EXHIBIT 10.15

CERTAIN INFORMATION IN THIS EXHIBIT IS SUBJECT TO A REQUEST FOR CONFIDENTIAL
TREATMENT. IN ACCORDANCE WITH RULE 406 UNDER THE SECURITIES ACT OF 1933, AS
AMENDED, SUCH INFORMATION HAS BEEN OMITTED AND FILED SEPARATELY WITH THE
SECURITIES AND EXCHANGE COMMISSION. THE LOCATION OF SUCH OMITTED INFORMATION HAS
BEEN INDICATED WITH AN ASTERISK(*).

                                                              CONFIDENTIAL
                                                          TANOX BIOSYSTEMS, INC.

                             SUPPLEMENTAL AGREEMENT

Tanox and Ciba currently are engaged in settling certain litigation with
Genentech and Roche. In connection with the settlement of such litigation,
Tanox, Ciba and Genentech (and possibly certain Roche entities) have entered
Into an agreement (the "Multiparty Transaction" under which Genentech merges its
anti-IgE antibody project with the anti-IgE antibody project of Tanox/Ciba which
is being pursued under a Development and License Agreement between Tanox and
Ciba dated May 11, 1990 (the "D&L Agreement").

Tanox and Ciba also are party to a Loan Agreement dated effective December 13,
1994, pursuant to which Ciba has agreed to loan up to $10 million for the
construction, validation, and start-up of a mammalian cell culture pilot
manufacturing facility located in Houston, Texas (the "Pilot Plant").

To further facilitate the cooperation between them in connection with the
Multiparty Transaction and the development of the respective anti-IgE antibody
product ("Anti-IgE Product") project, Tanox and Ciba intend to enter into an
agreement which modifies and amends their agreements to the extent of the
provisions set forth below.

1.0  DEVELOPMENT IN CERTAIN FAR EAST COUNTRIES

     1.1 MERGER OF RIGHTS IN EA COUNTRIES. Pursuant to the D & L Agreement,
     Tanox has retained certain commercialization rights for the Anti-IgE
     Product in the territories of Taiwan, Hong Kong, Singapore, China and Korea
     (the "EA Countries"). Ciba also has certain commercialization rights for
     Anti-IgE Product in such EA Countries. As part of this Addendum to the D &
     L Agreement, Tanox and Ciba agree to merge their rights in the EA Countries
     into a combined development and commercialization program for the Anti-IgE
     Product, in which each shall share all development and commercialization
     costs as set out in Section 1.4 below and net profits before taxes from
     commercialization of the Anti-IgE Product on a 50:50 basis.

     1.2 TANOX DEVELOPMENT COMPANY. The parties acknowledge that all or a
     portion of Tanox's rights and responsibilities for development and
     commercialization of the Anti-IgE Product in the EA Countries may be
     undertaken by (a) company(ies) formed by Tanox for such purpose ("EA
     Company"), which may be financed in part by third parties as shareholders
     and in which Tanox shall have the control, i.a. by having a majority of the
     voting rights as shareholder.

     1.3 DEVELOPMENT AND COMMERCIALIZATION ACTIVITIES. The parties shall
     cooperate fully in connection with the development and commercialization of
     the Anti-IgE Product. Tanox shall assume primary responsibility in
     coordination with the Steering Committee (as defined in the D & L
     Agreement) for pursuing any additional
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Supplemental Agreement
June 10, 1996
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     development activities in the EA Countries which may be necessary or
     beneficial in connection with obtaining product registration and marketing
     approval in such countries. Ciba shall have the right to market and sell
     the Anti-IgE Product following marketing approval. Tanox shall have the
     right to supply product to the EA Countries at cost from its pilot facility
     in Houston, Texas, and/or from a facility owned and operated by the EA
     Company. To the extent Tanox is unable to supply all the product which may
     be required for the EA Countries, such product also may be purchased from
     Genentech or supplied by Ciba, as applicable.

     1.4 COST AND PROFIT SHARING. All costs associated with development
     activities undertaken for commercialization of the Anti-IgE Product in the
     EA Countries, which may include production of clinical materials, clinical
     trials and other activities for obtaining marketing authorization, and
     supporting activities/services and fees, shall be shared between Ciba and
     Tanox or the EA Company on a 50:50 basis. All costs for the
     manufacturing/purchase of the Anti-IgE Product and the costs for marketing
     and sales in the EA Countries, including all supporting and auxillary
     activities and royalties to third parties (if any) also shall be shared
     between Ciba and Tanox or the EA Company on a 50:50 basis. All net profits
     before taxes from commercialization of the Anti-IgE Product in the EA
     Countries shall be shared between Ciba and Tanox or the EA Company on a
     50:50 basis.

2.0  REIMBURSEMENT OF CIBA'S PATENT LITIGATION COSTS

     2.1 REIMBURSABLE COSTS. Tanox agrees that it will reimburse Ciba in the
     manner agreed below for * of its ex-pocket expenditures for
     attorneys' fees and expenses, document discovery, and compensation for
     expert's opinions which are directly related to Ciba's defense of
     Genentech's patent claims against it for infringement of the U.S. "Cabilly"
     patent and/or Ciba's counterclaims against Genentech for invalidation of
     such patent. Ciba agrees that reimbursable ex-pocket expenditures shall not
     include any expenditures which have been incurred by Ciba following
     February 15, 1996, and shall not include any expenditures relating to
     representation of Ciba's interests outside the scope of the patent claims
     and counterclaims as defined above, including, without limitation, any
     attorneys' fees incurred in connection with representation of Ciba in the
     multiparty settlement discussions or bilateral discussions between Tanox
     and Ciba.

     2.2 DOCUMENTATION. Promptly following execution of the agreement
     contemplated hereby, Ciba agrees to submit copies to Tanox of appropriate
     invoices or other documentation for such ex-pocket expenditures to be
     claimed for reimbursement hereunder so that Tanox can confirm that all such
     charges are
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June 10, 1996
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     properly within the scope of the ex-pocket expenditures agreed to be
     reimbursable hereunder.

     2.3 REIMBURSEMENT BY OFFSET. All amounts due Ciba hereunder shall be paid
     solely by Ciba's offset of such amounts against royalties payable to Tanox
     by Ciba under the D & L Agreement or which are attributable to Ciba based
     on its profit sharing ratios under the Multiparty Transaction.

3.0  LOAN REPAYMENT OBLIGATION

     3.1 EFFECTIVENESS OF PROVISIONS. If the Multiparty Transaction is
     concluded, then as of such date, repayment of that certain $10 million loan
     ("Loan") from Ciba to Tanox for construction, validation and start-up of
     the Pilot Plant shall be governed by the provisions of this Paragraph 3.

     3.2 UTILIZATION OF FACILITY. The parties agree that (i) if Genentech's
     anti-IgE antibody is selected for development as of the Selection Date, as
     defined in the Outline of Terms entered into in connection with the
     Multiparty Transaction or (ii) if the parties to the Outline of Terms
     otherwise determine that activities currently contemplated for the Pilot
     Plant should be shifted to Genentech in whole or in part in connection with
     its agreement, on Ciba's request, to undertake commercial production of up
     to * of Anti-IgE Product, then Tanox shall have 120 days from receipt of
     written notice of the occurrence of either such event to notify Ciba of its
     intent to pursue one of the following alternatives:

     (a)  Tanox may notify Ciba that it has decided to shut down the Pilot Plant
          and terminate production activities effective as of a date mutually
          agreeable to the parties ("Shut-down Date"), subject to reactivation,
          as Tanox may wish, within five (5) years from the Shut-down Date
          (however, Tanox can continue to perform routine maintenance activities
          at its cost beneficial to reactivation of the facility); or

     (b)  Tanox may notify Ciba that it has decided to continue operation of the
          Pilot Plant at its own commercial and technical responsibility, in
          which case such decision shall become effective immediately on Ciba's
          receipt of such notice ("Continuation Date")

     3.3  SHUT-DOWN ALTERNATIVE. If Tanox has notified Ciba that it has decided
          to shut down the Pilot Plant, subject to reactivation as provided
          hereinafter, then at the expiration five (5) years from the Shut-down
          Date or on satisfaction of Tanox's obligations to Ciba following the
          Disposal Date, as hereinafter defined, whichever is the first to
          occur, Ciba shall forgive, without further
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Supplemental Agreement
June 10, 1996
Page 4

          action on its part, the principal balance and accrued interest
          remaining outstanding on the Loan at such time. Thereafter, Tanox
          shall have no further obligations under the Loan. During such
          five-year period, if Tanox should sell or otherwise dispose of the
          Pilot Plant (the "Disposal Date", then Tanox shall be obligated to pay
          to Ciba, in reduction of the outstanding principal and any unforgiven
          interest, the proceeds from such disposal net of all reasonable costs
          accrued during such period in connection with the maintenance of the
          shut-down Pilot Plant. Tanox shall pay such net proceeds to Ciba
          within 90 days following the Disposal Date. During such five-year
          period, if Tanox should decide to reactivate the Pilot Plant for
          production activities, then the provisions of Paragraph 3.4 below
          shall immediately become effective.

     3.4  CONTINUATION ALTERNATIVE. If Tanox has notified Ciba that it has
          decided to continue operation of (or reactivate, as the case may be)
          the Pilot Plant, then Tanox will be obligated at the end of each
          calendar year, beginning as of the first year end following the
          Continuation Date (or date of reactivation), to account for the
          operations of the Pilot Plant and determine Net Cash Flow available
          from operations as defined in Paragraph 3.5 below. On or before March
          31 of the succeeding year, Tanox will be obligated to apply 75% of Net
          Cash Flow first to payment of accrued interest for such year and then
          to reduction of the outstanding principal balance on the Loan. Tanox
          will be entitled to retain the remaining 25% of Net Cash Flow. If the
          75% Net Cash Flow payments due as set out above should not equal the
          Loan amount including accrued interest, at the end of 10 years from
          the Continuation Date (or date of reactivation), and following payment
          by Tanox of the amount to be applied against the Loan and accrued
          interest through such date based on an interim accounting for such
          final period, Ciba shall forgive, without further action on its part,
          the principal balance and all accrued interest for such period that
          remains outstanding on the Loan and Tanox shall have no further
          obligations under the Loan.


     3.5  ACCOUNTING FOR THE PILOT PLANT. At the end of each calendar year
          during which Pilot Plant operations are continued or reactivated, the
          Pilot Plant's Net Cash Flow will be computed and any payments required
          under Paragraph 3.4 above will be made on or before March 31 of the
          succeeding year. Net Cash Flow for purposes of this agreement will be
          the Pilot Plant's "net cash provided from operating activities" (as
          illustrated in SFAS No.95), *
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June 10, 1996
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          *. Allocable costs to the Pilot Plant shall be apportioned in a manner
          consistent with GAAP and Tanox's normal practices, which have been
          reviewed in connection with the preparation of Tanox's annual audited
          financial statements.

     3.6  INTEREST EXPENSE. As set out in the Loan Agreement, interest shall
          begin as of the Commencement Date (as defined in the Loan Agreement)
          for the Pilot Plant. If the Pilot Plant is shut down, then at the end
          of each calendar year in which the Pilot Plant remains shut down
          (provided no disposal has occurred), Ciba agrees to forgive all the
          interest accrued on the Loan for such year. If Tanox disposes of the
          Pilot Plant, the net proceeds of such disposal, as provided in
          Paragraph 3.3 above, shall first be applied to payment of accrued
          interest on the Loan for the year in which disposal occurs and then to
          reduction of the outstanding principal balance.

The contents of this supplemental agreement represent the bona fide intent of
the parties who shall negotiate and enter into a more detailed agreement in
line with the provisions set forth above.

Ciba-Geigy Limited                               Tanox Biosystems, Inc.
By: H. F. MOHR                                   By: DAVID ANDERSON
Name: Dr. H. F. Mohr                             Name: David Anderson
Title: Head Pharma Licensing                     Title: Executive Vice President
Date:                                            Date: July 8, 1996

By: HERBERT GUT
Name: Dr. Herbert Gut
TItle: Senior Division Counsel
Date: July 6, 1996