<PAGE>


                                                                     EXHIBIT 4.3


                                                       NO. _____________________


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


<TABLE>
<S>             <C>
                              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, and
                -----------           ---------------------------------    ----------------------
                                      (type, class & series, if any)

                            shares of                                   of                        shares outstanding,
                -----------           ---------------------------------    ----------------------
                                      (type, class & series, if any)
</TABLE>

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
----------------------------------------------------------------------------------------------------------------------------------
       TYPE:                NUMBER OF SHARES                TYPE                NUMBER OF SHARES                 PAR VALUE
----------------------------------------------------------------------------------------------------------------------------------
<S>                         <C>                          <C>                    <C>                              <C>
      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
----------------------------------------------------------------------------------------------------------------------------------
       TYPE:                NUMBER OF SHARES                TYPE                NUMBER OF SHARES                 PAR VALUE
----------------------------------------------------------------------------------------------------------------------------------
<S>                         <C>                           <C>                   <C>                              <C>
     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:                                                   .
                     --------------------------------------------------

<TABLE>
<S>                                                                                                       <C>
SIGNED UNDER THE PENALTIES OF PERJURY, this         13TH            day of NOVEMBER          , 2001
                                            -----------------------        ------------------  ---------- ,


  /s/ Mitchel Sayare                                                                                      , *President
---------------------------------------------------------------------------------------------------------


  /s/ Jonathan L. Kravetz                                                                                 *Clerk
---------------------------------------------------------------------------------------------------------
</TABLE>

*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
                               -------------------------------------------------