capital lease agreement remains.

  Six Months Ended December 31, 1995

        The Company's revenues decreased approximately 8% from approximately
$344,000 for the six months ended December 31, 1994 to approximately $315,000
in the same period in 1995. Interest income totalled approximately $319,000,
or approximately 93%, of revenues for the six months ended December 31, 1994,
compared to approximately $57,000, or 18% of revenues, in the corresponding
1995 period. This decrease is attributable to the significant decrease in cash
balances available for investment in the 1995 period.  Revenues for the six
month period ended December 31, 1995 include development revenues of
approximately $223,000, or 71% of total revenues, which represents revenue
earned under the Small Business Innovation Research Program of the National
Institutes of Health. Also included in revenues for the 1995 period are $7,500
of licensing revenues received by the Company's 72%-owned subsidiary, Apoptosis
Technology, Inc., on the signing of a licensing agreement. Revenues in both
periods include a gain on sale of assets which resulted from the sale/leaseback
agreement for certain equipment at the Canton, Massachusetts facility executed
in fiscal 1994 which had been deferred and recorded as income through December
1995.

        Total expenses decreased approximately 26% from approximately $11.9
million for the six months ended December 31, 1994 to approximately $8.9
million in the corresponding 1995 period. Without the one-time charge for
disposal of the Canton facility and equipment, the Company's total expenses
would have decreased approximately 40%, from approximately $11.9 million to
approximately $7.2 million.  

        Research and development costs constituted the primary component of the
Company's total ongoing expenses (81% and 78% for the six months ended December
31, 1994 and 1995, respectively), decreasing approximately 42% from
approximately $9.7 million for the six months ended December 31, 1994 to
approximately $5.6 million for the corresponding 1995 period.  As in the three
months ended December 31, this decrease is primarily the result of the
Company's restructuring plan implemented in December 1994 and that portion of
the facilities costs savings attributable to the facility sublease allocated to
research and development.

                                     11


<PAGE>   12
        General and administrative expenses decreased approximately 50% from
approximately $1.9 million for the six months ended December 31, 1994 to
approximately $1.0 million in the corresponding 1995 period. As in the three
months ended December 31, this decrease results  principally from savings
associated with the restructuring plan implemented in December 1994 and that
portion of facilities costs savings attributable to the facility sublease
allocated to general and administrative expenses.

        Interest expense increased 104% from approximately $290,000 for the six
months ended December 31, 1994 to approximately $590,000 for the corresponding
1995 period. This increase is due to issuance costs associated with, and
accrued interest on, the Company's 7% subordinated convertible debentures
issued in an August 1995 private offering, offset somewhat by lower interest
costs associated with the decreasing principal balance of the Company's capital
lease agreements.

LIQUIDITY AND CAPITAL RESOURCES

        Since July 1, 1993 the Company has financed its operating deficit of
approximately $52.1 million from various sources, including net proceeds of
approximately $13.0 million raised in its February 1994 public offering, net
proceeds of approximately $3.3 million raised in its August 1995 private
offering to foreign investors and from the exercise of stock options. Since
July 1, 1993 the Company has received approximately $1.3 million of interest
income. At December 31, 1995 approximately $0.9 million of cash and cash
equivalents remained available.

        In February 1994 the Company sold in a public offering 2,012,500 shares
of its Common Stock. Net proceeds to the Company amounted to $13,242,250. In
March 1994 the Company executed a sale/leaseback agreement to finance
approximately $4.0 million of equipment at its Canton, Massachusetts facility. 
The transaction included warrants to purchase Common Stock, which expire in
April 1999.

        In August 1995 the Company issued $3.6 million of 7% subordinated
convertible debentures, due July 31, 1996, in a private placement to a small
number of foreign investors. Net proceeds to the Company amounted to
approximately $3.3 million. As of December 13, 1995, debentures totalling
$3.24 million plus accrued interest thereon had been converted to shares of the
Company's Common Stock. On December 22, 1995 conversion notices for the
remaining $360,000 principal balance were received by the Company. However,
because Nasdaq Stock Market regulations prohibit certain issuances of stock
without the prior approval of shareholders and such approval had not been
obtained, the Company was unable to convert the debentures at 


                                     12



<PAGE>   13


that time. Subsequent to December 31, 1995 the Company sought and
received an exception to the shareholder approval requirement from the Nasdaq
Stock Market, the shareholders were notified of the exception and in early 1996
the remaining convertible debentures were converted into shares of the
Company's Common Stock. In total, 2,753,269 shares were issued to the holders
of the $3.6 million 7% subordinated convertible debentures for both principal
and interest. 

        In the period since July 1, 1993 approximately $8.1 million was
expended on property and equipment, including equipment sold and leased back by
the Company, principally for construction of the Company's manufacturing
facilities. No significant amounts are expected to be expended on property and
equipment in fiscal 1996.

        Pursuant to its agreements with ATI, ImmunoGen has agreed to obtain or
furnish an additional $3.0 million in equity for ATI on such terms and
conditions as may be mutually agreed to by ATI and the providers of such
additional equity. The Company anticipates that approximately $650,000 of
funding may be required by ATI during calendar year 1996 in order for ATI to
satisfy certain contractual obligations.
                                
        The Company anticipates that its existing capital resources will enable
it to maintain its current and planned operations through February 1996. 
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. Although management
continues to pursue additional funding arrangements, no assurance can be given
that such financing will in fact be available to the Company. If the Company
is unable to obtain financing on acceptable terms in order to maintain
operations, it could be forced to curtail or discontinue its operations.

                                     13
                                

<PAGE>   14
                               IMMUNOGEN, INC.

 
                        PART II - OTHER INFORMATION
                         ---------------------------


Item 1. Legal Proceedings
        -----------------

        Not applicable.


Item 2. Changes in Securities
        ---------------------
        
        Not applicable.


Item 3. Defaults Upon Senior Securities
        -------------------------------
       
        Not applicable.


Item 4. Submission of Matters to a Vote of Security Holders
        ---------------------------------------------------

        Not applicable.


Item 5. Other Information
        -----------------

        Not applicable.


Item 6. Exhibits and Reports on Form 8-K                   
        ---------------------------------
                                                                    

        (a) Exhibits      
                  
            Exhibit 10.29   Leasehold Mortgage and Collateral
                            Assignment of Lessee's Interest dated
                            January 1, 1996 between OraVax, Inc.,
                            as assignor, and the Registrant, as assignee

            Exhibit 10.30   Assignment of Lease dated January 1,
                            1996 between the Registrant, as assignor,
                            and OraVax, Inc., as assignee


                                     14


<PAGE>   15
            Exhibit 10.31   Assignment of Equipment Lease and
                            Lessor's Consent dated January 1, 1996
                            by and among the Registrant, as lessee,
                            OraVax, Inc., as assignee, and Aberlyn
                            Capital Managment Limited Partnership,
                            as lessor
                    
        (b)  No reports on Form 8-K were filed during the three months
             ended December 31, 1995. 


                                     15

<PAGE>   16

                                 SIGNATURES
                                 ----------

     Pursuant to the requirements of the Securities Exchange Act of 1934, the 
 registrant has duly caused this report to be signed on its behalf by the
 undersigned thereunto duly authorized.


                                           IMMUNOGEN, INC.





 Date:   February 14, 1996                 By:  /s/ Mitchel Sayare
                                              ----------------------------
                                                Mitchel Sayare
                                                Chief Executive Officer
                                                (principal executive officer)


 Date:   February 14, 1996                 By:  /s/ Frank J. Pocher           
                                              ---------------------------  
                                                Frank J. Pocher     
                                                Vice President and
                                                Chief Financial Officer  
                                                (principal financial officer)
                    

                                     16
 

<PAGE>   17

<TABLE>

                               IMMUNOGEN, INC.


                                EXHIBIT INDEX
                                    
<CAPTION>                                    
                                                           Sequentially Numbered
                                                           ---------------------
Exhibits                                                            Page
- - --------                                                            ----
<S>                <C>                                               <C>
10.29              Leasehold Mortgage and Collateral
                   Assignment of Lessee's Interest dated
                   January 1, 1996 between OraVax, Inc.,
                   as assignor, and the Registrant, as assignee      18

10.30              Assignment of Lease dated January 1,
                   1996 between the Registrant, as assignor,
                   and OraVax, Inc., as assignee                     25

10.31              Assignment of Equipment Lease and
                   Lessor's Consent dated January 1, 1996
                   by and among the Registrant, as lessee,
                   OraVax, Inc., as assignee, and Aberlyn
                   Capital Management Limited Partnership,
                   as lessor                                         33


</TABLE>



                                     17






<PAGE>   1
                 LEASEHOLD MORTGAGE AND COLLATERAL ASSIGNMENT
                 --------------------------------------------
                             OF LESSEE'S INTEREST
                             --------------------


        THIS LEASEHOLD MORTGAGE AND COLLATERAL ASSIGNMENT OF LESSEE'S INTEREST
(this "Mortgage") is made as of the 1st day of January, 1996 by ORAVAX, INC., a
Delaware corporation (the "Assignor"), with a mailing address at 38 Sidney
Street, Cambridge, Massachusetts 02139, to IMMUNOGEN, INC., its successors and
assigns (the "Assignee"), with a mailing address at 128 Sidney Street,
Cambridge, Massachusetts 02139.  

        Reference is made hereby to a certain Assignment of Lease (the
"Assignment"), of even date, by and between Assignee and Assignor, which
relates to Assignee's assignment to Assignor of all of Assignee's right, title
and interest in and to a lease (the "Space Lease") between Assignee and AEW #1
Corporation ("Landlord"), covering certain premises known as 90 Shawmut Road in
Canton, Massachusetts. Pursuant to the Lease Assignment, Assignor undertook
certain obligations, including without limitation the performance of all
obligations of Assignee under the Space Lease, and the payment of Two Million
Two Hundred Ninety Thousand Dollars ($2,290,000.00) in installments
(collectively, the "Obligations"). The Lease and the Assignment, as well as any
other instrument or agreement
 executed and/or delivered in connection
therewith, may be referred to as the "Instruments."

        In order to secure further the prompt payment of all Obligations due in
respect of the Assignment, Assignor does hereby (I) assign, transfer, and set
over unto the Assignee (a) all right, title and interest of Assignor in, to and
under the Space Lease (including without limitation any leasehold improvements
therein, to the extent owned by Assignor), now existing or which Assignor may
from time to time hereafter obtain during the term of this Mortgage, together
with any and all extensions, amendments, replacements, substitutions or
modifications thereof; (b) all of Assignor's contractual rights now existing or
hereafter arising between Assignor and any successor landlord, together with
the right to dispose of or otherwise act with respect to such rights in the
event of bankruptcy or insolvency of any such landlord (the "Contractual
Rights"), and (c) all of Assignor's rights, whether evidenced by permit,
agreement or otherwise, to operate the premises demised under the Lease (the
"Premises") (the "Operating Rights") and (II) grant to Assignee all of
Assignor's right, title and interest in, under and to (x) the Lease (y) the
Contractual Rights, and (z) the Operating Rights, with MORTGAGE COVENANTS and
with all the rights of a secured party under the Uniform Commercial Code as in
effect from time to time in the Commonwealth of Massachusetts.

        The Premises are more particularly described in Exhibit A attached
hereto, and consist in part of real property located at 90 Shawmut Road in
Canton, Massachusetts.

        The rights granted in this Mortgage shall become operative or may be
exercised at the option of the Assignee upon the occurrence of any default
under this Mortgage or the other Instruments beyond any applicable cure period
(hereinafter, an "Event of Default"), provided that in no event shall an Event
of Default be deemed to exist until any default by Assignor has


<PAGE>   2
continued for a period of (i) fifteen (15) days after written notice
thereof by Assignee in the case of a payment due from Assignor to Assignee
under the Lease Assignment, and (ii) thirty (30) days after written notice
thereof by Assignee in the case of any default not involving such a payment,
or, if shorter, any period of time allowed by landlord under the Lease for
Assignee to cure a failure by Assignor. So long as there shall exist no default
in the payment of the indebtedness secured hereby or in the performance of any
obligation, covenant or agreement herein or in the Instruments, or a default
contained in the Lease on the part of the Assignor to be performed, which
continues beyond any cure period contained herein or therein, the Assignor
shall have the right to use and enjoy all rights and privileges granted to it
pursuant to the terms of the Lease.  

        Assignor hereby authorizes Assignee, its employees and agents, at the
Assignee's option, upon or at any time after an Event of Default and with such
notice as shall be required by law, if any, to enter upon the Premises and take
possession thereof and exercise all other rights and privileges granted to
Assignor pursuant to the Lease.  

        Assignor also authorizes Assignee, its employees and agents, at its
option after such Event of Default, to enforce all or any of such contractual
rights as may have been assigned hereby, and Assignor hereby irrevocably
appoints Assignee its attorney in fact, coupled with an interest, to do all
acts pertaining thereto in its place and stead.  

        Assignor also authorizes Assignee, its employees and agents, upon such
entry, at its option, to take over and assume the management, operation and
maintenance of the Premises as provided for pursuant to the terms of the Lease
and in connection therewith to perform all acts and to expend such monies as
Assignee may deem advisable, in the same manner and to the same extent as
Assignor might do including the right to enter into subleases with subtenants
for the management, operation and maintenance of the Premises described in the
Lease. Assignor hereby releases all claims against Assignee arising out of or
in connection with such management, operation and maintenance and the exercise
of its other rights hereunder.  

        Assignor agrees, represents, covenants and warrants to Assignee that
(1) Assignor has not executed any prior assignment of its rights thereunder,
(2) Assignor will not modify, amend or terminate the Lease without the prior
written consent of Assignee, which consent shall not be unreasonably withheld
provided that any such modification or amendment does not adversely affect the
security granted to Assignee hereunder, and that in the event of a proposed
termination, Assignee is furnished with substitute collateral reasonably
satisfactory to Assignee, (3) Assignor will not do any act which would destroy
or impair the benefits to Assignee of this Mortgage, and will promptly pay and
perform all of its obligations under the terms of the Lease, (4) Assignor is
the owner of a valid and subsisting interest as tenant under the Lease prior to
the expiration or any applicable cure periods set forth in the Lease, (5) the
Assignor has full power and lawful authority to mortgage and grant the security
interests in the manner and form herein done, (6) the Assignor will preserve
the leasehold estate created by the Lease, and will forever

                                     -2-

<PAGE>   3
warrant and defend the same to the Assignee and will forever warrant
and defend the validity and priority of the lien hereof to the Assignee against
the claims of all persons and parties whomsoever (except as otherwise provided
in the Assignment), (7) the Assignor will not, without the Assignee's prior
written consent, enter into any agreement terminating, modifying or amending
the Lease or releasing the Landlord thereunder from any obligations imposed
upon it thereby, and any such termination, modification or amendment of the
Lease or release of the Landlord without the prior written consent thereto by
the Assignee (which consent shall not be unreasonably withheld provided that
any such modification or amendment does not adversely affect the security
granted to Assignee hereunder, and that in the event of a proposed termination,
Assignee is furnished with substitute collateral reasonably satisfactory to
Assignee), shall be void and of no force and effect, (8) if the Assignor
receives a notice of default under the Lease, it shall immediately cause a copy
of such notice to be given to the Assignee, (9) Assignor will send Assignee
copies of all statements and notices of a material nature, in addition to
notices of default, which are required to be given under any Lease or received
with respect thereto, (10) the interests of Assignor and Landlord under the
Lease shall not be merged and thereby extinguished without Assignee's prior
written consent, (11) notwithstanding any provisions of the Lease to the
contrary, no notice of cancellation of the Lease shall be given by the Assignor
until Assignee has given its prior written consent thereto, unless all
obligations of the Assignor pursuant to the Instruments have been paid or
performed in full and Assignee has acknowledged such payment or performance in
writing signed by an officer of the Assignee duly authorized to do so, and (12)
Assignor will not further mortgage or otherwise encumber the leasehold estate
or other interests hereby mortgaged or assigned.

        Assignor shall not, without having obtained the prior written consent
of Assignee, which consent shall not be unreasonably withheld provided such
release, consent or waiver will not adversely affect the security granted
Assignee hereunder, release Landlord from any liability under the Lease or
otherwise, or consent to, suffer or permit or waive any act or omission on the
part of the Landlord. In addition, Assignor acknowledges that, under the terms
of that certain Consent to Assignment of Lease and Leasehold Mortgage to be
executed and delivered by Landlord in respect of the Assignment, Landlord
requires that Assignee remain liable to Landlord for all obligations and
liabilities of Assignor under the Lease. Therefore, Assignor agrees with
Assignee: (i) not to amend or modify, or agree to or acquiesce in any amendment
to or modification of, the Lease, the effect of which amendment or modification
would be to increase or extend the obligations or liability of the Assignor or
Assignee under the Lease, as so amended or modified, without Assignee's prior
written consent (which shall not be unreasonably withheld or delayed, provided
that Assignor shall have given Assignee such information as Assignee may
reasonably request regarding Assignor's financial standing and condition as to
allow Assignee to determine that Assignor has resources sufficient under the
circumstances to meet such increased obligations); and (ii) to immediately give
Assignee a true and complete copy of any notice or other correspondence
received by Assignor in respect of the Lease, including without limitation any
notice or correspondence concerning any failure, breach or default, whether
actual or alleged, on the part of Assignor under the Lease.

                                     -3-

<PAGE>   4
        The Assignee shall not be liable for any loss sustained by the Assignor
resulting from any act or omission of the Assignee in exercising its rights
hereunder or under the Lease unless such loss is caused by the bad faith of the
Assignee. Assignee shall not be obligated to perform or discharge any
obligation, duty or liability under the Lease (except as otherwise provided in
the Assignment), and Assignor hereby indemnifies and holds Assignee harmless
from any liability, loss, or damage which it might incur under the Lease
(except as otherwise provided in the Assignment), by reason of this Mortgage or
the exercise of its rights hereunder, or from any other claims or demands which
may be asserted against Assignee by reason of any alleged obligation or
undertaking on its part to be performed or discharged under the Lease (except
as otherwise provided in the Assignment) prior to completion of the exercise of
its rights hereunder or in defense of any such claims or demands, excluding,
however, any liability, loss, damage, claim or demand arising out of Assignee's
bad faith. The amount of any indemnified liability, loss, damage, claim or
demand, including costs, expenses and reasonable attorneys' fees, shall be
secured hereby.  Upon the failure of the Assignor to do so, the Assignee may,
at its option, declare all sums secured hereby immediately due and payable and
shall cause any non-reimbursed amounts to be added to the debt secured hereby. 
If Assignee incurs any such liability, loss or damage in the defense of any
such claims or demands, Assignor shall immediately, upon demand, reimburse
Assignee for the amount thereof, including costs, expenses and reasonable
attorneys' fees. It is further understood that, prior to completion of the
exercise of its rights hereunder, this Agreement shall not operate to place
responsibility upon the Assignee for the control, care, management or repair of
the Premises, nor for the carrying out of any of the terms and conditions of
the Lease; nor shall it operate to make the Assignee responsible or liable for
any negligence in the management, upkeep, repair or control of the Premises
resulting in loss or injury or death to any tenant, licensee, employee or
stranger.  

        Entry by Assignee upon the Premises under the terms of this Mortgage
shall not constitute Assignee a "mortgagee in possession" in contemplation of
law, except at the option of Assignee expressed in writing.

        The provisions of this Mortgage shall be binding upon Assignor and
Assignor's legal representatives, successors and assigns and shall inure to the
benefit of the Assignee and its successors and assigns. To the maximum extent
permitted by law, this instrument shall be governed by the law of the
Commonwealth of Massachusetts.

        This Mortgage shall remain in full force and effect so long as any
obligations under the Instruments shall remain outstanding and only a discharge
hereof appearing of record in the Norfolk County Registry of Deeds or Land
Court or other written acknowledgment signed by an officer of the Assignee duly
authorized to do so shall operate as a release of Assignee's rights and
interest hereunder.

        The mortgage granted hereby is upon the STATUTORY CONDITION and also
upon the other conditions herein set forth, all of which shall be binding on
Assignor and those claiming

                                     -4-

<PAGE>   5
under Assignor.  For any breach of the aforesaid STATUTORY CONDITION or of any
of the other conditions herein set forth, Assignee or any subsequent holder of
this instrument shall have the STATUTORY POWER OF SALE in addition to any other
remedy or remedies provided herein.

        Assignee may elect to exercise any or all of its rights hereunder and
the partial exercise of its rights hereunder at any one time shall not
thereafter preclude the exercise, at any later time or times, of its other
rights hereunder.

        The mortgage lien granted hereby includes and attaches to all of
Assignor's rights and remedies at any time arising under or pursuant to Section
365(h) of the Bankruptcy Code, 11 U.S.C. Sec. 365(h), including, without
limitation, all of its rights to remain in possession of the Premises
thereunder.

        Assignor shall not, without prior written consent of Assignee, elect to
treat the Lease as terminated or elect to remain in possession of the Premises
under Section 365(h)(1) of the Bankruptcy Code, 11 U.S.C. Sec. 365(h)(1).  Any
such election made without Assignee's prior written consent shall be void.

        Assignor hereby unconditionally assigns, transfers and sets over to
Assignee all of Assignor's claims and rights to the payment of damages arising
from any rejection of the Lease under the Bankruptcy Code, 11 U.S.C. Sec. 101
et seq. (the "Bankruptcy Code"). Assignee shall have the right to proceed in
its own name or in the name of Assignor in respect of any claim, suit, action
or proceeding relating to the rejection of the Lease, including, without
limitation, the right to file and prosecute, to the exclusion of Assignor, any
proofs of claim, complaints, motions, applications, notices and other
documents, in any case in respect of Landlord under the Bankruptcy Code. This
assignment constitutes a present, irrevocable and unconditional assignment of
the foregoing claims, rights and remedies, and shall continue in effect until
all of the indebtedness and obligations secured by this Mortgage shall have
been satisfied and discharged in full. Any amounts received by the Assignee as
damages arising out of the rejection of the Lease as aforesaid shall be applied
first to all costs and expenses of Assignee (including, without limitation,
reasonable attorneys' fees) incurred in connection with the exercise of any of
its rights or remedies hereunder.

        If, pursuant to Section 365(h)(2) of the Bankruptcy Code, Assignor
seeks to offset against the rent reserved in the Lease the amount of any
damages caused by the non-performance by the Landlord of any of Landlord's
obligations under the Lease after the rejection by Landlord of the Lease under
the Bankruptcy Code, Assignor shall, prior to effecting such offset, notify the
Assignee of its intent so to do, setting forth the amounts proposed to be so
offset and the basis therefor. Assignee shall have the right to object to all
or any part of such offset, and, in the event of such objection, Assignor shall
not effect any offset of the amounts so objected to by Assignee. If Assignee
has failed to object as aforesaid within 30 days after notice from the

                                     -5-

<PAGE>   6
Assignor in accordance with the first sentence of this paragraph,
Assignor may proceed to effect such offset in the amounts set forth in
Assignor's notice. Neither the Assignee's failure to reject as aforesaid nor
any objection or other communication between Assignee and Assignor relating to
such offset shall constitute an approval of any such offset by Assignee. 
Assignor shall indemnify and save Assignee harmless from and against any and
all claims, demands, actions, suits, proceedings, damages, losses, costs and
expenses of every nature whatsoever (including, without limitation, attorneys'
fees) arising from or relating to any offset by Assignor against the rent
reserved in the Lease.

        If any action, proceeding, motion or notice shall be commenced or filed
in respect of the Landlord of the Premises in connection with any case under
the Bankruptcy Code, the Assignee shall have the option, to the exclusion of
Assignor, exercisable upon notice from Assignee to Assignor, to conduct and
control any such litigation with counsel of Assignee's choice. Assignee may
proceed in its own name or in the name of Assignor in connection with any such
litigation, and Assignor agrees to execute any and all powers, authorizations,
consents or other documents required by the Assignee in connection therewith. 
Assignor shall, upon demand, pay to Assignee all costs and expenses (including
reasonable attorneys' fees) paid or incurred by Assignee in connection with the
prosecution or conduct of any such proceedings. Any such costs or expenses not
paid by Assignor as aforesaid shall be secured by the lien of this Mortgage and
shall be added to the principal amount of the indebtedness secured hereby. 
Assignor shall not commence any action, suit, proceeding or case, or file any
application or make any motion, in respect of the Lease in any such case under
the Bankruptcy Code without the prior written consent of Assignee.

        Assignor shall, promptly after obtaining knowledge thereof, notify
Assignee orally of any filing by or against Landlord of a petition under the
Bankruptcy Code. Assignor shall thereafter forthwith give written notice of
such filing to Assignee setting forth any information available to Assignor as
to the date of such filing, the court in which such petition was filed, and the
relief sought therein. Assignor shall promptly deliver to Assignee, following
receipt, any and all notices, summonses, pleadings, applications and other
documents received by Assignor in connection with any such petition and any
proceedings relating thereto or in connection therewith.

        Assignor and Assignee (by its acceptance of this Agreement) hereby
irrevocably waive any and all rights to a trial by jury in any action,
proceeding or counterclaim arising out of or in any way related to this
Agreement.

                                     -6-


<PAGE>   7
        IN WITNESS WHEREOF, the undersigned has caused this Mortgage to be
signed as an instrument under seal as of January 1, 1996.


                            ORAVAX, INC.


                            By: /s/ Lance K. Gordon                          
                               -------------------------------------------------
                            Lance K. Gordon
                            Its: President and Chief Executive Officer, hereunto
                            duly authorized


                            By: /s/ Keith S. Ehrlich                          
                               -------------------------------------------------
                            Keith S. Ehrlich
                            Its: Vice President, Treasurer and Chief Financial
                            Officer, hereunto duly authorized


                        COMMONWEALTH OF MASSACHUSETTS


Middlesex, ss.

        On this 22nd day of January, 1996, before me personally appeared Lance
K. Gordon and Keith S. Ehrlich, the President and Chief Executive Officer, and
the Vice President, Treasurer and Chief Financial Officer, respectively, of
Oravax, Inc., and acknowledged this instrument to be the free act and deed of
Oravax, Inc.

                             /s/ Julie M. Klinger
                             ---------------------                        
                             Notary Public
                             My commission expires:

                          [stamp:  JULIE M. KLINGER
                                Notary Public
                     My Commission Expires Feb. 22, 2002]

                                     -7-



<PAGE>   1
                             ASSIGNMENT OF LEASE


        THIS ASSIGNMENT OF LEASE (this "Agreement"), dated as of January 1,
1996, by and between IMMUNOGEN, INC. ("Tenant"), a Massachusetts corporation,
and ORAVAX, INC. ("Assignee"), a Delaware corporation.

        WHEREAS, by a lease (the "Lease") dated as of June 30, 1992, AEW #1
CORPORATION ("Landlord") leased to Tenant that certain 31,113 square-foot
building (the "Premises") known as and numbered 90 Shawmut Road, Canton,
Massachusetts; and

        WHEREAS, on and subject to the terms and conditions set forth herein,
Tenant now desires to assign all of its right, title and interest in and to the
Lease to Assignee, and Assignee desires to accept such assignment, effective as
of the date hereof; and

        NOW, THEREFORE, in consideration of the foregoing, and for other good
and valuable consideration, each to the other paid, the receipt and sufficiency
of which are hereby acknowledged, Landlord, Tenant and Assignee hereby agree as
follows:

1.      ASSIGNMENT; REPRESENTATION; INDEMNITY. (a) That, effective as
        of the date hereof, Tenant hereby assigns to Assignee all of Tenant's
        right, title and interest in and to the Lease, including without
        limitation all security and other deposits currently being held by
        Landlord in respect of the Lease, and Tenant's right
 to purchase the
        Premises as set forth in Section 2.6 of the Lease, all upon the terms
        and conditions herein set forth. A true copy of the Lease, together
        with any amendments, is annexed as Exhibit A.

        (b) Tenant represents and warrants to Assignee that, as of the
        date hereof, it has not received from Landlord any written notice from
        Landlord to the effect that any default (or event or circumstance
        which, with the passage of time or the giving of notice, would
        constitute a default) exists on the part of Tenant under the Lease, and
        to the best of Tenant's actual knowledge, no such default, event or
        circumstance in fact exists; and Tenant agrees with Assignee to
        indemnify and hold Assignee harmless from and against any and all costs
        and obligations to have been paid or performed by Tenant under the
        Lease on or prior to the date hereof.

        (c) Tenant agrees with Assignee to indemnify and hold Assignee
        harmless from and against any and all loss, cost, damage and expense
        suffered by Assignee (including without limitation reasonable
        attorneys' fees and costs of defense) as a result of any claim under or
        in respect of the Lease, which claim relates to the period prior to the
        date hereof.

        (d) Tenant has not dealt with any broker or other person or
        firm to whom a commission or fee is or may be due in respect of this
        assignment, and Tenant hereby agrees to


<PAGE>   2
        indemnify and hold the Assignee and Landlord harmless from and
        against any and all loss, cost, damage and expense (including without
        limitation reasonable attorneys' fees and costs) suffered by the other
        as a result of any claim against Assignee or Landlord that a fee or
        commission is due on account of a relationship between the claimant and
        the Tenant.

2.      ACCEPTANCE AND ASSUMPTION OF OBLIGATIONS; INDEMNITY. (a) Assignee
        accepts from Tenant the above assignment, and hereby agrees
        with Landlord and Tenant faithfully to assume and perform each and
        every obligation of Tenant under the Lease, including without
        limitation the obligation to pay rent, additional rent, additional
        charges, common area charges, real estate taxes and any and all other
        costs and expenses, however labelled or designated, and to observe all
        terms and conditions thereof, including without limitation the
        prohibition against further subletting or assignment, with the same
        force and effect as if Assignee were the Tenant originally named under
        the Lease.

        (b) Assignee agrees to accept the Premises in their "As Is"
        condition as of December 29, 1995, and represents that Assignee has
        inspected the Premises and any common areas demised under the Lease,
        and is satisfied as to their suitability for Assignee's intended uses.
        Neither Tenant nor Landlord shall be liable for the performance of any
        work to prepare the Premises for Assignee's use, or for the
        reimbursement of any cost or expense incurred by Tenant in connection
        with any such work. Notwithstanding any provision hereof to the
        contrary, Assignee shall be required upon the expiration or earlier
        termination of the Lease to return the Premises to Landlord in such
        condition as may be required in the Lease, and Tenant shall have no
        liability for any restoration of or repair to the Premises. The removal
        of any fixtures, equipment, additions, alterations or improvements,
        whether made by Tenant or Assignee, shall be the sole responsibility of
        Assignee.

        (c) Assignee agrees with Tenant to indemnify and hold Tenant
        harmless from and against any and all loss, cost, damage and expense
        suffered by Tenant (including without limitation reasonable attorneys'
        fees and costs of defense) as a result of any claim under or in respect
        of the Lease, which claim relates to the period on or subsequent to the
        date hereof. Notwithstanding the foregoing, Assignee shall not be
        liable to indemnify Tenant from any such loss, cost, damage or expense
        arising from any tort claim in the Premises occurring prior to the date
        on which Tenant first delivers possession of the Premises to Assignee.

        (d)  Assignee has not dealt with any broker or other person or
        firm to whom a commission or fee is or may be due in respect of this
        assignment, and Assignee hereby agrees to indemnify and hold the Tenant
        and Landlord harmless from and against any and all loss, cost, damage
        and expense (including without limitation reasonable attorneys' fees

                                     -2-


<PAGE>   3

        and costs) suffered by the other as a result of any claim
        against Tenant or Landlord that a fee or commission is due on account
        of a relationship between the claimant and the Assignee.

        (e) Assignee acknowledges that, under the terms of that certain
        Consent to Assignment of Lease and Leasehold Mortgage to be executed
        and delivered by Landlord in respect of this Assignment, Landlord
        requires that Tenant remain liable to Landlord for all obligations and
        liabilities of Assignee under the Lease. Therefore, Assignee agrees
        with Tenant: (i) not to amend or modify, or agree to or acquiesce in
        any amendment to or modification of, the Lease, the effect of which
        amendment or modification would be to increase or extend the
        obligations or liability of the Assignee under the Lease, as so amended
        or modified, without Tenant's prior written consent (which shall not be
        unreasonably withheld or delayed, provided that Assignee shall have
        given Tenant such information as Tenant may reasonably request
        regarding Assignee's financial standing and condition as to allow
        Tenant to determine that Assignee has resources sufficient under the
        circumstances to meet such increased obligations); and (ii) to
        immediately give Tenant a true and complete copy of any notice or other
        correspondence received by Assignee in respect of the Lease, including
        without limitation any notice or correspondence concerning any failure,
        breach or default, whether actual or alleged, on the part of Assignee
        under the Lease.

3.      EQUIPMENT LEASE.  Assignee agrees that, as a material part of the 
        assignment and assumption transaction represented hereby,
        Assignee and Tenant have entered into an Assignment and Assumption of
        Equipment Lease of even date (the "Equipment Assignment"), whereby
        Assignee has agreed to accept and assume from Tenant an assignment of
        all of Tenant's right, title and interest in and to that certain Master
        Lease Agreement dated as of March 31, 1994, by and between Tenant and
        Aberlyn Capital Management Limited Partnership (the "Equipment Lease").

4.      TENANT'S PAYMENTS FOR EQUIPMENT; COLLECTION. (a) In addition to
        assuming Tenant's obligations under the Equipment Lease, and as
        a material inducement to Tenant's entering into this Assignment,
        Assignee has agreed to purchase from Tenant, and Tenant has agreed to
        sell to Assignee, the leasehold improvements in the Premises, together
        with certain other equipment, fixtures and personal property belonging
        to Tenant, and assign to Assignee all of Tenant's right, title and
        interest in and to the leasehold improvements heretofore made by Tenant
        in the Premises. In consideration thereof, Tenant promises to pay to
        Tenant the sum of Two Million Two Hundred Ninety Thousand Dollars
        (US$2,290,000.00), which shall be paid in installments as follows:

          1.    Assignee promises to pay to Tenant (or to Tenant's
                order) Five Hundred Thousand Dollars (US$500,000.00), to be

                                     -3-


<PAGE>   4
                paid to Tenant simultaneously with Tenant's execution
                hereof;

          2.    Assignee promises to pay to Tenant (or to Tenant's
                order) One Hundred Fifty Thousand Dollars (US$150,000.00) on
                the later to occur of (x) 5:00 p.m., Boston time, on February
                2, 1996, or (y) the date on which Tenant delivers to Assignee's
                address set forth in Section 8 hereof all drawings, plans,
                warranties and other technical information relevant to the
                Premises and in the possession or under the control of Tenant
                or its agents or employees (Tenant hereby represents to
                Assignee that Tenant has in its possession or under its control
                all drawings, plans, warranties and other technical information
                necessary for the proper operation of the Premises for the
                purposes for which the Premises were constructed);

          3.    Assignee promises to pay to Tenant (or to Tenant's order)
                Three Hundred Thirty Thousand Dollars (US$330,000.00)
                on the earlier to occur of (x) 5:00 p.m., Boston time, on
                July 1, 1997, or (y) the date on which Assignee has closed
                one or more contemplated equity financings having
                aggregate gross proceeds of not less than Twenty Million
                Dollars (US$20,000,000.00); and

          4.    Assignee promises to pay to Tenant (or to Tenant's order)
                One Million Three Hundred Ten Thousand Dollars
                (US$1,310,000.00) in four installments, the first
                installment for Two Hundred Sixty Thousand Dollars
                ($260,000) being due and payable October 1, 1998, and the
                remaining installments of Three Hundred Fifty Thousand
                Dollars ($350,000) each being due and payable January 1,
                1999, April 1, 1999, and July 1, 1999.

        All payments referred to above shall be paid in immediately
        available federal funds, either by wire transfer or by cashier's check
        drawn on Fleet Bank of Massachusetts, N.A., BayBank, N.A., The First
        National Bank of Boston or another national banking association in
        Boston or New York City reasonably acceptable to Tenant. The payments
        referred to in clauses (3) and (4) above shall be subject to adjustment
        to the extent that any equipment leased by Tenant under the Equipment
        Lease is missing or damaged, all as more particularly set forth in the
        Equipment Assignment.


                                     -4-


<PAGE>   5
        (b) In the event that Assignee shall fail to make any such
        payment when due, Assignee shall pay to Tenant, in addition to the
        amount due, a late charge equal to five percent (5%) thereof and, if
        such failure shall continue for fifteen (15) days after written notice
        thereof from Tenant to Assignee, Tenant shall have the right (but not
        the obligation), without limiting any other rights or remedies Tenant
        may have, to: (i) declare the entire amount then unpaid (including
        without limitation late charges) to be immediately due and payable in
        full; (ii) to terminate this Assignment and declare the same null, void
        and without further force or effect (without, however, in any way
        affecting Tenant's liability for the period between the date hereof and
        the date of such termination) and peaceably re-enter the Premises and
        repossess the same and any leasehold improvements owned by Assignee;
        and (iii) to exercise its rights under the Mortgage (as hereinafter
        defined). In addition to the $2,290,000 to be paid as aforesaid,
        Assignee shall, simultaneously with Assignee's execution hereof, pay
        over to Tenant the $40,000.00 security deposit currently being held by
        Landlord, as referenced in Section 3(c) above. 

        (c)  The Assignee agrees to pay all costs of suit and other
        expenses of collection, including reasonable fees and expenses of
        attorneys at both the trial and appellate levels, in the event that
        this instrument is placed in the hands of any attorney for collection
        or suit is brought thereon.

        (d)  The Assignee hereby waives presentment, protest and
        demand, notice of protest, demand and dishonor and non-payment hereof,
        and to the extent permitted by law, waives and releases all rights of
        redemption, valuation, appraisement, notice of election to mature or to
        declare due the whole of the indebtedness evidenced hereby, and to the
        extent permitted by law, errors, defects and imperfections in any
        proceedings instituted by the Tenant under the terms hereof or benefits
        that might accrue to the Assignee by virtue of any present or future
        laws providing for any stay of execution, exemption from civil process,
        or extension of time for payment. Further, Assignee agrees that its
        liability hereunder shall remain unimpaired, notwithstanding any
        extension of the time of payment or other indulgence granted by the
        Tenant, or the release of all or any part of any security for the
        liability of any party which may assume the obligation to make payment
        of the indebtedness evidenced hereby, or the performance and the
        obligations of the Assignee hereunder. The foregoing shall not be
        deemed to constitute a waiver by Assignee of any rights that may be
        available to Assignee under applicable law to offset against any such
        payment obligation the amount of any judgment obtained by Assignee
        against Tenant as a result of Tenant's failure or refusal to perform
        its obligations hereunder.

        (e)  In no event shall either party, by any act of omission or
        commission, be deemed to waive any of its rights or remedies hereunder
        unless such waiver is in writing signed by such party, and no waiver of
        any one event shall be construed as continuing or as a bar to or waiver
        of such right or remedy on a subsequent event.

                                     -5-


<PAGE>   6
        (f) Any legal action or proceeding with respect to the
        collection of the indebtedness evidenced hereby may, at the option of
        the Tenant, be brought in the courts of The Commonwealth of
        Massachusetts or of the United States of America for the District of
        Massachusetts. By execution and delivery hereof, the Assignee accepts,
        generally and unconditionally, the jurisdiction of the aforesaid
        courts. The Assignee irrevocably consents to the service of process of
        any of the aforementioned courts in any such action or proceeding by
        the mailing of copies thereof by certified mail, postage prepaid, to
        the party at its address set forth herein. 

        (g) Assignee's obligation to make each such payment as and when
        due shall be secured by a Leasehold Mortgage and Assignment of Lessee's
        Interest (the "Mortgage"), by Assignee in favor of Tenant, covering
        Assignee's interest in the Lease, such Mortgage to be in the form
        attached hereto as EXHIBIT B.

5.      NOTICES, ETC.  All notices, requests, consents and other communications
        hereunder shall be in writing, shall be addressed to the receiving
        party's address set forth below or to such other address as a
        party may designate by notice hereunder, and shall be either (i)
        delivered by hand, (ii) made by telex, telecopy or facsimile
        transmission, (iii) sent by overnight courier, or (iv) sent by
        registered or certified mail, return receipt requested, postage
        prepaid.

        If to the Landlord:

        AEW #1 Corporation
        265 Franklin Street
        Boston, MA 02110

        With a copy to:

        Stephen M. Nolan, Esq.
        Hill & Barlow
        One International Place
        Boston, MA 02109

        If to the Tenant:

        Immunogen, Inc.
        128 Sidney Street
        Cambridge, MA 02139
        Attention: Mr. Frank Pocher

        With a copy to:


                                     -6-


<PAGE>   7

        Stephen T. Langer, Esq.
        Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
        One Financial Center
        Boston, MA 02111

        If to Assignee:

        Oravax, Inc.
        38 Sidney Street
        Cambridge, MA 02139
        Attention:

        With a copy to:

        Jeffrey Hermanson, Esq.
        Hale and Dorr
        60 State Street
        Boston, MA 02109

        All notices, requests, consents and other communications
        hereunder shall be deemed to have been given either (i) if by hand, at
        the time of the delivery thereof to the receiving party at the address
        of such party set forth above, (ii) if made by telex, telecopy or
        facsimile transmission, at the time that receipt thereof has been
        acknowledged by electronic confirmation or otherwise, (iii) if sent by
        overnight courier, on the next business day following the day such
        notice is delivered to the courier service, or (iv) if sent by
        registered or certified mail, on the 5th business day following the day
        such mailing is made. The Assignee shall give the holder prompt notice
        of any change in the Assignee's principal address.

6.      ACCOUNTING; SERVICE AGREEMENT. At or before the execution hereof, Tenant
        has delivered to Assignee an accounting, in reasonable detail,
        of Tenant's prior investment in the Premises and the leasehold
        improvements therein. Tenant agrees to negotiate in good faith with
        Assignee a mutually acceptable service agreement, pursuant to which
        Tenant shall provide a reasonable level of consulting services to
        Assignee to assist Assignee in making a smooth transition to the
        Premises.

7.      HEADINGS AND CAPTIONS.  The headings and captions of the various 
        subdivisions of this Agreement are for convenience of reference
        only and shall in no way modify, or affect the meaning or construction
        of any of the terms or provisions hereof.

8.      GOVERNING LAW.  This Agreement and the rights and obligations of the 
        parties hereunder shall be construed in accordance with and governed by
        the law of the

                                     -7-

<PAGE>   8
        Commonwealth of Massachusetts, without giving effect to the
        conflict of law principles thereof.

9.      UNENFORCEABILITY, ETC. If any provision hereof or the application 
        thereof to any person or circumstances shall to any extent be
        invalid or unenforceable, the remainder hereof, or the application or
        such provision to persons or circumstances other than those as to which
        it is held invalid or unenforceable, shall not be affected thereby, and
        each provision hereof shall be valid and enforced to the fullest extent
        permitted by law.

10.     RATIFICATION. Except as hereinabove specifically assigned, the Lease is
        hereby ratified and confirmed.

        IN WITNESS WHEREOF, Landlord and Tenant have signed and sealed this 
        Amendment as of the day and year first above written.


                               IMMUNOGEN, INC.



                               By: /s/ Frank J. Pocher
                                  -----------------------------------------    
                                   Frank J. Pocher, Vice President and Chief
                                   Financial Officer


                               ORAVAX, INC.



                               By: /s/ Lance K. Gordon
                                  -----------------------------------------    
                                   Name: Lance K. Gordon
                                   President and Chief Executive Officer


                               By: /s/ Keith S. Ehrlich
                                  -----------------------------------------    
                                   Name: Keith S. Ehrlich
                                   Vice President, Treasurer and Chief
                                   Financial Officer



                                     -8-




<PAGE>   1
                        ASSIGNMENT OF EQUIPMENT LEASE
                            AND LESSOR'S CONSENT


        THIS ASSIGNMENT OF EQUIPMENT LEASE AND LESSOR'S CONSENT (this
"Agreement"), dated as of January 1, 1996, by and among IMMUNOGEN, INC.
("Lessee"), a Massachusetts corporation, ORAVAX, INC. ("Assignee"), a Delaware
corporation, and ABERLYN CAPITAL MANAGEMENT LIMITED PARTNERSHIP, a Delaware
limited partnership ("Lessor").

        WHEREAS, by a Master Lease Agreement (the "Lease") dated as of March
31, 1994, Lessor leased to Lessee that certain equipment and other personal
property (the "Equipment") more particularly described in Lease Schedule No.
001 and Lease Schedule No. 002, each of which is attached to and made a part of
the Lease; and

        WHEREAS, on and subject to the terms and conditions set forth herein,
Lessee now desires to assign all of its right, title and interest in and to the
Lease to Assignee, and Assignee desires to accept such assignment, effective as
of the date hereof; and

        NOW, THEREFORE, in consideration of the foregoing, and for other good
and valuable consideration, each to the other paid, the receipt and sufficiency
of which are hereby acknowledged, Lessor, Lessee and Assignee hereby agree as
follows:

1.      ASSIGNMENT; REPRESENTATION; INDEMNITY. (a) That, effective as
        of the date
 hereof, Lessee hereby assigns to Assignee all of Lessee's
        right, title and interest in and to the Lease, including without
        limitation all security and other deposits currently being held by
        Lessor in respect of the Lease, and Lessee's right to purchase the
        Equipment as set forth in Section 2.6 of the Lease and in the Lease
        Schedules, all upon the terms and conditions herein set forth. A true
        copy of the Lease, together with any amendments, is annexed as EXHIBIT
        A.

        (b) Lessee represents and warrants to Assignee that, as of the
        date hereof, it has not received from Lessor any written notice from
        Lessor to the effect that any Event of Default (or event or
        circumstance which, with the passage of time or the giving of notice,
        would constitute an Event of Default) exists on the part of Lessee
        under the Lease, and to the best of Lessee's actual knowledge, no such
        Event of Default, event or circumstance in fact exists; and Lessee
        agrees with Assignee to indemnify and hold Assignee harmless from and
        against any and all costs and obligations to have been paid or
        performed by Lessee under the Lease on or prior to the date hereof.

        (c) Lessee agrees with Assignee to indemnify and hold Assignee
        harmless from and against any and all loss, cost, damage and expense
        suffered by Assignee (including without limitation reasonable
        attorneys' fees and costs of defense) as a result of any claim under or
        in respect of the Lease, which claim relates to the period prior to the
        date hereof.

        (d) Lessee has not dealt with any broker or other person or
        firm to whom a commission or fee is or may be due in respect of this
        assignment, and Lessee hereby agrees to indemnify and hold the Assignee
        and Lessor harmless from and against any and all loss, cost, damage and
        expense (including without limitation reasonable attorneys' fees and
        costs) suffered by the other as a result 


<PAGE>   2
        of any claim against Assignee or Lessor that a fee or commission is due
        on account of a relationship between the claimant and the Lessee.

        (e)  Lessee represents and warrants to Lessor that, as of the
        date hereof, the Equipment is free from all liens and encumbrances made
        by Lessee except the Lease.

2.      ACCEPTANCE AND ASSUMPTION OF OBLIGATIONS; INDEMNITY. (a)
        Assignee accepts from Lessee the above assignment, and hereby agrees
        with Lessor and Lessee faithfully to assume and perform each and every
        obligation of Lessee under the Lease, including without limitation the
        obligation to pay Rent, additional rent, Supplemental Rent, additional
        charges, taxes and any and all other costs and expenses, however
        labelled or designated, and to observe all terms and conditions
        thereof, including without limitation the prohibition against further
        subletting or assignment, with the same force and effect as if Assignee
        were the Lessee originally named under the Lease.

        (b) Subject to the provisions of Section 8 hereof, Assignee
        agrees to accept the Equipment in its "As Is" condition as of December
        29, 1995, and represents that Assignee has inspected the Equipment, and
        is satisfied as to its suitability for Assignee's intended uses.
        Notwithstanding any provision hereof to the contrary, Assignee shall be
        required upon the expiration or earlier termination of the Lease to
        return the Equipment to Lessor in such condition as may be required in
        the Lease, and Lessee shall have no liability in connection therewith.
        Assignee agrees to pay to Lessee, simultaneously with the execution
        hereof, an amount equal to the last month's rental payments due under
        the Lease, which have been paid in advance by the Lessee. Assignee
        agrees that such amount is $96,595.14.

        (c) Assignee agrees with Lessee to indemnify and hold Lessee
        harmless from and against any and all loss, cost, damage and expense
        suffered by Lessee (including without limitation reasonable attorneys'
        fees and costs of defense) as a result of any claim under or in respect
        of the Lease, which claim relates to the period on or subsequent to the
        date hereof.

        (d) Assignee has not dealt with any broker or other person or
        firm to whom a commission or fee is or may be due in respect of this
        assignment, and Assignee hereby agrees to indemnify and hold the Lessee
        and Lessor harmless from and against any and all loss, cost, damage and
        expense (including without limitation reasonable attorneys' fees and
        costs) suffered by the other as a result of any claim against Lessee or
        Lessor that a fee or commission is due on account of a relationship
        between the claimant and the Assignee.

        (e)  Assignee represents and warrants to Lessor that to the
        best of its knowledge there is no sales, use or other tax required to
        be paid in connection with the assignment effected hereby, and Assignee
        shall pay any such tax if the same is later found applicable.

        (f)  Assignee acknowledges that as of the date hereof, there
        remains unpaid under the Lease aggregate rent payments in the amount of
        $3,091,044.48.

3.      LESSOR'S CONSENT. (a) Lessor hereby consents to the Assignment of the 
        Lease to Assignee on the following terms and conditions:

                                     -2-

<PAGE>   3
        (i)     Neither the giving of this consent nor anything
                contained herein shall be construed to modify, waive, impair or
                affect any of the covenants, agreements, terms, provisions,
                obligations or conditions contained in the Lease (except as may
                herein be expressly provided), or to waive any breach thereof,
                or any rights of Lessor against any person, firm, association
                or corporation liable or responsible for the performance
                thereof, or to increase the obligations or diminish the rights
                of Lessor under the Lease, or to increase the rights or
                diminish the obligations of the tenant thereunder, or to, in
                any way, be construed as giving Assignee any greater rights
                than the original Lessee named in the Lease would be entitled
                to, and all covenants, agreements, terms, provisions and
                conditions of the Lease are hereby mutually declared to be in
                full force and effect.

        (ii)    The giving of this consent shall not be construed
                either as a consent by Lessor to, or as permitting, any other
                or further assignment of the Lease, whether in whole or in
                part, or any subletting of the Equipment or any part thereof,
                or as a waiver of the requirement of obtaining Lessor's consent
                thereto, to the extent required under the Lease.

        (iii)   The giving of this Consent shall not result in any
                liability on the part of Lessor for the payment of any
                commissions or fees in connection with the proposed assignment
                transaction herein contemplated by Lessee and Assignee; Lessor
                hereby represents that it has dealt with no broker or other
                party to whom a commission is due as a result of this
                Agreement.

        (b) Lessor represents and warrants to Lessee and Assignee that
        as of the date hereof, no Event of Default (or event or circumstance
        which, with the passage of time or the giving of notice, would
        constitute an Event of Default) exists on the part of Lessee under the
        Lease and the Lease has not been amended except as disclosed on Exhibit
        A. Lessor is not currently holding any security deposits from Lessee or
        any other amounts for the account of Lessee, but Lessor acknowledges
        that the last month's rental payment under the Lease has been paid in
        advance. 

4.      RELEASE. Effective as of the date hereof, (i) Lessor hereby
        releases Lessee from any and all further liability or obligation under
        or in respect of the Lease, which liability or obligation first arises
        on or subsequent to the date hereof, and (ii) Lessee hereby releases
        Lessor from any and all claims, actions or causes of action,
        liabilities or obligations, of whatever type or nature, known or
        unknown, arising out of or in respect of the Lease or Lessee's use of
        the Equipment and first arising prior to the date hereof.

5.      SPACE LEASE.  Assignee agrees that, as a material part of the
        assignment and assumption transaction represented hereby, Assignee and
        Lessee have entered into an Assignment and Assumption of Lease of even
        date (the "Space Assignment"), whereby Assignee has agreed to accept
        and assume from Lessee an assignment of all of Lessee's right, title
        and interest in and to that certain Lease dated as of June 30, 1992, by
        and between Lessee and AEW #1 Corporation (the "Space Lease").

                                     -3-

<PAGE>   4
6.      SECURED PARTIES, ETC. Lessor represents that there exist no
        holders of chattel mortgages or security interests on the Equipment,
        and no further consents or approvals are required for Lessor to enter
        into this Agreement.

7.      NOTICES, ETC.  All notices, requests, consents and other
        communications hereunder shall be in writing, shall be addressed to the
        receiving party's address set forth below or to such other address as a
        party may designate by notice hereunder, and shall be either (i)
        delivered by hand, (ii) made by telex, telecopy or facsimile
        transmission, (iii) sent by overnight courier, or (iv) sent by
        registered or certified mail, return receipt requested, postage
        prepaid.


        If to the Lessor:

        Aberlyn Capital Management Limited Partnership
        1000 Winter Street
        Waltham, MA 02154
        Attention: Douglas Brian


        If to the Lessee:

        Immunogen, Inc.
        128 Sidney Street
        Cambridge, MA 02139
        Attention: Mr. Frank Pocher


        With a copy to:

        Stephen T. Langer, Esq.
        Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
        One Financial Center
        Boston, MA 02111


        If to Assignee:

        Oravax, Inc.
        38 Sidney Street
        Cambridge, MA 02139
        Attention:

                                     -4-

<PAGE>   5
        With a copy to:

        Jeffrey Hermanson, Esq.
        Hale and Dorr
        60 State Street
        Boston, MA 02109

        All notices, requests, consents and other communications
        hereunder shall be deemed to have been given either (i) if by hand, at
        the time of the delivery thereof to the receiving party at the address
        of such party set forth above, (ii) if made by telex, telecopy or
        facsimile transmission, at the time that receipt thereof has been
        acknowledged by electronic confirmation or otherwise, (iii) if sent by
        overnight courier, on the next business day following the day such
        notice is delivered to the courier service, or (iv) if sent by
        registered or certified mail, on the 5th business day following the day
        such mailing is made.

8.      INSPECTION. At or before the execution hereof, Lessee and
        Assignee have performed a detailed inspection of the Equipment, and a
        complete listing of any missing or damaged (other than ordinary wear
        and tear) Equipment is annexed hereto as EXHIBIT B. If and to the
        extent that there exists any such missing or damaged Equipment,
        Assignee may deduct from the payment to be made to Lessee under Section
        4(3) and 4(4) of the Space Assignment an amount equal to the product of
        (i) the reasonable amount by which the value of any missing or damaged
        Equipment is diminished by reason of such damage below its reasonable
        value if not missing or undamaged, multiplied by a fraction, the
        numerator of which is the number of months remaining in the term of the
        Lease after December 31, 1995, and the denominator of which is the
        total number of months in the term of the Lease.

9.      HEADINGS AND CAPTIONS.  The headings and captions of the
        various subdivisions of this Agreement are for convenience of reference
        only and shall in no way modify, or affect the meaning or construction
        of any of the terms or provisions hereof.

10.     GOVERNING LAW.  This Agreement and the rights and obligations
        of the parties hereunder shall be construed in accordance with and
        governed by the law of the Commonwealth of Massachusetts, without
        giving effect to the conflict of law principles thereof.

11.     UNENFORCEABILITY, ETC. If any provision hereof or the
        application thereof to any person or circumstances shall to any extent
        be invalid or unenforceable, the remainder hereof, or the application
        or such provision to persons or circumstances other than those as to
        which it is held invalid or unenforceable, shall not be affected
        thereby, and each provision hereof shall be valid and enforced to the
        fullest extent permitted by law.

12.     RATIFICATION. Except as hereinabove specifically assigned and amended,
        the Lease is hereby ratified and confirmed.


                                     -5-

<PAGE>   6

     

        IN WITNESS WHEREOF, Lessor, Lessee and Assignee have signed and sealed
this Amendment as of the day and year first above written.


                               ABERLYN CAPITAL MANAGEMENT LIMITED
                               PARTNERSHIP

                               By:  Aberlyn Capital Management Company, Inc., 
                                    its general partner



                               By: /s/ Diana M. Spano
                                  -----------------------------------------
                                   Name: Diana M. Spano
                                   (Vice) President



                               IMMUNOGEN, INC.



                               By: /s/ Frank J. Pocher
                                  -----------------------------------------
                                   Frank J. Pocher, Vice President and Chief
                                   Financial Officer


                               ORAVAX, INC.
                               


                               By: /s/ Lance K. Gordon
                                  -----------------------------------------
                                   Name: Lance K. Gordon
                                   President and Chief Executive Officer



                               By: /s/ Keith S. Ehrlich
                                  -----------------------------------------
                                   Name: Keith S. Ehrlich
                                   Vice President, Treasurer and Chief Financial
                                   Officer

                                     -6-

<PAGE>   7

     

        IN WITNESS WHEREOF, Lessor, Lessee and Assignee have signed and sealed
this Amendment as of the day and year first above written.


                               ABERLYN CAPITAL MANAGEMENT LIMITED
                               PARTNERSHIP

                               By:  Aberlyn Capital Management Company, Inc.,
                                    its general partner



                               By: 
                                  -----------------------------------------
                                   Name: 
                                   (Vice) President



                               IMMUNOGEN, INC.



                               By: /s/ Frank J. Pocher
                                  -----------------------------------------
                                   Frank J. Pocher, Vice President and Chief
                                   Financial Officer


                               ORAVAX, INC.



                               By: /s/ Lance K. Gordon
                                  -----------------------------------------
                                   Name: Lance K. Gordon
                                   President and Chief Executive Officer



                               By: /s/ Keith S. Ehrlich
                                  -----------------------------------------
                                   Name: Keith S. Ehrlich
                                   Vice President, Treasurer and Chief Financial
                                   Officer

                                     -6-
                            



<TABLE> <S> <C>

<ARTICLE> 5
<LEGEND>
THIS SCHEDULE CONTAINS SUMMARY FINANCIAL INFORMATION EXTRACTED FROM THE
FINANCIAL STATEMENTS OF THE IMMUNOGEN CORPORATION FOR THE THREE MONTHS ENDED
DECEMBER 31, 1995, AND IS QUALIFIED IN ITS ENTIRETY BY REFERENCE TO SUCH
FINANCIAL STATEMENTS.
</LEGEND>
<CIK> 0000855654
<NAME> IMMUNOGEN CORPORATION
<MULTIPLIER> 1
<CURRENCY> U.S. DOLLARS
       
<S>                             <C>
<PERIOD-TYPE>                   3-MOS
<FISCAL-YEAR-END>                          JUN-30-1996
<PERIOD-START>                             OCT-01-1995
<PERIOD-END>                               DEC-31-1995
<EXCHANGE-RATE>                                      1
<CASH>                                         877,406
<SECURITIES>                                         0
<RECEIVABLES>                                        0
<ALLOWANCES>                                         0
<INVENTORY>                                          0
<CURRENT-ASSETS>                             1,880,483
<PP&E>                                      14,549,927
<DEPRECIATION>                               9,465,785
<TOTAL-ASSETS>                               8,648,325
<CURRENT-LIABILITIES>                        3,650,641
<BONDS>                                              0
<COMMON>                                       150,742
<PREFERRED-MANDATORY>                                0
<PREFERRED>                                          0
<OTHER-SE>                                           0
<TOTAL-LIABILITY-AND-EQUITY>                 8,648,325
<SALES>                                              0
<TOTAL-REVENUES>                               315,133
<CGS>                                                0
<TOTAL-COSTS>                                        0
<OTHER-EXPENSES>                             8,851,388
<LOSS-PROVISION>                                     0
<INTEREST-EXPENSE>                             592,272
<INCOME-PRETAX>                            (8,536,255)
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