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                                                               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 ("[**] TESTING") of [**] 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 [**] 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 [**]
          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 [**] testing
          [**] 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 [**] TESTING as
          invoiced by [**] and [**] plus [**] percent ([**]%) for administrative
          costs. Invoicing and payment shall

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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
          [**] TESTING.

     4.   SIEGFRIED will provide a copy of the [**] and [**] invoices for [**]
          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 [**] TESTING [**] TESTING") of those
quantities of the PRODUCT to be used by MOMENTA in support of the ANDA
covering the PRODUCT. The [**] TESTING shall be conducted under the terms and
conditions of the Agreement, with the following modifications:

     6.   SIEGFRIED shall contract the services of [**] 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 [**]
          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 [**] 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 [**] 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 [**] TESTING.

     9.   SIEGFRIED will provide a copy of the [**] and [**] invoices for [**]
          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 [**] 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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          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
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Agreed and accepted:

MOMENTA PHARMACEUTICALS, INC.


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