SEC Form 4

FORM 4		UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940	OMB APPROVAL OMB Number: 3235-0287 Estimated average burden hours per response: 0.5
	Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

1. Name and Address of Reporting Person* RAPPUOLI RINO (Last) (First) (Middle) 4560 HORTON STREET (Street) EMERYVILLE CA 94608 (City) (State) (Zip)	2. Issuer Name and Ticker or Trading Symbol CHIRON CORP [ CHIR ]	5. Relationship of Reporting Person(s) to Issuer (Check all applicable) Director 10% Owner X Officer (give title below) Other (specify below) V.P., Chief Scientific Officer
	3. Date of Earliest Transaction (Month/Day/Year) 04/18/2006
	4. If Amendment, Date of Original Filed (Month/Day/Year)	6. Individual or Joint/Group Filing (Check Applicable Line) X Form filed by One Reporting Person Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	3. Transaction Code (Instr. 8)		4. Securities Acquired (A) or Disposed Of (D) (Instr. 3, 4 and 5)			5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4)	6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
			Code	V	Amount	(A) or (D)	Price
Common Stock	04/18/2006		M		2,812	A	$19.4375	16,960	D
Common Stock	04/18/2006		S		2,812	D	$47.95	14,148	D
Common Stock	04/18/2006		M		20,000	A	$43.8125	34,148	D
Common Stock	04/18/2006		S		20,000	D	$47.95	14,148	D
Common Stock	04/18/2006		M		20,000	A	$45.7188	34,148	D
Common Stock	04/18/2006		S		20,000	D	$47.95	14,148	D
Common Stock	04/18/2006		M		10,000	A	$44.84	24,148	D
Common Stock	04/18/2006		S		10,000	D	$47.95	14,148	D
Common Stock	04/18/2006		M		15,000	A	$42.535	29,148	D
Common Stock	04/18/2006		S		15,000	D	$47.95	14,148	D
Common Stock	04/18/2006		M		11,563	A	$36.335	25,711	D
Common Stock	04/18/2006		S		11,563	D	$47.95	14,148	D
Common Stock	04/18/2006		M		15,573	A	$33.88	29,721	D
Common Stock	04/18/2006		S		15,573	D	$47.95	14,148	D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 3)	2. Conversion or Exercise Price of Derivative Security	3. Transaction Date (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	4. Transaction Code (Instr. 8)		5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5)		6. Date Exercisable and Expiration Date (Month/Day/Year)		7. Title and Amount of Securities Underlying Derivative Security (Instr. 3 and 4)		8. Price of Derivative Security (Instr. 5)	9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)	10. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares
Employee Stock Option (Right to Buy)	$19.4375	04/18/2006		M			2,812	02/19/1999(1)	02/18/2008	Common Stock	2,812	(2)	0	D
Employee Stock Option (Right to Buy)	$43.8125	04/18/2006		M			20,000	01/02/2002(3)	01/01/2011	Common Stock	20,000	(2)	5,000	D
Employee Stock Option (Right to Buy)	$45.7188	04/18/2006		M			20,000	02/16/2002(4)	02/15/2011	Common Stock	20,000	(2)	0	D
Employee Stock Option (Right to Buy)	$44.84	04/18/2006		M			10,000	01/11/2003(5)	01/10/2012	Common Stock	10,000	(2)	15,000	D
Employee Stock Option (Right to Buy)	$42.535	04/18/2006		M			15,000	02/21/2003(6)	02/20/2012	Common Stock	15,000	(2)	0	D
Employee Stock Option (Right to Buy)	$36.335	04/18/2006		M			11,563	02/20/2004(7)	02/19/2013	Common Stock	11,563	(2)	3,437	D
Employee Stock Option (Right to Buy)	$33.88	04/18/2006		M			15,573	02/22/2006(8)	02/21/2015	Common Stock	15,573	(9)	41,927	D

Explanation of Responses:

1. The option was originally granted for 15,000 shares on February 19, 1998. The option vested and became exercisable for one-fourth of the total shares on the first anniversary of the grant date and vested and became exercisable for the balance of the option shares in a series of thirty-six successive equal monthly installments upon the Reporting Person's completion of each additional month of service with the Issuer thereafter.

2. Employee Stock Option granted under the Issuer's 1991 Stock Option Plan in a transaction exempt under Rule 16b-3(d).

3. The option was originally granted for 25,000 shares on January 2, 2001. The option vests after seven years of service with the Issuer or upon earlier achievement of specified performance objectives. Certain performance objectives were achieved during the initial three-year period measured from the grant date, and the option accordingly vested in part and became exercisable as follows: 5,000 shares on January 2, 2002; 5,000 shares on January 2, 2003; and 10,000 shares on January 2, 2004. The option will vest and become exercisable for the balance of the option shares upon the Reporting Person's completion of seven years of service with the Issuer or upon earlier achievement of specified performance objectives.

4. The option was originally granted for 20,000 shares on February 16, 2001. The option vested and became exercisable for one-fourth of the total shares on the first anniversary of the grant date and vested and became exercisable for the balance of the option shares in a series of thirty-six successive equal monthly installments upon the Reporting Person's completion of each additional month of service with the Issuer thereafter.

5. The option was originally granted for 25,000 shares on January 2, 2001. The option vests after seven years of service with the Issuer or upon earlier achievement of specified performance objectives. Certain performance objectives were achieved during the initial two-year period measured from the grant date, and the option accordingly vested in part and became exercisable as follows: 2,500 shares on January 11, 2003 and 7,500 shares on January 11, 2004. The option will vest and become exercisable for the balance of the option shares upon the Reporting Person's completion of seven years of service with the Issuer or upon earlier achievement of specified performance objectives.

6. The option was originally granted for 15,000 shares on February 21, 2002. The option vested and became exercisable for one-fourth of the total shares on the first anniversary of the grant date and vested and became exercisable for the balance of the option shares in a series of thirty-six successive equal monthly installments upon the Reporting Person's completion of each additional month of service with the Issuer thereafter.

7. The option was originally granted for 15,000 shares on February 20, 2003. The option vested and became exercisable for one-fourth of the total shares on the first anniversary of the grant date and vests and becomes exercisable for the balance of the option shares in a series of thirty-six successive equal monthly installments upon the Reporting Person's completion of each additional month of service with the Issuer thereafter.

8. The option was originally granted for 57,500 shares on February 22, 2005. The option vested and became exercisable for one-fourth of the total shares on the first anniversary of the grant date and vests and becomes exercisable for the balance of the option shares in a series of thirty-six successive equal monthly installments upon the Reporting Person's completion of each additional month of service with the Issuer thereafter.

9. Employee Stock Option granted under the Issuer's 2004 Stock Compensation Plan in a transaction exempt under Rule 16b-3(d).

Remarks:

	Rino Rappuoli by Naima Robinson as Atty-in-Fact	04/19/2006
	** Signature of Reporting Person	Date
Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 4 (b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.