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                                                                   Exhibit 10.52

                               AMENDMENT NO. 4 TO
                                    AGREEMENT

         This Amendment No. 4 ("AMENDMENT NO. 4") is entered effective March 5,
2003 (the "AMENDMENT EFFECTIVE DATE") pursuant to and amending that certain
Agreement between Gen-Probe Incorporated, a Delaware corporation ("GEN-PROBE")
and Chiron Corporation, a Delaware corporation ("CHIRON"). Capitalized terms
used but not defined herein shall have the meanings set forth in the Agreement.


                                    Recitals

         A. The parties entered into the Agreement as of June 11, 1998 pursuant
to which, among other things, the parties described their respective rights and
obligations with respect to the development, manufacture, marketing and
distribution of Products in the Blood Screening and Clinical Diagnostic Fields.
The Agreement was amended by the following: (i) June 11, 1998 Collaboration
Agreement, (ii) June 11, 1998 Supplemental letter agreement, (iii) June 26, 1998
Addendum to Collaboration Agreement, (iv) June 30, 1998 Supplemental letter
agreement, (v) June 30, 1998 Consent, (vi) December 1, 1998 letter agreement re
American Red Cross revenues, (vii) December 7, 1999 Amendment, (viii) February
1, 2000 Second Amendment, (ix) June 7, 2001 Supplemental Agreement No. 1
(Customer training and Support), (x) October 30, 2001 Confidentiality and Joint
Interest Agreement, (xi) April 4, 2001 Agreement re Arbitration and (xii) April
1, 2002 Amendment No. 3 (warehousing and shipping). In addition, the parties
submitted a number of disputes under the Agreement to arbitration pursuant to
Section 13 of the Agreement, and in resolution of those disputes, executed a
Definitive Written Settlement Agreement, dated December 5, 2001, and a Short
Form Agreement, dated November 16, 2001.

         B. The parties have previously revised Article 14 of the Agreement,
memorialized in that certain December 7, 1999 Amendment. The parties have
discussed a further revision to and clarification of their respective rights and
obligations with respect to Article 14, and have agreed to make certain
additional changes to the Agreement in connection therewith, in accordance with
the provisions of this Amendment No. 4.

                                    Agreement

         NOW, THEREFORE, for and in consideration of the mutual covenants and
agreements set forth in this Amendment No. 4, the parties agree as follows:

         1. ULTRIO ON TIGRIS. Article 14 of the Agreement shall be and is hereby
amended to read as follows:

         "In consideration for the rights originally granted to Chiron by this
Agreement with respect to TMA and HPA, Chiron shall pay to Gen-Probe, or shall
cause Bayer Corporation to pay to Gen-Probe, the following nonrefundable amounts
upon the occurrence of each event set forth below:

         (1)      ten million dollars ($10,000,000) upon the Effective Date; and



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         (2)      eight and one-half million dollars ($8,500,000) upon the first
         to occur of the following:

                  (a)(i) Gen Probe certifies to Chiron that Gen-Probe has
                  completed manufacture of a Lot of the Initial Blood Screening
                  Assay for the qualitative detection of both HCV and HIV-1 at
                  Gen-Probe's Willow Court facility, which Lot is manufactured
                  in accordance with FDA Good Manufacturing Processes and
                  includes not less than 400,000 tests labeled for IVD
                  distribution outside the United States; and (ii) Gen-Probe has
                  released such tests for foreign sale; and (iii) Gen-Probe
                  delivers to Chiron a copy of the Certificate of Analysis with
                  respect to such Lot, in form reasonably satisfactory to
                  Chiron, signed by Gen-Probe's quality assurance organization;
                  or

                  (b) Gen-Probe certifies to Chiron that it has successfully
                  completed validation of the Enhanced Semi-Automated System
                  ("ESAS") for use with the Initial Blood Screening Assay,
                  including both instrument hardware and software, and delivers
                  or makes available to Chiron a copy of the Validation Report
                  with respect to both the instrument and the software, signed
                  by Gen-Probe's quality assurance organization.

         (3)      six and one-half million dollars ($6,500,000) upon the date on
         which the following has occurred: (a) thirty (30) days has passed since
         submission to the FDA of an Investigational New Drug application for a
         Future Blood Screening Assay for qualitative detection of HCV, HIV-1,
         and hepatitis B virus ("HBV"), specifying the use of such assay on the
         Tigris Instrument in the Blood Screening Field; provided that no
         clinical hold notice has been issued by the FDA as to such application,
         and no agreement has been reached with the FDA to delay the clinical
         trial, within such thirty-day period; and (b) Gen-Probe has installed a
         Tigris Instrument pursuant to the IND referred to above, at the
         clinical site and completed technologist training and proficiency
         testing with a panel of specimens and control materials with respect to
         the instrument and the Future Blood Screening Assay for the qualitative
         detection of HCV, HIV-1, and HBV.

         (4)      ten million dollars ($10,000,000) upon receipt of the required
         marketing approval from the FDA to market the Future Blood Screening
         Assay for the qualitative detection of HCV, HIV-1 and HBV in the United
         States for use in Blood Screening Field on the Tigris Instrument.

         2.       NO OTHER AMENDMENT. Except as is expressly set forth in this
Amendment No. 4, all other terms and conditions of the Agreement shall continue
in full force and effect.

         3.       COUNTERPARTS. This Amendment No. 4 may be executed in
counterparts, each of which shall be an original, and all of which together
shall constitute one and the same instrument.




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         IN WITNESS WHEREOF, the parties have caused this Amendment No. 4 to be
executed and the persons signing below warrant that they are duly authorized to
sign for and on behalf of the respective parties.


GEN-PROBE:                                 CHIRON:


GEN-PROBE INCORPORATED,                    CHIRON CORPORATION,
a Delaware corporation                     a Delaware corporation


By: /s/ Henry L. Nordhoff                  By: /s/ Jack Goldstein
    ---------------------                      ------------------
Its:  President & Chief Executive           Its: President, Chiron Blood
      Officer                                    Testing
Date: March 5, 2003                        Date: March 18, 2003





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