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                                                                  Exhibit 10.3.1

                            FIRST AMENDMENT TO LEASE

     This FIRST AMENDMENT TO LEASE AGREEMENT (this "FIRST AMENDMENT") is entered
into as of the 31st day of March, 1999, by and between FIRST SECURITY BANK,
N.A., not in its individual capacity, but solely as Trustee under the Owner
Trust Agreement, dated as of October 21, 1998, as lessor ("LESSOR"), and GELTEX
PHARMACEUTICALS, INC., a Delaware corporation, as lessee ("LESSEE").

                                    RECITALS:

     A.   Lessor and Lessee entered into that certain Lease Agreement, dated as
of October 21, 1998 (the "LEASE"), pursuant to which Lessee leased from Lessor
that certain property located in Waltham, Massachusetts, described in Schedule A
annexed to the Lease, together with all Improvements thereon (the "PROPERTY").
Capitalized terms used herein and not specifically defined herein shall have the
respective meanings set forth in the Lease.

     B.   Lessor and Lessee desire to modify certain terms and conditions of the
Lease as set forth herein.

     NOW, THEREFORE, in consideration of the mutual agreements and covenants
contained herein, the parties hereto agree as follows:

     1.   The last sentence of the second paragraph of Section (1) of Schedule C
annexed to the Lease, which reads as follows, shall be deleted in its entirety:

     "For purposes of this Lease, including without limitation Schedule G,
Lessor and Lessee hereby agree that such land acquisition costs are equal to
$9,000,000."

     2.   NO OTHER MODIFICATIONS. Lessor and Lessee hereby acknowledge and agree
that, except as provided in this First Amendment, the Lease has not been
modified, amended, canceled, terminated, released, superseded or otherwise
rendered of no force or effect. All prior negotiations and agreements by Lessor
and Lessee with respect to the subject matter hereof are fully merged herein and
superseded by the terms of this First Amendment.

     3.   CONFIRMATION AND RATIFICATION. The Lease as hereby amended is hereby
ratified and confirmed by Lessor and Lessee, and every provision, covenant,
condition, obligation, right, term and power contained in and under the Lease,
as hereinabove amended, shall continue in full force and effect.

     4.   COUNTERPARTS. This First Amendment may be executed in counterparts.

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     5.   GOVERNING LAW. This First Amendment shall be governed by the laws of
the Commonwealth of Massachusetts.

     6.   SUCCESSORS AND ASSIGNS. This First Amendment shall be binding on the
successors and assigns of each of Lessor and Lessee.

                            [SIGNATURE PAGE FOLLOWS]

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     IN WITNESS WHEREOF, the undersigned have executed this First Amendment as
of the day and year first above written.


                                LESSOR:

                                FIRST SECURITY BANK, N.A., not in its
                                individual capacity except as expressly set
                                forth herein, but solely as Trustee under the
                                Owner Trust Agreement dated as of October 21,
                                1998


                                By: /s/ Val T. Orton
                                   --------------------------------------
                                   Name:  Val T. Orton
                                   Title: Vice President

                                LESSEE:

                                GELTEX PHARMACEUTICALS, INC.


                                By: /s/ Paul J. Mellett, Jr.
                                   --------------------------------------
                                   Name:  Paul J. Mellett, Jr.
                                   Title: Vice President, Finance and
                                          Administration; Chief Financial
                                          Officer