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                                                                  EXHIBIT 10.723
                          DEBENTURE PURCHASE AGREEMENT
 
    DEBENTURE PURCHASE AGREEMENT dated as of June 22, 1990 by and between CHIRON
CORPORATION, a corporation organized and existing under the laws of the State of
Delaware  ("Seller"),  and  CIBA-GEIGY,  LIMITED,  a  corporation  organized and
existing under the laws of Switzerland ("Purchaser").
 
                                  WITNESSETH:
 
    WHEREAS, Seller has  filed a Registration  Statement on Form  S-3 (File  No.
33-34918)  with the Securities and  Exchange Commission to register $121,500,000
aggregate principal amount of convertible  subordinated debentures due 2015  (as
amended at the time it becomes effective, including any information deemed to be
a part thereof pursuant to Rule 430A, the "Registration Statement");
 
    WHEREAS,  Seller has  entered into  an Underwriting  Agreement of  even date
herewith (the "Underwriting Agreement") with Morgan Stanley & Co.  Incorporated,
Robertson,  Stephens  &  Company and  Montgomery  Securities  (collectively, the
"Underwriters") providing for  an underwritten public  offering of  $100,000,000
aggregate  principal  amount  of convertible  subordinated  debentures  due 2015
($115,000,000 if the Underwriters' overallotment  option is exercised in  full);
and
 
    WHEREAS, Purchaser wishes to purchase, and Seller wishes to sell, $6,500,000
aggregate  principal amount of debentures  covered by the Registration Statement
(the "Debentures"), subject to the terms and conditions set forth below.
 
    NOW, THEREFORE, in  consideration of the  mutual covenants contained  herein
and  of other  good and valuable  consideration, the receipt  and sufficiency of
which are hereby acknowledged, Purchaser and Seller hereby agree as follows:
 
                             I.  SALE OF DEBENTURES
 
    1.1  DEBENTURES TO  BE SOLD.   Subject to the terms  and conditions of  this
Agreement,  at the Closing (as hereinafter defined), Seller will sell, issue and
deliver the Debentures to Purchaser.
 
    1.2  CONSIDERATION.  Subject to the terms and conditions of this  Agreement,
at the Closing Purchaser will deliver to Seller the aggregate purchase price for
the  Debentures of $6,500,000 (the "Purchase Price") which price represents 100%
of the price at which  the debentures to be sold  concurrently by the Seller  to
the  Underwriters pursuant  to the  Underwriting Agreement  will be  sold to the
public. The  Purchase  Price  will  be  paid by  wire  transfer  to  an  account
designated by Seller.
 
    1.3    CLOSING.    The  Closing of  the  transactions  contemplated  by this
Agreement (the "Closing")  will be contingent  upon and will  take place at  the
same  time and the same place  as the closing referred to  in Section III of the
Underwriting Agreement. At the Closing:
 
    (a) Purchaser shall deliver to Seller the following:
 
        (i) the Purchase Price; and
 
        (ii) the certificate(s) described in Paragraph 6.4.
 
    (b) Seller shall deliver to Purchaser the following:
 
        (i) a debenture certificate representing the Debentures;
 
        (ii) a copy of the prospectus in the form first used to confirm sales of
    the debentures by the Underwriters (the "Prospectus");
 
       (iii) the certificates described in Paragraph 5.4;
 
       (iv) the opinions of counsel described in Paragraph 5.5; and
 
        (v) the accountant's comfort letter described in Paragraph 5.6.
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                II.  REPRESENTATIONS AND WARRANTIES OF PURCHASER
 
    Purchaser hereby represents, covenants and warrants to Seller as follows:
 
    2.1   AUTHORIZATION.   Purchaser has  taken all  action required  by law  to
authorize  the execution  and delivery  of this  Agreement and  the transactions
contemplated hereby.  Upon execution,  this  Agreement is  a valid  and  binding
obligation  of Purchaser enforceable  in accordance with  its terms, except that
(i) such enforcement may be  subject to bankruptcy, insolvency,  reorganization,
moratorium  or  other  similar  laws  now or  hereafter  in  effect  relating to
creditors' rights, and (ii) the remedies of specific performance and  injunctive
relief  may be subject to equitable defenses  and to the discretion of the court
before which any proceeding may be brought.
 
    2.2  NO VIOLATION.  Neither the execution and delivery of this Agreement nor
the consummation  of  the  transactions  contemplated  hereby  will  violate  or
conflict  with, or constitute a default under,  or cause the acceleration of the
maturity of any  debt obligation  pursuant to,  any agreement  or commitment  to
which Purchaser is a party or by which Purchaser is bound.
 
    2.3  INFORMATION.  Purchaser acknowledges that Purchaser (i) received a copy
of Seller's prospectus (subject to completion) issued May 16, 1990, and (ii) has
had the opportunity to obtain any additional information necessary to verify the
information  received.  Purchaser  understands  the  speculative  nature  of the
Debentures and the financial risks with respect thereto.
 
    2.4   LITIGATION.    There  is  no  action,  suit,  inquiry,  proceeding  or
investigation  by or  before any  court or  governmental or  other regulatory or
administrative agency  or commission  pending  or, to  the best  of  Purchaser's
knowledge,  threatened  against  Purchaser,  that  questions  or  challenges the
validity of this Agreement.
 
    2.5   CONSENTS  AND  GOVERNMENTAL  APPROVALS.    No  consent,  approval,  or
authorization  of, or declaration, filing or registration with, any governmental
or regulatory authority is required in connection with Purchaser's execution and
delivery of this  Agreement and  consummation of  the transactions  contemplated
hereby.
 
    2.6   INVESTMENT INTENT.  Purchaser is  acquiring the Debentures for its own
account for investment and not with a present view to, or for sale in connection
with, any distribution of  the Debentures or of  the Common Stock issuable  upon
conversion thereof.
 
                 III.  REPRESENTATIONS AND WARRANTIES OF SELLER
 
    Seller hereby represents, covenants and warrants to Purchaser as follows:
 
    3.1    AUTHORIZATION.   Seller  has  taken  all action  required  by  law to
authorize the execution and delivery of  this Agreement and consummation of  the
transactions contemplated hereby. Upon execution, this Agreement will be a valid
and  binding  obligation of  Seller enforceable  in  accordance with  its terms,
except that  (i) such  enforcement  may be  subject to  bankruptcy,  insolvency,
reorganization,  moratorium or  other similar  laws now  or hereafter  in effect
relating to creditors' rights, and (ii) the remedies of specific performance and
injunctive relief may be subject to equitable defenses and to the discretion  of
the court before which any proceeding may be brought.
 
    3.2  NO VIOLATION.  Neither the execution and delivery of this Agreement nor
the  consummation  of  the  transactions  contemplated  hereby  will  violate or
conflict with, or constitute a default  under, or cause the acceleration of  the
maturity  of any  debt obligation  pursuant to,  any agreement  or commitment to
which Seller is a party or by which Seller is bound.
 
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    3.3   DEBENTURES.   Upon issuance  the Debentures  will be  entitled to  the
benefits  of  the  Indenture  dated  as  of  July  2,  1990  between  Seller and
Manufacturers Hanover Trust  Company of California.  Except for this  Agreement,
there  are no outstanding options, rights, or agreements of any kind relating to
the issuance, sale or transfer of the Debentures.
 
    3.4   LITIGATION.    There  is  no  action,  suit,  inquiry,  proceeding  or
investigation  by or  before any  court or  governmental or  other regulatory or
administrative agency  or  commission  pending  or,  to  the  best  of  Seller's
knowledge,  threatened against Seller that  questions or challenges the validity
of this Agreement.
 
    3.5   CONSENTS  AND  GOVERNMENTAL  APPROVALS.    No  consent,  approval,  or
authorization  of, or declaration, filing or registration with, any governmental
or regulatory authority is  required in connection  with Seller's execution  and
delivery  of this  Agreement and  consummation of  the transactions contemplated
hereby.
 
                                 IV.  COVENANTS
 
    4.1  COVENANTS  OF PURCHASER.   Purchaser hereby covenants  and agrees  with
Seller that it will use its best efforts to insure that the conditions set forth
in  Article VI hereof are satisfied prior to the Closing insofar as such matters
are within Purchaser's control.
 
    4.2  COVENANTS OF SELLER.  Seller hereby covenants and agrees with Purchaser
that (i) it will use its best efforts to insure that the conditions set forth in
Article V  hereof are  satisfied insofar  as such  matters are  within  Seller's
control,  and (ii)  after the Closing,  Seller shall  from time to  time, at the
request of Purchaser, execute and  deliver such other instruments and  documents
and  take  such  other  actions  as Purchaser  may  reasonably  request  to more
effectively consummate the transactions contemplated by this Agreement.
 
                   V.  CONDITIONS TO PURCHASER'S OBLIGATIONS
 
    Each and every obligation of Purchaser under this Agreement on or before the
Closing shall be subject to the satisfaction, on or before the Closing, of  each
of the following conditions, unless waived in writing by Purchaser:
 
    5.1   REPRESENTATIONS AND WARRANTIES.  The representations and warranties of
Seller contained herein shall be true  and accurate in all material respects  at
and  as of  the date  when made  and at  and as  of the  Closing as  though such
representations and warranties were made at and as of such date.
 
    5.2   PERFORMANCE.   Seller  shall  have  performed and  complied  with  all
agreements,  obligations  and  conditions  required  by  this  Agreement  to  be
performed or complied with on or prior to the Closing.
 
    5.3  PROCEEDING  OR LITIGATION.   No suit,  action, investigation,  inquiry,
appeal  or other proceeding by any governmental body or other person or legal or
administrative  proceeding  shall  have  been  instituted  or  threatened   that
questions the validity or legality of the transactions contemplated hereby.
 
    5.4    CERTIFICATES.    Seller  shall  have  furnished  Purchaser  with such
certificates of  officers  of  Seller  dated the  Closing  date  (i)  evidencing
compliance  with the conditions set forth in this Article V as may be reasonably
requested by  Purchaser, and  (ii) stating  that no  stop order  suspending  the
effectiveness of the Registration Statement is in effect, and no proceedings for
such  purpose are  pending before or  threatened by the  Securities and Exchange
Commission, and that there has not occurred any material adverse change, or  any
development   involving   a  prospective   material   adverse  change,   in  the
 
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condition, financial or otherwise, or  in the earnings, business or  operations,
of the Seller and its subsidiaries, taken as a whole, from that set forth in the
Registration Statement and the Prospectus, as amended or supplemented.
 
    The  officer signing and delivering such certificates may rely upon the best
of his knowledge as to proceedings threatened.
 
    5.5  OPINIONS OF COUNSEL.
 
    (a) Seller  shall  have furnished  Purchaser  with an  opinion  of  Brobeck,
Phleger  & Harrison  dated the date  of the Closing  to the effect  set forth in
Section  IV(b)  (i)-(vii)  and  (x)-(xiv)  of  the  Underwriting  Agreement.  In
addition, such opinion will state:
 
        (i) that this Agreement has been duly authorized, executed and delivered
    by Seller and is a valid and binding agreement of Seller; and
 
        (ii)  that the execution and delivery  by Seller of, and the performance
    of its obligations under, this  Agreement, the Debentures and the  Indenture
    will  not contravene any  provision of applicable law  or the certificate of
    incorporation or  by-laws of  Seller,  or, to  the  best knowledge  of  such
    counsel,  any material agreement or other  instrument binding upon Seller or
    any of its subsidiaries or any judgment, order or decree of any governmental
    body, agency or court having jurisdiction over Seller or any of its property
    or any  of  its subsidiaries  or  any of  their  property, and  no  consent,
    approval,  authorization or order of  or qualification with any governmental
    body or agency is required for the performance by Seller of its  obligations
    under  this Agreement, the Debentures and  the Indenture, except such as are
    specified and have been obtained.
 
    (b)   Seller  shall have  furnished  Purchaser  with an  opinion  of  Robert
Blackburn,  Esq., counsel to  the Company, and  an opinion of  Jane L. Stratton,
Esq., counsel to the Company, each dated the date of Closing, to the effect  set
forth  in Section  IV(d) and  Section IV(e),  respectively, of  the Underwriting
Agreement.
 
    5.6  COMFORT LETTER.   Seller shall have  furnished Purchaser with a  letter
dated  the date of  Closing from Ernst &  Young, independent public accountants,
containing the same  statements and  information with respect  to the  financial
statements  and certain  financial information  contained in  or incorporated by
reference into the Prospectus as are contained in the letter delivered by  Ernst
&  Young  to the  Underwriters  pursuant to  Section  IV(f) of  the Underwriting
Agreement.
 
                    VI.  CONDITIONS TO SELLER'S OBLIGATIONS
 
    Each and every obligation  of Seller under this  Agreement on or before  the
Closing  shall be subject to the satisfaction, on or before the Closing, of each
of the following conditions, unless waived in writing by Seller:
 
    6.1  REPRESENTATIONS AND WARRANTIES.  The representations and warranties  of
Purchaser  contained herein shall be true  and accurate in all material respects
at and  as of  the date  when made  at  and as  of the  Closing as  though  such
representations and warranties were made at and as of such date.
 
    6.2   PERFORMANCE.   Purchaser  shall have  performed and  complied with all
agreements,  obligations  and  conditions  required  by  this  Agreement  to  be
performed or complied with on or prior to the Closing.
 
    6.3   PROCEEDING  OR LITIGATION.   No suit,  action, investigation, inquiry,
appeal or other proceeding by any governmental body or other person or legal  or
administrative  proceedings  shall  have  been  instituted  or  threatened  that
questions the validity or legality of the transactions contemplated hereby.
 
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    6.4   CERTIFICATES.    Purchaser  shall  have  furnished  Seller  with  such
certificates  of  officers  of  Purchaser  dated  the  Closing  date  evidencing
compliance with the conditions set forth in this Article VI as may be reasonably
requested by Seller.
 
                                 VII.  HOLDBACK
 
    7.1  RESTRICTIONS ON PUBLIC SALE BY  THE PURCHASER.  In connection with  any
public  offering by Seller of its securities, Purchaser agrees not to effect any
public sale of the Debentures or  shares of Common Stock issued upon  conversion
of  the Debentures during  the ten (10)  business days prior  to, and during the
90-day period beginning on (i) the effective date of the registration  statement
filed  in  connection with  such  public offering,  or  (ii) if  applicable, the
commencement of public distribution of the securities of Seller pursuant to such
registration statement, whichever is  later, if and to  the extent requested  by
Seller  in the case of a nonunderwritten public offering or if and to the extent
requested by  Seller's  underwriter  in  the  case  of  an  underwritten  public
offering.
 
                              VIII.  MISCELLANEOUS
 
    8.1   ENTIRE  AGREEMENT; AMENDMENT.   This  Agreement sets  forth the entire
understanding of  the  parties with  respect  to the  transactions  contemplated
hereby.  This Agreement  may be amended,  modified and supplemented  only by the
written agreement of Purchaser and Seller.
 
    8.2  WAIVER OF  COMPLIANCE.  Any  failure of Purchaser on  the one hand,  or
Seller, on the other hand, to comply with any obligation, covenant, agreement or
condition  herein may be expressly waived in writing by Seller or Purchaser, but
such waiver or failure  to insist upon strict  compliance with such  obligation,
covenant,  agreement or condition shall not operate  as a waiver of, or estoppel
with respect to, any subsequent or other failure.
 
    8.3   NOTICES.   All  notices, requests,  demands and  other  communications
required  or permitted hereunder shall be in writing and shall be deemed to have
been duly given if (i) delivered by  hand, (ii) sent by certified or  registered
mail,  return receipt requested,  with postage prepaid,  or (iii) transmitted by
telefax:
 
            (a) If to Purchaser, to:
              CIBA-GEIGY, Limited
              CH 4002
              Basel, Switzerland
              Attn: Head of Pharma Division
              Telefax: 41-61-696-7487
              with a copy to:
              CIBA-GEIGY, Limited
              Legal Department
              Pharma Counsel CH 4002
              Basel, Switzerland
              Telefax: 41-61-696-5419
 
or to such  other person  or address  as Purchaser  shall furnish  to Seller  in
writing.
 
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            (b) If to Seller, to:
              Chiron Corporation
              4560 Horton Street
              Emeryville, CA 94608
              Attn: Chief Financial Officer
              Telefax: (415) 655-3282
              with a copy to:
              William G. Green, Esq.
              Brobeck, Phleger & Harrison
              One Market Plaza, Spear Street Tower
              San Francisco, CA 94105
              Telefax: (415) 442-1010
 
or  to such  other person  or address  as Seller  shall furnish  to Purchaser in
writing.
 
    8.4  ASSIGNMENT.  This Agreement and  all of the provisions hereof shall  be
binding upon and inure to the benefit of the parties hereto and their respective
successors  and permitted  assigns, but  neither this  Agreement nor  any of the
rights, interests or obligations  hereunder shall be assigned  by either of  the
parties hereto without the prior written consent of the other party.
 
    8.5   GOVERNING LAW.  This Agreement  shall be governed by and construed and
enforced in accordance with the laws of the State of Delaware.
 
    8.6   COUNTERPARTS.    This  Agreement  may  be  executed  in  two  or  more
counterparts,  each  of which  shall be  deemed  an original,  but all  of which
together shall constitute one and the same instrument.
 
    8.7   HEADINGS.    The headings  of  the  articles and  paragraphs  of  this
Agreement  are inserted  for convenience  only and  shall not  constitute a part
hereof or affect in any way the meaning or interpretation of this Agreement.
 
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    IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be duly
executed as of the day and year first above written.
 
                                          Purchaser:
                                          CIBA-GEIGY, LIMITED
 
                                          By /s/ F. R. BOCHUD
 
                                             -----------------------------------
                                             Name: Dr. F. Bochud
                                             Title: DEPUTY DIRECTOR
 
                                          By /s/ H. GUT
 
                                             -----------------------------------
                                             Name: Dr. H. Gut
                                             Title: VICE DIRECTOR
 
                                          Seller:
                                          CHIRON CORPORATION
 
                                          By
 
                                             -----------------------------------
                                             Name:
                                             Title:
 
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    IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be duly
executed as of the day and year first above written.
 
                                          Purchaser:
                                          CIBA-GEIGY, LIMITED
 
                                          By
 
                                             -----------------------------------
                                             Name:
                                             Title:
 
                                          By
 
                                             -----------------------------------
                                             Name:
                                             Title:
 
                                          Seller:
                                          CHIRON CORPORATION
 
                                          By /s/ DENNIS L. WINGER
 
                                             -----------------------------------
                                             Name: Dennis L. Winger
                                             Title: VICE PRESIDENT, FINANCE AND
                                             ADMINISTRATION
 
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    IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be duly
executed as of the day and year first above written.
 
                                          Purchaser:
                                          CIBA-GEIGY, LIMITED
 
                                          By /s/ F. R. BOCHUD
 
                                             -----------------------------------
                                             Name: Dr. F. Bochud
                                             Title: DEPUTY DIRECTOR
 
                                          By /s/ H. GUT
 
                                             -----------------------------------
                                             Name: Dr. H. Gut
                                             Title: VICE DIRECTOR
 
                                          Seller:
                                          CHIRON CORPORATION
 
                                          By
 
                                             -----------------------------------
                                             Name:
                                             Title:
 
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