<PAGE>


                                                                     EXHIBIT 4.3

                                                       NO. _____________________

                        THE COMMONWEALTH OF MASSACHUSETTS
--------                     WILLIAM FRANCIS GALVIN
Examiner                  Secretary of the Commonwealth
              One Ashburton Place, Boston, Massachusetts 02108-1512


                              ARTICLES OF AMENDMENT
                    (General Laws, Chapter 156B, Section 72)
--------
Name
Approved      We, MITCHEL SAYARE                                  , *President
                  ------------------------------------------------

              and JONATHAN KRAVETZ                                , *Clerk
                  ------------------------------------------------

              of IMMUNOGEN, INC.                                  ,
                 -------------------------------------------------
                          (EXACT NAME OF CORPORATION)

              located at 128 SIDNEY STREET, CAMBRIDGE, MA 02139 ,
                         -----------------------------------------------------
                            (STREET ADDRESS OF CORPORATION IN MASSACHUSETTS)

              certify that these Articles of Amendment affecting articles
              numbered:

                 3
              ----------------------------------------------------------------
               (NUMBERED THOSE ARTICLES 1, 2, 3, 4, 5 AND/OR 6 BEING AMENDED)

             of the Articles of Organization were duly adopted at a meeting
             held on NOVEMBER 13, 2001, BY VOTE OF:

             33,250,272 shares of COMMON STOCK of 39,680,326 shares outstanding,
             ----------           ------------    ----------
                                  (TYPE, CLASS
                                   & SERIES,
                                    IF ANY)

                     NO PREFERRED STOCK ISSUED & OUTSTANDING

                        shares of              of            shares outstanding,
             ----------           ------------    ----------
                                  (TYPE, CLASS
                                   & SERIES,
                                    IF ANY)

                        shares of              of            shares outstanding,
             ----------           ------------    ----------
                                  (TYPE, CLASS
                                   & SERIES,
                                    IF ANY)

C    [  ]
P    [  ] 1**being at least a majority of each type, class or series outstanding
          and entitled to vote thereon: / or  2** being at
M    [  ] least two-thirds of each type, class or series outstanding and
          entitled to vote thereon and of each type, class or
R.A. [  ] series of stock whose rights are adversely affected thereby:


-----------
P.C.

-------------------------------

*      DELETE THE INAPPLICABLE WORDS.

**     DELETE THE INAPPLICABLE CLAUSE.

1      FOR AMENDMENTS ADOPTED PURSUANT TO CHAPTER 156B, SECTION 70.

2      FOR AMENDMENTS ADOPTED PURSUANT TO CHAPTER 156B, SECTION 71.

NOTE:  IF THE SPACE PROVIDED UNDER ANY ARTICLE OR ITEM ON THIS FORM IS
       INSUFFICIENT, ADDITIONS SHALL BE SET FORTH ON ONE SIDE ONLY OF SEPARATE 8
       1/2 X 11 SHEETS OF PAPER WITH A LEFT MARGIN OF AT LEAST 1 INCH. ADDITIONS
       TO MORE THAN ONE ARTICLE MAY BE MADE ON A SINGLE SHEET SO LONG AS EACH
       ARTICLE REQUIRING EACH ADDITION IS CLEARLY INDICATED.

<PAGE>


To CHANGE the number of shares and the part value (if any) of any type, class or
series of stock which the corporation is authorized to issue, fill in the
following:

The total PRESENTLY authorized is:

<TABLE>
<CAPTION>
----------------------------------------------------- -----------------------------------------------------------------------------
              WITHOUT PAR VALUE STOCKS                                           WITH PAR VALUE STOCKS
<S>                 <C>                              <C>               <C>                               <C>
-------------------- -------------------------------- ----------------- --------------------------------- -------------------------
       TYPE:                NUMBER OF SHARES                TYPE                NUMBER OF SHARES                 PAR VALUE
-------------------- -------------------------------- ----------------- --------------------------------- -------------------------
      Common:                                             Common:       50,000,000                        $.01
-------------------- -------------------------------- ----------------- --------------------------------- -------------------------

-------------------- -------------------------------- ----------------- --------------------------------- -------------------------
     Preferred                                           Preferred:     5,000,000*                        $.01
-------------------- -------------------------------- ----------------- --------------------------------- -------------------------

-------------------- -------------------------------- ----------------- --------------------------------- -------------------------
</TABLE>

CHANGE the total authorized to:

<TABLE>
<CAPTION>
----------------------------------------------------- -----------------------------------------------------------------------------
              WITHOUT PAR VALUE STOCKS                                           WITH PAR VALUE STOCKS
<S>                 <C>                              <C>               <C>                               <C>
-------------------- -------------------------------- ----------------- --------------------------------- -------------------------
       TYPE:                NUMBER OF SHARES                TYPE                NUMBER OF SHARES                 PAR VALUE
-------------------- -------------------------------- ----------------- --------------------------------- -------------------------
      Common:                                             Common:       75,000,000                        $.01
-------------------- -------------------------------- ----------------- --------------------------------- -------------------------

-------------------- -------------------------------- ----------------- --------------------------------- -------------------------
     Preferred                                           Preferred:     5,000,000*                        $.01
-------------------- -------------------------------- ----------------- --------------------------------- -------------------------

-------------------- -------------------------------- ----------------- --------------------------------- -------------------------
</TABLE>


          *Preferred:           Series A Convertible Preferred     2,500
                                Shares $.01 par value
                                Series B Convertible Preferred     3,000
                                Shares $.01 par value
                                Series C Convertible Preferred     3,000
                                Shares $.01 par value
                                Series D Convertible Preferred     1,000
                                Shares $.01 par value
                                Series E Convertible Preferred     2,400
                                Shares $.01 par value


<PAGE>






The foregoing amendment(s) will become effective when these Articles of
Amendment are filed in accordance with General Laws, Chapter 156B, Section 6
unless these articles specify, in accordance with the vote adopting the
amendment, a LATER effective date not more than THIRTY DAYS after such filing,
in which event the amendment will become effective on such later date.

Late effective date:                                                   .
                     --------------------------------------------------

SIGNED UNDER THE PENALTIES OF PERJURY, this 13TH day of NOVEMBER   , 2001      ,
                                            ----        -----------     --------


/S/ MITCHEL SAYARE                                                  , *President
--------------------------------------------------------------------


/S/ JONATHAN L. KRAVETZ                                               *Clerk
--------------------------------------------------------------------

*DELETE THE INAPPLICABLE WORDS.


<PAGE>


                        THE COMMONWEALTH OF MASSACHUSETTS

                              ARTICLES OF AMENDMENT
                    (GENERAL LAWS, CHAPTER 156B, SECTION 72)


--------------------------------------------------------------------------------

I hereby approve the within Articles of Amendment and, the filing fee in the
amount of $________ having been paid, said articles are deemed to have been
filed with me this ____ day of ____________, 20 ______.

EFFECTIVE DATE:
               ----------------------------------------------------------------



                             WILLIAM FRANCIS GALVIN
                          SECRETARY OF THE COMMONWEALTH


                         TO BE FILLED IN BY CORPORATION
                      PHOTOCOPY OF DOCUMENT TO BE SENT TO:



        JONATHAN KRAVETZ, ESQUIRE
        -----------------------------------------------------------

        MINTZ, LEVIN, COHN, FERRIS, GLOVSKY AND POPEO, P.C.
        -----------------------------------------------------------

        ONE FINANCIAL CENTER, BOSTON, MA 02111
        -----------------------------------------------------------

        Telephone: 617 542-6000
        -----------------------------------------------------------