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                                                                Exhibit 10.13

February 8, 2005

BY OVERNIGHT COURIER

Viviana Wolinsky, Licensing Manager
Ernest Orlando Lawrence Berkeley National Laboratory
1 Cyclotron Road, MS 90R1070
Berkeley, California 94720-8125

Re:  LICENSE AGREEMENT/L-03-1370

Dear Ms. Wolinsky:

Reference is made to the License Agreement for Installing Novel Functional
Groups for Therapeutics between Momenta Pharmaceuticals, Inc. ("Momenta") and
the Regents of the University of California through the Ernest Orlando Lawrence
Berkeley National Laboratory ("Berkeley Lab") dated November 20, 2002, as
amended by letter agreement dated November 16, 2004 (the "Agreement").
Capitalized terms used herein and not otherwise defined shall have the meanings
given such terms in the Agreement.

Momenta has provided Berkeley Lab with a confidential strategic plan
pertaining to Licensed Products. Berkeley has accepted such strategic plan in
substitution for the milestones listed in Exhibit A to the Agreement with the
understanding that Momenta shall fund, or cause its Affiliates, sublicensees
and/or corporate partners to fund, no less than $2,000,000 toward the
research, development or commercialization of Licensed Products during
calendar year 2005.

In accordance with Section 3.3 of the Agreement, Momenta hereby notifies
Berkeley Lab that it elects to extend its rights to the Broader Field for the
remainder of the term of the Agreement and Berkeley Lab hereby accepts such
extension of its rights to the Broader Field.

If Momenta shall fail to expend the anticipated $2,000,000 during calendar
year 2005, Berkeley Lab and Momenta shall meet to discuss the reasons for
such failure. Thereafter, Berkeley Lab may elect to accept substitute
milestones achieved during calendar year 2005, negotiate additional
milestones for calendar year 2006, or terminate Momenta's rights to the
Broader Field. In the case of termination of Momenta's rights to the Broader
Field,

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Viviana Wolinsky, Licensing Manager
February 8, 2005
Page 2

Momenta shall nonetheless retain its exclusive license to the Specific
Applications Field which will then be selected in accordance with Section 3.4 of
the Agreement.

If the foregoing is acceptable to Berkeley Lab, please indicate your consent by
counter signing this letter and returning on fully-executed copy to me. We will
then forward the first payment of $33,333.33 to Berkeley Lab.

Very truly yours,

/s/ Susan K. Whoriskey

Susan K. Whoriskey
Vice President, Licensing & Business Development

Agreed and accepted:

The Regents of the University
Of California, through the
Ernest Orlando Lawrence
Berkeley National Laboratory

By:  /s/ V. Wolinsky
   ----------------------------------------------<Page>

                                                               Exhibit 10.15

Siegfried

                                                  SIEGFRIED (USA), INC.
                                                  33 Industrial Park Road
                                                  Pennsville, NJ 08070
                                                  USA

                                                  +1 616-895-4553
                                                  +1 616-835-0321mobile
                                                  +1 419-710-5687 fax
                                                  scott.powers@siegfried-usa.com

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

January 26, 2005

Dr. John E. Bishop
Vice President, Pharmaceutical Sciences & Manufacturing
Momenta Pharmaceuticals, Inc.
675 West Kendall Street
Cambridge, MA 02142

Dear John:

Reference is made to the Development and Production Agreement for Active
Pharmaceutical Ingredient between Siegfried (USA), Inc. and Siegfried Ltd.
("SIEGFRIED") and Momenta Pharmaceuticals, Inc. ("MOMENTA") dated October 10,
2003, as amended by Letter Agreements dated February 14, 2004, May 17, 2004 and
September 29, 2004 (the "Agreement"). Capitalized terms used herein and not
otherwise defined shall have the meanings given such terms in the Agreement.

MOMENTA hereby ratifies SIEGFRIED's engagement of the two subcontractors
identified below in connection with the testing ("RAW MATERIAL TESTING") of
certain raw materials used by SIEGFRIED in the manufacture of those
quantities of the PRODUCT to be used by MOMENTA in support of the ANDA
covering the PRODUCT. The RAW MATERIAL TESTING shall be deemed to have been
conducted under the terms and conditions of the Agreement, with the following
modifications:

     1.   SIEGFRIED has contracted the services of [**] to perform (a) the USP
          testing of [**], as per the previously provided quotation and scope of
          work received from [**], and (b) the [**] testing of [**].
          SIEGFRIED had previously approved [**] as a contract laboratory per
          SIEGFRIED's internal procedures.

     2.   SIEGFRIED has contracted the services of [**] to perform
          microbiological testing of [**] out of the facilities of [**].
          SIEGFRIED had previously approved this facility of [**] as a
          contract laboratory per SIEGFRIED's internal procedures.

     3.   MOMENTA shall pay SIEGFRIED the actual costs of RAW MATERIAL TESTING
          as invoiced by [**] and [**] plus [**] percent ([**]%) for
          administrative costs. Invoicing and payment shall

www.siegfried-usa.com              Page 1 of 3

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Siegfried

          be in accordance with Section 10.1 of the Agreement. For purposes of
          Section 9.4 of the Agreement, the foregoing fee shall be attributed to
          RAW MATERIAL TESTING.

     4.   SIEGFRIED will provide a copy of the [**] and [**] invoices for RAW
          MATERIAL TESTING when invoicing MOMENTA.

     5.   SIEGFRIED shall remain primarily responsible to Momenta for the work
          of [**] and [**]. Further, SIEGFRIED represents and warrants to
          Momenta that it has or will have in place prior to (a) any final
          payment to [**], a binding agreement obligating [**] to terms and
          conditions comparable to those set forth in Sections 7 and 8 of the
          Agreement and sufficient in order for SIEGFRIED to comply with its
          obligations to MOMENTA under Sections 7 and 8 of the Agreement and (b)
          any final payment to [**], a binding agreement obligating [**] to
          terms and conditions comparable to those set forth in Sections 7 and 8
          of the Agreement and sufficient in order for SIEGFRIED to comply with
          its obligations to MOMENTA under Sections 7 and 8 of the Agreement.
          Copies of such agreements shall be furnished to MOMENTA in order for
          MOMENTA to ascertain such compliance.

MOMENTA hereby consents to SIEGFRIED's engagement of the two subcontractors
identified below in connection with the release and stability testing ("API
RELEASE AND STABILITY TESTING") of those quantities of the PRODUCT to be used
by MOMENTA in support of the ANDA covering the PRODUCT. The API RELEASE AND
STABILITY TESTING shall be conducted under the terms and conditions of the
Agreement, with the following modifications:

     6.   SIEGFRIED shall contract the services of [**] to perform
          microbiological testing of the PRODUCT. Such testing shall be
          conducted (a) out of the facilities of [**] and (b) pursuant to a
          quotation and scope of work that will be furnished to and approved
          in advance by MOMENTA. SIEGFRIED has previously approved this
          facility of [**] as a contract laboratory per SIEGFRIED's internal
          procedures.

     7.   SIEGFRIED shall contract the services of [**] to perform the release
          testing of the PRODUCT as per the previously provided quotation and
          scope of work received from [**]. The expedited services quoted
          therein by [**] shall be used by SIEGFRIED only if authorized by
          MOMENTA. In addition, SIEGFRIED shall contract the services of [**] to
          perform the stability testing of the PRODUCT. Such testing shall be
          conducted pursuant to a quotation and scope of work that will be
          furnished to and approved in advance by MOMENTA. SIEGFRIED has
          previously approved [**] as a contract laboratory per SIEGFRIED's
          internal procedures.

     8.   MOMENTA shall pay SIEGFRIED the actual costs of API RELEASE AND
          STABILITY TESTING as invoiced by [**] and [**] plus [**]percent
          ([**]%) for administrative costs. Invoicing and payment shall be in
          accordance with Section 10.1 of the Agreement. For purposes of
          Section 9.4 of the Agreement, the foregoing fee shall be attributed
          to API RELEASE AND STABILITY TESTING.

     9.   SIEGFRIED will provide a copy of the [**] and [**] invoices for API
          RELEASE AND STABILITY TESTING when invoicing MOMENTA.

     10.  SIEGFRIED shall remain primarily responsible to Momenta for the work
          of [**] and [**]. Further, SIEGFRIED represents and warrants to
          Momenta that it has or will have in place prior to the commencement of
          any API RELEASE AND STABILITY TESTING by (a) [**] a binding agreement
          obligating [**] to terms and conditions comparable to those set forth
          in Sections 7 and 8

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Siegfried

          of the Agreement and sufficient in order for SIEGFRIED to comply with
          its obligations to MOMENTA under Sections 7 and 8 of the Agreement and
          (b) [**] a binding agreement obligating [**] to terms and conditions
          comparable to those set forth in Sections 7 and 8 of the Agreement and
          sufficient in order for SIEGFRIED to comply with its obligations to
          MOMENTA under Sections 7 and 8 of the Agreement. Copies of such
          agreements shall be furnished to MOMENTA in order for MOMENTA to
          ascertain such compliance. Further an advance copy of the quality
          agreement between [**] and SIEGFRIED shall be furnished to MOMENTA for
          comment by MOMENTA.

If the foregoing is in conformity with your understanding, please execute both
copies of this Letter Agreement and return one fully executed copy to me.

Sincerely,

/s/ Scott Powers

Scott Powers

Offered by:

SIEGFRIED (USA), INC.

By:    /s/ Scott Powers              Date:   January 26, 2005
   -----------------------                -------------------

SIEGFRIED LTD.

By:  /s/ Dennis P. Bauer             Date:   January 27, 2005
   -----------------------                -------------------

Agreed and accepted:

MOMENTA PHARMACEUTICALS, INC.

By:  /s/ John E. Bishop              Date:   31 Jan 05
   -----------------------                -----------------------

                                   Page 3 of 3

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