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                                                                   EXHIBIT 10.13

                                SECOND AMENDMENT
                                       TO
                             THE HEADS OF AGREEMENT


This Second Amendment to the Heads of Agreement (the "Amendment") is made as of
this 7th day of August, 1998, by and between Genelabs Technologies, Inc.
("Genelabs") and SmithKline Beecham p.l.c. ("SKB") and amends that certain Heads
of Agreement dated August 27, 1992, between Genelabs and SKB, as amended by a
letter agreement dated September 15, 1993 and by that Amendment No. 1 dated as
of October 1, 1995 (as amended, the "Agreement").

The parties now wish to amend the Agreement to (i) terminate Genelabs'
co-exclusive license under certain patents and know-how to make, have made, use
and sell HEV Vaccines, either alone or in combination with other vaccines, in
Asia and PAKIPI, and (ii) change the terms of payment by SKB to Genelabs of two
development milestones. In partial consideration for these amendments, SKB has
agreed to pay to Genelabs U.S. $1 million.

Unless otherwise defined herein, all capitalized terms used in this Amendment
shall have the same meaning as such terms are defined in the Agreement.

NOW, THEREFORE, the parties have agreed as follows:

1.  Termination of Co-exclusive License.

Genelabs hereby grants to SKB an exclusive, worldwide license, under PATENTS and
KNOW-HOW, to make, have made, use and sell HEV Vaccine. Accordingly, the
Agreement is amended as follows:

         (a)      The definition of "SKB TERRITORY", set forth in Paragraph
                  1.16, is amended to mean all countries of the world.

         (b)      The restrictions provided for under Paragraph 2.01, last
                  sentence, to LICENSEE'S rights to sub-license are hereby
                  terminated.

         (c)      Genelabs' co-exclusive license, set forth in Paragraph 2.02,
                  under PATENTS and KNOW-HOW to make, have made, use and sell
                  HEV Vaccine alone or in combination with other vaccines in
                  ASIA and PAKIPI is hereby terminated.

         (d)      Consistent with such termination of Genelabs' co-exclusive
                  license, the rights and licenses granted to Genelabs in
                  Paragraphs 2.03, 2.04, 2.05, 2.06, 2.09 and 2.10 (as such
                  Paragraph relates to the rights contained in Article 2) are
                  hereby terminated. The provisions of Paragraph 2.10 continue
                  in effect to the extent they apply to the rights granted to
                  Genelabs pursuant to Paragraph 6.03.


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2.  Amendment of Two Development Milestone Payments.

         (a)      Paragraph 4.02(a) is hereby amended to change the minimum
                  payment from [*]. Paragraph 4.02 (a) shall read in its
                  entirety as follows:

                           "US [*] upon LICENSEE entering into Phase I of
                           clinical trials with the HEV Vaccine; however, if the
                           price (the `STOCK PRICE') of LICENSOR's common stock
                           in the First Stock Purchase (as defined in Article
                           4.08 below) in the Stock Purchase Agreement (as
                           defined in Article 4.08 below) is less than [*] and
                           the stock purchase is made as provided for in the
                           Stock Purchase Agreement, this milestone will be
                           reduced by the product of [*], but under no
                           circumstance will the milestone be less than [*]."

         (b)      Paragraph 4.02(b) is hereby amended to read in its entirety as
                  follows:

                           "US [*] upon the earlier of (i) demonstration in
                           humans of protection by the HEV Vaccine against
                           disease, or (ii) three months following the
                           successful completion of a Phase III trial in any
                           country".

3. Payment by SKB.

In consideration for the foregoing amendments, SKB agrees to pay to Genelabs the
amount of US $1,000,000 (one million US dollars). Such amount shall be due and
payable promptly upon the execution of this Amendment.

Except as expressly amended by this Amendment, all terms and conditions of the
Agreement remain in full force and effect.

IN WITNESS WHEREOF, this Amendment has been executed as of the date first
written above by the duly authorized representatives of the parties hereto.


GENELABS TECHNOLOGIES, INC.            SMITHKLINE BEECHAM p.l.c.

By:    /s/ GILBERT R. MINTZ, Ph.D.     By: /s/ President and General  Manager  
    ------------------------------         ----------------------------------


Title:  Vice President, Business Development    Title:  President and General
                                                         Manager


[*]      Confidential treatment requested pursuant to a request for confidential
         treatment filed with the Securities and Exchange Commission. Omitted
         portions have been filed separately with the Commission.