<PAGE>   1
               Confidential materials omitted and filed separately with the
                  Securities and Exchange Commission.
                        Asterisks denote such omissions.



                      DATED this 20th day of December 1996

                                     BETWEEN

                               BIOTRANSPLANT INC.
                                     ("BTI")

                                      -and-

                            CASTELLA RESEARCH PTY LTD
                                  ("Castella")

                                     - and-

                             SECURE SCIENCES PTY LTD
                                   ("Secure")

                                     - and-

                           STEM CELL SCIENCES PTY LTD
                                 ("the Company")




                AGREEMENT TO FURTHER VARY SHAREHOLDERS' AGREEMENT



                                 HOLDING REDLICH

<PAGE>   2

                             LAWYERS AND CONSULTANTS

         350 Williams Street                Level 12, Chifley Tower
         Melbourne VIC 3000                 2 Chifley Squre
                                            Sydney NSW 2000

         Tel (03) 9321 9999                      Tel (02) 234 4444
         Fax (03) 9321 9900                      Fax (02) 234 4400
         Ref:  CLSCS109.106


<PAGE>   3

                                    CONTENTS

<TABLE>
<S>  <C>                                                                 <C>
1.   DEFINITIONS AND INTERPRETATION                                      5

     1.1  Definitions                                                    5

     1.2  Interpretation                                                 5

     1.3  Recitals                                                       5

2.   ISSUE OF SHARES                                                     6

     2.1  Issue of Shares to BTI                                         6

     2.2  Issue of Shares to Castella                                    6

     2.3  Issue of Shares to Secure                                      6

3.   ISSUE OF OPTIONS                                                    6

     3.1  Issue of Option to BTI                                         6

     3.2  Issue of Option to Castella                                    7

     3.3  Issue of Option to Secure                                      7

4.   VARIATION OF SHAREHOLDERS' AGREEMENT                                7

     4.1  Variation                                                      7

     4.2  Confirmation of Shareholders' Agreement                        7

     4.3  Acknowledgement                                                7
</TABLE>


<PAGE>   4


         AGREEMENT TO FURTHER VARY SHAREHOLDERS' AGREEMENT dated the
                     day of                , 1996

         BETWEEN:

                   BIOTRANSPLANT INCORPORATED a corporation organised and
                   existing under the laws of the State of Delaware and having
                   its principal office 13 Pipp Street, Building 96, Navy Yard,
                   Charlestown, MA, United States of America
                                     ("BTI")

         AND:      CASTELLA RESEARCH PTY LTD (ACN 006 125 364) of
                   587 Whitehorse Road, Mont Albert, Victoria, Australia in its
                   own capacity and as trustee of the M.R. Brandon Family Trust
                                  ("Castella")

         AND:      SECURE SCIENCES PTY LTD (ACN 064 139 948) of Level 10,
                   420 St. Kilda Road, Melbourne, Victoria, Australia in its own
                   capacity and as trustee of the Secure Sciences Unit Trust
                                   ("Secure")

         AND:      STEM CELL SCIENCES PTY LTD (ACN 063 293 130) of Level 10,
                   420 St. Kilda Road, Melbourne, Victoria, Australia
                                 ("the Company")

         RECITALS



         A.   By a Shareholders' Agreement dated 5 April, 1994 between BTI,
              Castella, Secure and the Company (the "Shareholders' Agreement")
              the Shareholders recorded their agreement as to how the Company
              would be owned, controlled and funded by them.

         B.   Pursuant to Clause 2.5 of the Shareholders' Agreement the Company
              issued to BTI the First BTI Option and the Second BTI Option.

         C.   The First BTI Option was to be exercised on or before the first
              anniversary of the Effective Date.

         D.   Pursuant to Clause 2.5(b) of the Shareholders' Agreement if the
              First BTI Option was not exercised on or before the first
              anniversary of the Effective Date, the Second BTI Option would
              lapse.
<PAGE>   5

          Confidential materials omitted and filed separately with the
                       Securities and Exchange Commission.
                        Asterisks denote such omissions.

         E.   BTI did not exercise the First BTI Option on or before the first
              anniversary of the Effective Date (the anniversary occurring on 5
              April, 1995) and accordingly, the Second BTI Option lapsed.

         F.   Each of Castella and Secure exercised the options granted to them
              pursuant to Clause 2.5(a)(iii) and (v), respectively, by the
              first anniversary of the Effective Date.

         G.   Pursuant to a Variation of the Shareholders' Agreement dated 2
              February, 1996 between BTI, Castella, Secure and the Company ("the
              First Variation"):

              (a)  BTI subscribed for and the Company issued to BTI a further
                   ******* ****** in the capital of the Company; and

              (b)  the Company issued to BTI an option to subscribe for a
                   further ****** Shares in the capital of the Company at a
                   price of ****** per share at any time on or before 1 July,
                   1996 ("the New Option").

         H.   Neither Castella or Secure exercised the options granted to them
              pursuant to, respectively, Clause 2.5(a)(iv) and (vi) of the
              Shareholders' Agreement (respectively, "the Second Castella
              Option" and "the Second Secure Option") by the second anniversary
              of the Commencement Date.

         I.   BTI did not exercise the New Option by 1 July, 1996.

         J.   As a result of the exercise of the option by Castella and Secure,
              the failure by BTI to exercise the First BTI Option, the failure
              by Castella to exercise the Second Castella Option, the failure by
              Secure to exercise the Second Secure Option and the issue of
              shares to BTI pursuant to the First Variation, the issued capital
              of the Company is owned in the following proportions:

                   BTI:      30%

                   Castella: ******

                   Secure:   ******

         K.   BTI has agreed to provide an amount of equity capital to the
              Company and prior to the date of this Agreement, has subscribed
              that capital to the Company.


<PAGE>   6

         L.   In line with the intent of the Shareholders' Agreement that on the
              provision by BTI of equity capital to the Company such shares in
              the Company would be issued to maintain the shareholdings of BTI,
              Castella and Secure in the Company in the ratios set out in
              Recital J, the Shareholders have agreed to cause the Company to
              allot to BTI a certain addition number of "A" shares in the
              capital of the Company at a premium and to allot to Castella and
              Secure certain additional "B" Shares in the capital of the Company
              at par.

         M.   The Shareholders and the Company have also agreed that the Company
              will allot to BTI, Castella and Secure fresh options on the terms
              set out in this Agreement.

         N.   Clause 18.5 of the Shareholders' Agreement provides that the
              Shareholders' Agreement may not be modified, amended, added to or
              otherwise varied except by a document in writing signed by each of
              the parties or signed on behalf of each party by a director under
              hand.

         O.   The parties wish by this Agreement to record the matters set out
              in Recitals K to M and to vary the Shareholders' Agreement to the
              extent necessary to take account of those matters.

         IT IS AGREED

         1.   DEFINITIONS AND INTERPRETATION

         1.1  Definitions

              In this Agreement (including in the Recitals), unless the contrary
              intention appears, each defined word and expression has the
              meaning assigned to that word or expression in the Shareholders'
              Agreement.

         1.2  Interpretation

              Clauses 1.3 and 1.4 of the Shareholders' Agreement are
              incorporated in and form part of this Agreement as if each
              referenced to "this Agreement" in those clauses were a reference
              to this Agreement to vary the Shareholders' Agreements.

         1.3  Recitals

              The parties acknowledge and agree that the Recitals are true and
              correct and accurately reflect the circumstances in which this
              Agreement was entered into.

<PAGE>   7


          Confidential materials omitted and filed separately with the
                       Securities and Exchange Commission.
                        Asterisks denote such omissions.


         2.   ISSUE OF SHARES

         2.1  Issue of Shares to BTI

              (a)  On the date of this Agreement BTI must subscribe for, and be
                   issued with ****** in the capital of the Company issued at a
                   price of ****** per share (being $1.00 par and ****** premium
                   per share) for a total consideration of ******.

              (b)  The Company acknowledges that prior to the date of this
                   Agreement BTI has tendered and the Company has received
                   payment for the "A" Shares to be issued to it pursuant to
                   this Clause 2.1.

         2.2  Issue of Shares to Castella

              On the date of this Agreement Castella must subscribe for and be
              issued with ****** in the capital of the Company issued at the par
              price of $1.00 per share for a total consideration of ******
              payable on the date of this Agreement.

         2.3  Issue of Shares to Secure

              On the date of this Agreement Secure must subscribe for and be
              issued with ****** in the capital of the Company issue at the par
              price of $1.00 per share for a total consideration of ******
              payable on the date of this Agreement.

         3.   ISSUE OF OPTIONS

         3.1  Issue of Options to BTI

              (a)  On the date of this Agreement, the Company must issue to BTI
                   an option (the Option Terms attaching) to subscribe for,
                   fully pay up and be issued with a further ****** in the
                   capital of the Company for a total consideration of US******
                   at any time on or before 31 December, 1996 ("the option").

              (b)  If BTI exercises the option the Company will issue BTI ******
                   in the capital of the Company at a price per share of A$1.00
                   (being the par value) plus a premium per share calculated in
                   A$ as at the date of the exercise of the option.


<PAGE>   8

          Confidential materials omitted and filed separately with the
                       Securities and Exchange Commission.
                        Asterisks denote such omissions.


         3.2  Issue of Option to Castella

              On the date of this Agreement, the Company must issue to Castella
              an option (with the Option Terms attaching) to subscribe for,
              fully pay up and be issued with a further ****** in the capital of
              the Company issued at a price of $1.00 per share for a total
              consideration of ****** at any time on or before 31 December,
              1996.

         3.3  Issue of Option to Secure

              On the date of this Agreement, the Company must issue to Secure an
              option (the Options Terms attaching) to subscribe for, fully pay
              up and be issued with a further ****** in the capital of the
              Company issued at a price of $1.00 per share for a total
              consideration of ****** at any time on or before 31 December,
              1996.

         4.   VARIATION OF SHAREHOLDERS' AGREEMENT

         4.1  Variation

              In consideration of the mutual agreements made by each party under
              this Agreement, the parties agree, pursuant to cause 18.5 of the
              Shareholders' Agreement, to vary clause 2.5 of the Shareholders'
              Agreement to the extent necessary to give effect to the provisions
              of Clauses 2 and 3 of this Agreement.

         4.2  Confirmation of Shareholders' Agreement

              The parties confirm that the terms and conditions of the
              Shareholders' Agreement (as varied by this Agreement) remain in
              full force and effect.

         4.3  Acknowledgements

              Without limiting Clause 4.2 but for the removal of doubt, the
              parties acknowledge that:

              (a)  pursuant to clause 5.4(a)(i) of the Shareholders' Agreement,
                   all paragraphs of clause 5.3 of the Shareholders' Agreement
                   except paragraphs (e), (f), (p) and (v) ceased to operate on
                   the first anniversary of the Effective Date and nothing in
                   this Agreement will operate to or

<PAGE>   9


                   be construed as bring back into effect those paragraphs of
                   clause 5.3 of the Shareholders' Agreement which have ceased
                   to operate; and

              (b)  BTI did not exercise the second BTI Option and nothing in
                   this Agreement will be interpreted to mean that BTI did
                   exercise the Second BTI Option and the Shareholders'
                   Agreement will be construed and operate accordingly.

         SIGNED AS AN AGREEMENT on the date first appearing

         SIGNED by BIOTRANSPLANT INC.       )

         by its director ELLIOT LEBOWITZ    )           /s/ Elliot Lebowitz

         in the presence of:                )


         (Witness)


         SIGNED by CASTELLA RESEARCH        )

         PTY LTD by its director MAL BRANDON)           /s/ Mal Brandon

         in the presence of:                )


         (Witness)


         SIGNED by SECURE SCIENCES PTY LTD  )

         by its director PETER MOUNTFORD    )           /s/ Peter Mountford

         in the presence of:                )


         (Witness)


         SIGNED by STEM CELL SCIENCES       )

         PTY LTD by its director MAL BRANDON)           /s/ Mal Brandon

         in the presence of:                )


         (Witness)