<PAGE>
                                                                   Exhibit 10.24

[*] CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
    BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND
    EXCHANGE COMMISSION PURSUANT TO RULE 406 OF THE SECURITIES EXCHANGE ACT OF
    1933, AS AMENDED.

[CYTOKINETICS LOGO]                        280 East Grand Avenue
                                           South San Francisco, CA 94080
                                           Tel (650) 624-3000 Fax (650) 624-3010

July 28, 2003

Glaxo Group Limited, doing business as GlaxoSmithKline
709 Swedeland Road
King of Prussia, Pennsylvania 19406
Attn: Pradip K. Bhatnagar, Ph.D., Director, Genetic & Discovery Alliances

RE: FEASIBILITY STUDY UNDER THAT CERTAIN COLLABORATION AND LICENSE AGREEMENT BY
AND BETWEEN GLAXO GROUP LIMITED, A GLAXOSMITHKLINE COMPANY, ("GSK") AND
CYTOKINETICS, INC. ("CK") OF EVEN DATE JUNE 20, 2001 (THE "COLLABORATION
AGREEMENT")

Dear Pradip:

Pursuant to this letter amendment to the Collaboration Agreement (the
"Feasibility Study Letter Amendment"), GSK and CK desire to revise the
definition of Feasibility Study applicable under the Collaboration Agreement as
agreed by the JRC on July 25, 2003, all on the terms set forth herein.

Now therefore, GSK and CK agree as follows:

1.  All capitalized terms not defined herein shall have the meaning ascribed to
    them in the Collaboration Agreement.

2.  For purposes of the Collaboration Agreement, "Feasibility Study" shall have
    the meaning, effective as of the Effective Date, set forth in the revised
    Exhibit 6.3.2, attached hereto and incorporated herein. For clarity, it is
    understood that the definition of Feasibility Study may only be modified or
    amended pursuant in a writing signed by both Parties referencing the
    Collaboration Agreement and expressly modifying or amending the definition
    of Feasibility Study.

3.  Except as specifically modified or amended hereby, the Collaboration
    Agreement shall remain in full force and effect and, as modified or amended,
    is hereby ratified, confirmed and approved. No provision of this Feasibility
    Study Letter Amendment may be modified or amended except expressly in a
    writing signed by both Parties nor shall any terms be waived except
    expressly in a writing signed by the Party charged therewith. This
    Feasibility Study Letter Amendment shall be governed in accordance with the
    laws of the [*], without regard to principles of conflicts of laws.

* Certain information on this page has been omitted and filed separately with
  the Commission. Confidential treatment has been requested with respect to the
  omitted portions.
<PAGE>
Pradip K. Bhatnagar, Ph.D.
July 28, 2003
Page Two

Please sign and return two copies of this letter if you agree to the foregoing
terms.

Sincerely,

/s/ Robert I. Blum
---------------------------
Robert I. Blum
Senior Vice President, Finance and Corporate Development
Chief Financial Officer
Cytokinetics, Inc.

Agreed and accepted:

GLAXO GROUP LIMITED

/s/ Pradip K. Bhatnagar
--------------------------

Name: Pradip K. Bhatnagar
      --------------------

Title: Director
       -------------------

cc:  Vice President, Business Development, Glaxo Group Limited, doing business
     as GlaxoSmithKline
     Senior Vice President and Assistant General Counsel-R&D Legal Operations,
     GlaxoSmithKline Corporate Legal Department
     Kenneth A. Clark, Esq., Wilson Sonsini Goodrich & Rosati Professional
     Corporation
<PAGE>
                                 EXHIBIT 6.3.2
                            [Revised July 25, 2003]

                               Feasibility Study
                        (effective as of June 20, 2001)


                                      [*]


* Certain information on this page has been omitted and filed separately with
  the Commission. Confidential treatment has been requested with respect to the
  omitted portions.