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                                                                 EXHIBIT 10.21

                              FIRST AMENDMENT
                              ---------------

     FIRST AMENDMENT to CREDIT AGREEMENT (this "First Amendment") dated as of 
March 20, 1996 between CHIRON CORPORATION (the "Borrower"), and SWISS BANK 
CORPORATION, SAN FRANCISCO BRANCH (the "Bank").  Capitalized terms used 
herein and not otherwise defined herein shall have their respective meanings 
provided in the Credit Agreement.

                              W I T N E S S E T H:

     WHEREAS, the Borrower and the Bank have entered into a Revolving Credit 
Agreement dated as of March 24, 1995 (the "Credit Agreement"); and

     WHEREAS, the parties to the Credit Agreement desire to amend the Credit 
Agreement;

     NOW, THEREFORE, it is agreed:

     1.   The definition of "Expiry Date" in Section 1 of the Credit 
Agreement is hereby amended by deleting the date "March 22, 1996" therein and 
inserting in lieu thereof the date "March 24, 1997".

     2.   This First Amendment is limited as specified and shall not 
constitute a modification, acceptance or waiver of any other provision of the 
Credit Agreement or any other credit document.

     3.   From and after the date hereof, all references in the Credit 
Agreement and each of the other credit documents to the Credit Agreement 
shall be deemed to be references to the Credit Agreement as amended hereby.

     4.   In order to induce the Bank to enter into this First Amendment, the 
Borrower hereby makes each of the representations, warranties and agreements 
contained in Section 12 of the Credit Agreement on the date hereof, after 
giving effect to this First Amendment.

     5.   This First Amendment shall become effective as of the date first 
above written when each of the parties hereto have executed a counterpart of 
this First Amendment.

     6.   This First Amendment may be executed on separate counterparts by 
the parties hereto, each of which when so executed and delivered shall be an 
original, but all of which shall constitute one and the same instrument.

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     7.   THIS FIRST AMENDMENT AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES 
HEREUNDER SHALL BE CONSTRUED IN ACCORDANCE WITH AND GOVERNED BY THE LAWS OF 
THE STATE OF CALIFORNIA.

     THE PARTIES HERETO HEREBY IRREVOCABLY AND UNCONDITIONALLY WAIVE TRIAL BY 
JURY.

     IN WITNESS WHEREOF, the Borrower and the Bank have caused their duly 
authorized officers to execute and deliver this First Amendment as of the 
date first above written.

                                             CHIRON CORPORATION


                                             By /s/ J.E. Kent
                                               ----------------------------
                                               Name:
                                               Title: Treasurer


                                             SWISS BANK CORPORATION,
                                               SAN FRANCISCO BRANCH


                                             By /s/ Nang S. Peechaphand
                                               ----------------------------
                                               Name:  Nang S. Peechaphand
                                               Title: Associate Director
                                                      Accounting


                                             By /s/ Hans-Ueli Surber
                                               ----------------------------
                                               Name:   Hans-Ueli Surber
                                               Title:  Executive Director 
                                                       Merchant Banking

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