<PAGE>
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

                                 UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549
                            ------------------------

                                  FORM 10-K/A
                          AMENDMENT NO. 1 TO FORM 10-K
                 FOR ANNUAL AND TRANSITION REPORTS PURSUANT TO
          SECTIONS 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

(MARK ONE)

<TABLE>
<C>        <S>
   /X/     ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
           SECURITIES EXCHANGE ACT OF 1934
</TABLE>

                  FOR THE FISCAL YEAR ENDED DECEMBER 31, 2000
                                       OR

<TABLE>
<C>        <S>
   / /     TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
           SECURITIES EXCHANGE ACT OF 1934
</TABLE>

        FOR THE TRANSITION PERIOD FROM ______________ TO ______________

                        COMMISSION FILE NUMBER 000-28324
                            ------------------------

                           BIOTRANSPLANT INCORPORATED

             (Exact name of registrant as specified in its charter)

<TABLE>
<S>                                          <C>
                 DELAWARE                                    04-3119555
      (State or other jurisdiction of             (IRS Employer Identification No.)
      incorporation or organization)

          CHARLESTOWN NAVY YARD,                                02129
          BLDG. 75, THIRD AVENUE,                            (Zip Code)
              CHARLESTOWN, MA
 (Address of principal executive offices)
</TABLE>

Registrant's telephone number, including area code: (617) 241-5200

Securities registered pursuant to Section 12(b) of the Act: NONE

Securities registered pursuant to Section 12(g) of the Act: COMMON STOCK, $.01
PAR VALUE
                                                (Title of each class)
                            ------------------------

    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 by check mark if disclosure of delinquent filers pursuant to Item
405 of Regulation S-K is not contained herein, and will not be contained, to the
best of registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of this Form 10-K or any amendment to the
Form 10-K. / /

    The aggregate market value of voting Common Stock held by non-affiliates of
the registrant was $44,937,925 based on the last reported sale price of the
Common Stock on the Nasdaq consolidated transaction reporting system on
March 21, 2001.

    Number of shares of the registrant's class of Common Stock outstanding as of
March 21, 2001: 11,795,120.

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
<PAGE>

    This Amendment No. 1 on Form 10-K/A to the Registrant's Annual Report on
Form 10-K for the year ended December 31, 2000 is being filed to add the
information required to be set forth in Part III.


                                    PART III

ITEM 10. DIRECTORS AND EXECUTIVE OFFICERS


    Information regarding executive officers of BioTransplant is furnished in
Part I of this Annual Report on Form 10-K under the heading "Executive Officers
of the Registrant."



    Set forth below is the name, age and position of each director of
BioTransplant.



<TABLE>
<CAPTION>
NAME                                      AGE            POSITION AT BIOTRANSPLANT
----                                    --------   --------------------------------------
<S>                                     <C>        <C>
Elliot Lebowitz, Ph.D.................  60         President, Chief Executive Officer and
                                                   Director
Donald R. Conklin.....................  64         Director
William W. Crouse.....................  58         Director
James C. Foster, J.D..................  50         Director
Daniel O. Hauser, Ph.D................  63         Director
Michael S. Perry, D.V.M., Ph.D........  41         Director
</TABLE>



    ELLIOT LEBOWITZ, PH.D. has served as President and Chief Executive Officer
and as a member of the Board of Directors of BioTransplant since April 1991.
From 1985 to 1991, he served as Vice President for Research and Development at
C.R. Bard, Inc., a medical device company, directing internal and collaborative
research and development programs for Bard's Vascular Systems, Cardiosurgery and
Cardiopulmonary Divisions. From 1981 until 1985, Dr. Lebowitz served as Director
of Long Range Research and Development at DuPont Corporation, a diversified
health care company, developing immunopharmaceuticals. From 1977 until 1981, he
served as Division Manager of the Medical Products Division of New England
Nuclear Corporation, which developed, manufactured and sold radiopharmaceuticals
for in vivo diagnosis. Earlier in his career, Dr. Lebowitz served at Brookhaven
National Laboratories, a United States Department of Energy research facility,
where he developed Thallium-201, a radiopharmaceutical for the diagnosis of
coronary artery disease. Dr. Lebowitz was a founder of Diagnostic
Isotopes, Inc., a radiopharmaceutical company which was subsequently acquired by
Hoffmann-La Roche Inc., a pharmaceutical company. He was also a founder of
Procept, Inc., a biopharmaceutical company which focused on rational drug
design. He holds a B.A. from Columbia College and a Ph.D. from Columbia
University.



    DONALD R. CONKLIN has served as a director of BioTransplant since
January 1997. Mr. Conklin is currently retired. From February to December 1996,
he served as Chairman of Schering-Plough Health Care Products, a wholly-owned
subsidiary of Schering-Plough Corporation, a pharmaceutical company. From 1995
to February 1996, he served as President of Schering-Plough Health Care, and
from 1986 until September 1994, he served as Executive Vice President and
President of Schering-Plough Pharmaceuticals. Mr. Conklin also serves on the
board of directors of Alfacell Corporation, Ventiv Inc. and Vertex
Pharmaceuticals. He received his B.A. from Williams College and his M.B.A. from
Rutgers University.



    WILLIAM W. CROUSE has served as a director of BioTransplant since
June 1995. Since 1994, Mr. Crouse has served as Managing Director of HealthCare
Ventures LLC, a venture capital firm. Mr. Crouse served as Worldwide President
of Ortho Diagnostic Systems, a medical device company, and Vice President of
Johnson & Johnson International, a pharmaceutical company, from 1987 to 1994.
Mr. Crouse has more than 30 years experience in the pharmaceutical industry. He
also serves as a director of Dendreon Corporation, The New York Blood Center and
Liberty Science Center and is a Trustee of Lehigh University. Mr. Crouse
received his B.S. in finance and economics from Lehigh University and his M.B.A.
from Pace University.



    JAMES C. FOSTER, J.D. has served as a director of BioTransplant since
February 1992. Since 1992, he has served as President and Chief Executive
Officer of Charles River Laboratories, Inc., or CRL, a

<PAGE>

supplier of research animals and animal-related products and services.
Previously, he served in various other capacities with CRL. Mr. Foster received
his B.S. in psychology from Lake Forest College, his J.D. from the Boston
University School of Law and his M.A. in Science and Management from the
Massachusetts Institute of Technology.



    DANIEL O. HAUSER, PH.D. has served as a director of BioTransplant since
January 1994. Dr. Hauser is currently retired. From 1997 to 1998, he served as
the Senior Vice President of Preclinical Development & Project Management,
Operations in the United States for Novartis AG, a pharmaceutical corporation.
From 1992 until December 1996, he served as President of Sandoz Research
Institute and as Senior Vice President of Research and Development for Sandoz
Pharmaceutical Corporation, the predecessor of Novartis. From 1965 to 1992, he
served in various positions at the Pharma Division of Sandoz Pharma Ltd.
(Switzerland), including Senior Vice President from 1985 to 1992. Dr. Hauser
received his M.S. and Ph.D. in chemistry from the Swiss Federal Institute of
Technology (Switzerland) and was a Post-doctoral Research Fellow in the
Department of Chemistry at the Israel Institute of Technology (Haifa).



    MICHAEL S. PERRY, D.V.M., PH.D. has served as a director of BioTransplant
since January 1999. Since October 2000, he has served as Vice President, Global
Research and Development of Baxter Healthcare Corporation, a hospital supply and
medical technology company. From 1998 until September 2000, he served as
President and Chief Executive Officer of Genetic Therapy, Inc. and since 1997,
he has served as President and Chief Executive Officer of SyStemix, Inc. Genetic
Therapy and SyStemix are biopharmaceutical corporations which are wholly-owned
by Novartis. During 1997, Dr. Perry served as Vice President, Drug Regulatory
Affairs, North America for Novartis. From 1995 to 1996, he served as Vice
President, Drug Registration and Regulatory Affairs (North America) for Sandoz
Pharmaceutical Corporation, the predecessor of Novartis. From 1994 to 1995, he
served as Vice President, Drug Registration and Regulatory Affairs (USA) for
Sandoz. Dr. Perry received his Ph.D. in pharmacology (cardiopulmonary) and his
D.V.M. from the Ontario Veterinary College, University of Guelph (Canada).


SECTION 16(a) BENEFICIAL OWNERSHIP REPORTING COMPLIANCE


    Based solely on its review of copies of reports filed by all officers of
BioTransplant who are persons required to file reports, referred to as reporting
persons, pursuant to Section 16(a) of the Securities Exchange Act of 1934, as
amended, BioTransplant believes that during fiscal 2000 all filings required to
be made by its reporting persons were timely made in accordance with the
requirements of the Exchange Act, except for the following: Daniel O. Hauser
received 3,000 shares of common stock on February 22, 2000 upon the exercise of
stock options and sold 3,000 shares of common stock on March 3, 2000, and the
Form 5 reporting these transactions was not filed until February 14, 2001; James
Hope, BioTransplant's Senior Vice President of Development, received 6,000
shares of common stock on March 2, 2000 upon the exercise of stock options and
sold 6,000 shares of common stock on March 2, 2000, and the Form 5 reporting
these transactions was not filed until February 14, 2001; and Richard V.
Capasso, BioTransplant's Vice President, Finance and Treasurer, received 5,000
shares of common stock on March 2, 2000 upon the exercise of stock options and
sold 5,000 shares of common stock on March 2, 2000, and the Form 5 reporting
these transactions was not filed until February 14, 2001.


ITEM 11. EXECUTIVE COMPENSATION


    SUMMARY COMPENSATION TABLE.  The following table sets forth information with
respect to the annual and long-term compensation for the last three fiscal years
of BioTransplant's Chief Executive Officer and its four other most highly
compensated executive officers at year end whose total annual salary and bonus
for 2000 exceeded $100,000, referred to collectively as the "named executive
officers." Dollar amounts in the "All Other Compensation" column consist of
matching contributions under BioTransplant's 401(K) plan. Dr. Greenstein has
resigned as Senior Vice President, Research of BioTransplant, effective
January 2001, in order to assume the position of Chief Executive Officer of
Immerge BioTherapeutics AG, and its wholly-owned subsidiary, Immerge
BioTherapeutics, Inc., the

<PAGE>

operating entities of BioTransplant's joint venture with Novartis.
Dr. Greenstein will continue to serve as an employee at BioTransplant but will
no longer have the duties or title of an officer of BioTransplant. Dr. Howard
Grossberg joined BioTransplant in December 1999 and resigned in February 2001.
Dr. Grossberg will serve as a consultant to BioTransplant.



                           SUMMARY COMPENSATION TABLE



<TABLE>
<CAPTION>
                                                                             LONG-TERM
                                                                            COMPENSATION
                                                                               AWARDS
                                                     ANNUAL COMPENSATION    ------------
                                                    ---------------------    SECURITIES
                                                                  BONUS      UNDERLYING       ALL OTHER
NAME AND PRINCIPAL POSITION                YEAR     SALARY ($)     ($)      OPTIONS (#)    COMPENSATION ($)
---------------------------              --------   ----------   --------   ------------   ----------------
<S>                                      <C>        <C>          <C>        <C>            <C>
Elliot Lebowitz, Ph.D..................    2000      $266,000    $40,000        64,500          $2,625
  Chief Executive Officer                  1999       252,956         --        67,098              --
                                           1998       252,956         --       100,000              --

James Hope, Ph.D.......................    2000       206,000     20,000        22,000           2,625
  Senior Vice President of Development     1999       195,038      5,000        23,000              --
                                           1998       182,619     22,423        26,800              --

Julia L. Greenstein, Ph.D..............    2000       227,000     33,000        24,500           2,625
  Senior Vice President of Research        1999       187,567         --        16,000              --
                                           1998       200,723     19,372        40,700              --

Mary White-Scharf, Ph.D................    2000       168,000     20,000        26,000           2,625
  Vice President of Research               1999       155,424         --        27,000              --
                                           1998       146,107     13,622        45,200              --

Howard Grossberg, M.D..................    2000       221,000     50,400        10,500           2,625
  Vice President of Medical Affairs        1999         6,800         --        50,000              --
</TABLE>



    OPTION GRANTS IN LAST FISCAL YEAR.  The following table sets forth
information regarding options to purchase common stock granted during the year
ended December 31, 2000 by BioTransplant to the named executive officers.
Options granted in 2000 become exercisable in four equal annual installments,
commencing twelve months after the vesting commencement date, which is typically
the date of grant. Amounts in the last two columns represent hypothetical gains
that could be achieved for options if exercised at the end of the option term.
These gains are based on assumed rates of stock price appreciation of 5% and 10%
compounded annually from the date options were granted to their expiration date.
Actual gains, if any, on stock option exercises will depend on the future
performance of the common stock on the date on which options are exercised.

<PAGE>

                       OPTION GRANTS IN LAST FISCAL YEAR



<TABLE>
<CAPTION>
                                                 INDIVIDUAL GRANTS                        POTENTIAL REALIZABLE
                             ---------------------------------------------------------      VALUE AT ASSUMED
                              NUMBER OF                                                   ANNUAL RATES OF STOCK
                             SECURITIES    PERCENT OF TOTAL                              PRICE APPRECIATION FOR
                             UNDERLYING    OPTIONS GRANTED    EXERCISE OR                      OPTION TERM
                               OPTIONS     TO EMPLOYEES IN    BASE PRICE    EXPIRATION   -----------------------
NAME                         GRANTED (#)   FISCAL YEAR (%)      ($/SH)         DATE        5% ($)      10% ($)
----                         -----------   ----------------   -----------   ----------   ----------   ----------
<S>                          <C>           <C>                <C>           <C>          <C>          <C>
Elliot Lebowitz, Ph. D.....     4,500             1.4%           $ 6.94      12/20/10     $ 19,634     $ 49,758
                               60,000            18.6              8.89       12/4/10      335,490      850,198

James Hope, Ph.D...........    20,000             6.2             14.25       9/11/10      179,235      454,217

                                2,000             0.6              6.94      12/20/10        8,727       22,115

Julia L. Greenstein,
  Ph.D.....................    21,000             6.5              7.63       1/31/10      100,702      255,198

                                3,500             1.1              6.94      12/20/10       15,271       38,701

Mary White-Scharf, Ph.D....    24,000             7.5             10.00       8/23/10      150,935      382,498

                                2,000             0.6              6.94      12/20/10        8,727       22,115

Howard Grossberg, M.D......     8,000             2.5              7.00      12/26/10       35,218       89,250

                                2,500             0.8              6.94      12/20/10       10,908       27,643
</TABLE>



    AGGREGATED OPTION EXERCISES AND YEAR-END OPTION TABLE.  The following table
sets forth information regarding options exercised by each named executive
officer during 2000 and exercisable and unexercisable stock options held as of
December 31, 2000 by each named executive officer. The value realized upon the
exercise of options represents the difference between the option exercise price
and the closing sale price of the common stock on the date of exercise. The
value of the unexercised in-the-money options at year end has been calculated
based on $8.69, which was the closing sales price of the common stock on the
Nasdaq National Market on December 29, 2000, the last trading day of
BioTransplant's 2000 fiscal year, less the applicable option exercise price.



                AGGREGATED OPTION EXERCISES IN LAST FISCAL YEAR
                       AND FISCAL YEAR-END OPTION VALUES



<TABLE>
<CAPTION>
                                                                  NUMBER OF SECURITIES
                                                                       UNDERLYING               VALUE OF UNEXERCISED
                                                                   UNEXERCISED OPTIONS          IN-THE-MONEY OPTIONS
                                                                 AT FISCAL YEAR-END (#)        AT FISCAL YEAR-END ($)
                            SHARES ACQUIRED   VALUE REALIZED   ---------------------------   ---------------------------
                            ON EXERCISE (#)        ($)         EXERCISABLE   UNEXERCISABLE   EXERCISABLE   UNEXERCISABLE
                            ---------------   --------------   -----------   -------------   -----------   -------------
<S>                         <C>               <C>              <C>           <C>             <C>           <C>
Elliot Lebowitz, Ph.D.....           --                --        282,138        179,928      $1,130,552      $526,119

James Hope, Ph.D..........       12,000          $145,631         84,707         61,675         349,190       162,323

Julia L. Greenstein,
  Ph.D....................        6,000            96,354         96,832         64,298         356,685       223,817

Mary White-Scharf,
  Ph.D....................       11,151           147,329         97,234         79,398         342,014       228,139

Howard Grossberg, M.D.....           --                --         12,500         48,000          35,938       125,692
</TABLE>



SEVERANCE AGREEMENTS WITH EXECUTIVE OFFICERS



    Under the terms of an agreement with Dr. Lebowitz, BioTransplant's President
and Chief Executive Officer, in the event of involuntary termination of
Dr. Lebowitz' employment, he is eligible to receive six months of base salary
from BioTransplant. Under the terms of agreements with Dr. Hope, Senior Vice
President of Development, Dr. White-Scharf, Vice President of Research, and
Mr. Capasso, Vice President, Finance and Treasurer, in the event of a
termination of employment without cause, the terminated officer is eligible to
receive six months of base salary, which will be discontinued if the officer
secures other employment. Furthermore, if at the end of the six-month period
following his termination Dr. Hope is unable to secure other employment, then
Dr. Hope and

<PAGE>

BioTransplant have agreed to negotiate an additional severance payment of up to
six months of base salary.



COMPENSATION OF DIRECTORS



    BioTransplant's non-employee directors who are not affiliated with Novartis
each receive $1,500, plus reasonable travel and out-of-pocket expenses, for each
meeting of the board of directors they attend.



    The board of directors intends to make awards of stock options to directors
as compensation for service on the board of directors under BioTransplant's 1997
stock incentive plan. Currently, the board of directors grants each director,
upon his or her initial election to the board of directors, an option to
purchase 15,000 shares of BioTransplant common stock at an exercise price equal
to the then fair market value. In addition, each director is eligible to receive
an option to purchase 6,000 shares of BioTransplant common stock, at an exercise
price equal to the then fair market value, upon his or her reelection to the
board of directors at each annual meeting of stockholders.



COMPENSATION COMMITTEE INTERLOCKS AND INSIDER PARTICIPATION



    The members of the Compensation Committee during 2000 were Messrs. Crouse
and Foster. No member of the Compensation Committee was at any time during 2000,
or formerly, an officer or employee of BioTransplant or any subsidiary of
BioTransplant.



    For a discussion of an arrangement between BioTransplant and Charles River
Laboratories, an entity of which Mr. Foster is President and Chief Executive
Officer, see "Item 13--Certain Relationships and Related Transactions."



    No executive officer of the Company has served as a director or member of
the Compensation Committee (or other committee serving an equivalent function)
of any other entity, one of whose executive officers served as a director of or
member of the Compensation Committee of BioTransplant.



ITEM 12. SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT



    The following table sets forth information as to the number of shares of our
common stock beneficially owned as of February 28, 2001 by:



    - each person that beneficially owns more than 5% of the outstanding shares
      of common stock;



    - each director of BioTransplant;



    - BioTransplant's Chief Executive Officer;



    - the four other named executive officers of BioTransplant; and



    - all BioTransplant executive officers and directors as a group.



    Except as indicated by the notes to the following table, the holders listed
below will have sole voting power and investment power over the shares
beneficially held by them. Beneficial ownership is determined according to the
rules of the Securities and Exchange Commission. The table below includes shares
subject to options and warrants which will be exercisable within 60 days
following

<PAGE>

February 28, 2001. All percentages assume that the options and warrants of the
particular person or group in question, and no others, have been exercised.



<TABLE>
<CAPTION>
                                                              BENEFICIAL OWNERSHIP
                                                              ---------------------
NAME OF BENEFICIAL OWNER                                       SHARES      PERCENT
------------------------                                      ---------   ---------
<S>                                                           <C>         <C>
5% BENEFICIAL OWNERS

Rho Management Trust II(1) .................................   857,815         7.3%
  c/o Rho Management Company, Inc.
  767 Fifth Avenue
  New York, New York 10153

Funds managed by Hambrecht & Quist Capital Management,         740,451         6.3
  Inc.(2) ..................................................
  50 Rowes Wharf
  Boston, Massachusetts 02110

S Squared Technology Corporation(3) ........................   752,500         6.4
  515 Madison Avenue
  Suite 4200
  New York, NY 10022

Joseph A. Cohen(4)..........................................   771,600         6.5

DIRECTORS AND NAMED EXECUTIVE OFFICERS

Elliot Lebowitz, Ph.D.(5)...................................   372,537         3.1
Donald R. Conklin(6)........................................    14,857           *
William W. Crouse(7)........................................   216,528         1.8
James C. Foster, J.D.(8)....................................    23,731           *
Daniel O. Hauser(9).........................................    13,875           *
Michael S. Perry, D.V.M., Ph.D.(10).........................     7,500           *
James Hope, Ph.D.(11).......................................    85,007           *
Julia L. Greenstein, Ph.D.(12)..............................   110,732           *
Mary White-Scharf, Ph.D.(13)................................   108,985           *
Howard Grossberg, M.D.(14)..................................    12,500           *
All directors and executive officers as a group                880,395         7.0
  (9 individuals)(15).......................................
</TABLE>


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


*   Beneficial ownership does not exceed 1% of the outstanding shares of
    BioTransplant common stock.



(1) Includes 38,466 shares of common stock which Rho Management Trust II has the
    right to acquire within 60 days of February 28, 2001 upon the exercise of
    warrants. Jan Philipp F. Reemtsma, Joshua Ruch and Fero Ventures Limited may
    be deemed to beneficially own the shares held by Rho Management Trust II and
    retain voting and dispositive rights for such shares.



(2) Includes 429,898 shares of common stock held by Hambrecht & Quist Health
    Care Investors, including 10,210 shares which H&Q Health has the right to
    acquire within 60 days of February 28, 2001 upon the exercise of warrants,
    and 310,553 shares of common stock held by Hambrecht & Quist Life Science
    Investors, including 17,722 shares which H&Q Life has the right to acquire
    within 60 days of February 28, 2001 upon the exercise of warrants.
    Hambrecht & Quist Capital Management Inc. serves as the investment advisor
    to H&Q Health and H&Q Life. The respective general partners of H&Q Health
    and H&Q Life exercise sole voting and investment power with respect to the
    shares held by each fund.



(3) This information is based solely on information included in a Schedule 13G
    dated February 12, 2001.



(4) This information is based solely on information included in a
    Schedule 13G/A dated February 8, 2001.



(5) Includes 298,463 shares of common stock which Dr. Lebowitz has the right to
    acquire within 60 days of February 28, 2001 upon the exercise of stock
    options.

<PAGE>

(6) Includes 11,875 shares of common stock which Mr. Conklin has the right to
    acquire within 60 days of February 28, 2001 upon the exercise of stock
    options.



(7) Includes the following shares of common stock of record:



       - 78,877 shares which HCV II, L.P. has the right to acquire within
         60 days of February 28, 2001 upon the exercise of warrants;



       - 99,705 shares which HCV III, L.P. has the right to acquire within
         60 days of February 28, 2001 upon the exercise of warrants;



       - 29,571 shares which HCV IV, L.P. has the right to acquire upon the
         exercise of warrants; and



       - 7,750 shares which Mr. Crouse has the right to acquire within 60 days
         of February 28, 2001 upon the exercise of stock options.



    Mr. Crouse is a general partner of HealthCare Partners II, L.P., HealthCare
    Partners III, L.P. and HealthCare Partners IV, L.P., the general partners,
    respectively, of HCV II, L.P., HCV III, L.P. and HCV IV, L.P. Mr. Crouse,
    together with the other general partners of HCV II, HCV III and HCV IV,
    respectively, shares voting and investment control with respect to the
    shares owned by HCV II, HCV III and HCV IV. The same individuals serve as
    general partners of HealthCare Partners II, L.P., HealthCare Partners III,
    L.P. and HealthCare Partners IV, L.P.



(8) Includes 2,856 shares of common stock owned by Charles River
    Laboratories, Inc. Mr. Foster, a director of BioTransplant, is the President
    and Chief Executive Officer of Charles River Laboratories and may be deemed
    to beneficially own the shares of Charles River Laboratories, although he
    disclaims beneficial ownership. Also includes 7,750 shares of common stock
    which Mr. Foster has the right to acquire within 60 days of February 28,
    2001 upon the exercise of stock options.



(9) Includes 13,250 shares of common stock which Dr. Hauser has the right to
    acquire within 60 days of February 28, 2001 upon the exercise of stock
    options.



(10) Includes 7,500 shares of common stock which Dr. Perry has the right to
    acquire within 60 days of February 28, 2001 upon the exercise of stock
    options.



(11) Includes 84,707 shares of common stock which Dr. Hope has the right to
    acquire within 60 days of February 28, 2001 upon the exercise of stock
    options. Includes 300 shares of common stock owned by Dr. Hope's minor
    children.



(12) Dr. Greenstein has resigned as Senior Vice President, Research of
    BioTransplant, effective January 1, 2001, in order to assume the position of
    Chief Executive Officer of Immerge BioTherapeutics AG and Immerge
    BioTherapeutics, Inc., the operating entities of BioTransplant's joint
    venture with Novartis. Dr. Greenstein will continue to serve as an employee
    at BioTransplant but will no longer have the duties or title of an officer
    of BioTransplant. Includes 110,732 shares of common stock which
    Dr. Greenstein has the right to acquire within 60 days of February 28, 2001
    upon the exercise of stock options.



(13) Includes 97,234 shares of common stock which Dr. White-Scharf has the right
    to acquire within 60 days of February 28, 2001 upon the exercise of stock
    options. Also includes 600 shares of common stock owned by Dr.
    White-Scharf's minor children.



(14) Represents 12,500 shares of common stock which Dr. Grossberg has the right
    to acquire within 60 days of February 28, 2001 upon the exercise of stock
    options. Dr. Grossberg resigned in February 2001 and will continue as a
    consultant to BioTransplant.



(15) Includes 774,057 shares of common stock which all directors and executive
    officers as a group may acquire upon the exercise of outstanding stock
    options and warrants exercisable within 60 days of February 28, 2001.

<PAGE>

ITEM 13. CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS



    In March 1991, BioTransplant entered into a supply agreement with Charles
River Laboratories. BioTransplant amended the agreement in 1998. Under the terms
of the agreement, as amended, CRL provides BioTransplant with miniature swine
and miniature swine organs for research and development purposes in exchange for
payments under a research and supply agreement. BioTransplant paid CRL $877,500,
$940,000 and $988,000 under this agreement in 1998, 1999 and 2000, respectively.
James C. Foster, President and Chief Executive Officer of CRL, is a director of
BioTransplant. BioTransplant expects to assign its rights in the field of
xenotransplantation under this agreement to its joint venture with Novartis.



    From 1993 through October 2000, BioTransplant was party to two collaboration
agreements with Novartis to research, develop and commercialize
xenotransplantation products. During the collaboration, BioTransplant received
an aggregate of $33.5 million in research funding and $16.5 million in license
fees and milestone payments from Novartis. In September 2000, BioTransplant
entered into an arrangement with Novartis to combine their respective expertise
in the field of xenotransplantation into a newly-formed, independently-run
company named Immerge BioTherapeutics AG and Novartis and BioTransplant
terminated their prior collaborations in xenotransplantation. In return for
contributing its technology, BioTransplant will retain a 33% share of the joint
venture and will receive royalty payments from Novartis sales of
xenotransplantation products, if any. David Sachs, M.D., Chairman of
BioTransplant's scientific advisory board and the Director of the
Transplantation Biology Research Center at Massachusetts General Hospital;
Elliot Lebowitz, the Chief Executive Officer of BioTransplant; Corinne Savill,
Chief Operating Officer of an affiliate of Novartis; and Clive Morris, Head of
Patents, Pharma Consumer Health and Global Generics at Novartis, are all
directors of Immerge BioTherapeutics AG and of its subsidiary Immerge
BioTherapeutics, Inc. Immerge BioTherapeutics began operations in January 2001.



    Dr. Perry, a director of BioTransplant, was formerly President and Chief
Executive Officer of Genetic Therapy and SyStemix, affiliates of Novartis.

<PAGE>
                                   SIGNATURES

    Pursuant to the requirements of Section 13 or 15(d) of the Securities
Exchange Act of 1934, the registrant has duly caused this Amendment No. 1 on
Form 10-K/A to be signed on its behalf by the undersigned, thereunto duly
authorized.


<TABLE>
<S>                                                    <C>  <C>
                                                       BIOTRANSPLANT INCORPORATED

                                                       By:          /s/ ELLIOT LEBOWITZ, PH.D.
                                                            -----------------------------------------
                                                                      Elliot Lebowitz, Ph.D.
Date: April 26, 2001                                          PRESIDENT AND CHIEF EXECUTIVE OFFICER
</TABLE>