=============================================================================
                                 UNITED STATES
                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549

                                 SCHEDULE 13D

                               (Amendment 3)1

      INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT TO RULE 13d-1(a)
                AND AMENDMENTS THERETO FILED PURSUANT TO RULE 13d-2(a)


                                Chiron Corporation
                ------------------------------------------------
                                (Name of Issuer)

                          Common Stock, $0.01 par value
                ------------------------------------------------
                         (Title of Class of Securities)

                                  170040109
                ------------------------------------------------
                                (CUSIP Number)

                             Allison Bennington, Esq.
                              ValueAct Capital
                        435 Pacific Avenue, Fourth Floor
                           San Francisco, CA 94133
                              (415) 362-3700
                ------------------------------------------------
               (Name, Address and Telephone Number of Person
              Authorized to Receive Notices and Communications)

                              With a Copy to:
                         Christopher G. Karras, Esq.
                               Dechert LLP
                          4000 Bell Atlantic Tower
                            1717 Arch Street
                    Philadelphia, Pennsylvania 19103-2793
                           Ph. (215) 994-4000

                              April 3, 2006
                ------------------------------------------------
           (Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to
report the acquisition that is the subject of this Schedule 13D, and is
filing this schedule because of Rule 240.13d-1(b)(e), 240.13d-1(f) or
240.13d-1(g) check the following box [ ].

NOTE: Schedules filed in paper format shall include a signed original and
five copies of the schedule, including all exhibits.  See Rule 240.13d-7(b)
for other parties to whom copies are to be sent

1The remainder of this cover page shall be filled out for a reporting
person's initial filing on this form with respect to the subject class of
securities, and for any subsequent amendment containing information which
would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be
deemed to be "filed" for the purpose of Section 18 of the Securities Exchange
Act of 1934 ("Act") or otherwise subject to the liabilities of that section
of the Act but shall be subject to all other provisions of the Act (however,
see the Notes).
=============================================================================


                                SCHEDULE 13D
--------------------------                          -------------------------
CUSIP NO. 170040109                                              Page 2 of 12
-----------------------------------------------------------------------------
1. NAME OF REPORTING PERSON/S.S. OR I.R.S. INDENTIFICATION NO. OF ABOVE
   PERSON (entities only)

   ValueAct Capital Master Fund, L.P.
-----------------------------------------------------------------------------
2. CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*                  (a) [X]
                                                                      (b) [ ]
-----------------------------------------------------------------------------
3. SEC USE ONLY

-----------------------------------------------------------------------------
4. SOURCE OF FUNDS*

   WC*
-----------------------------------------------------------------------------
5. CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED
   PURSUANT TO ITEMS 2(d) or 2(e)                                         [ ]
-----------------------------------------------------------------------------
6. CITIZENSHIP OR PLACE OF ORGANIZATION

   British Virgin Islands
-----------------------------------------------------------------------------
                    7.  SOLE VOTING POWER
                        0
   NUMBER OF       ----------------------------------------------------------
   SHARES           8.  SHARED VOTING POWER
   BENEFICIALLY         9,761,527**
   OWNED BY EACH   ----------------------------------------------------------
   PERSON WITH      9.  SOLE DISPOSITIVE POWER
                        0
                   ----------------------------------------------------------
                   10.  SHARED DISPOSITIVE POWER
                        9,761,527**
-----------------------------------------------------------------------------
11. AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

    9,761,527**
-----------------------------------------------------------------------------
12. CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
    CERTAIN SHARES                                                        [ ]
-----------------------------------------------------------------------------
13. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

   5.2%
-----------------------------------------------------------------------------
14. TYPE OF REPORTING PERSON

    PN
-----------------------------------------------------------------------------
*See Item 3
**See Item 2 and 5
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!
             INCLUDE BOTH SIDES OF THE COVER PAGE, RESPONSES TO ITEMS 1-7
               (INCLUDING EXHIBITS) OF THE SCHEDULE, AND THE SIGNATURE
                                ATTESTATION.

                                SCHEDULE 13D
--------------------------                          -------------------------
CUSIP NO. 170040109                                              Page 3 of 12
-----------------------------------------------------------------------------
1. NAME OF REPORTING PERSON/S.S. OR I.R.S. INDENTIFICATION NO. OF ABOVE
   PERSON (entities only)

   ValueAct Capital Partners Co-Investors, L.P.
-----------------------------------------------------------------------------
2. CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*                  (a) [X]
                                                                      (b) [ ]
-----------------------------------------------------------------------------
3. SEC USE ONLY

-----------------------------------------------------------------------------
4. SOURCE OF FUNDS*

   WC*
-----------------------------------------------------------------------------
5. CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED
   PURSUANT TO ITEMS 2(d) or 2(e)                                         [ ]
-----------------------------------------------------------------------------
6. CITIZENSHIP OR PLACE OF ORGANIZATION

   Delaware
-----------------------------------------------------------------------------
                    7.  SOLE VOTING POWER
                        0
   NUMBER OF       ----------------------------------------------------------
   SHARES           8.  SHARED VOTING POWER
   BENEFICIALLY         38,473**
   OWNED BY EACH   ----------------------------------------------------------
   PERSON WITH      9.  SOLE DISPOSITIVE POWER
                        0
                   ----------------------------------------------------------
                   10.  SHARED DISPOSITIVE POWER
                        38,473**
-----------------------------------------------------------------------------
11. AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

    38,473**
-----------------------------------------------------------------------------
12. CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
    CERTAIN SHARES                                                        [ ]
-----------------------------------------------------------------------------
13. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

    Less than 1%
-----------------------------------------------------------------------------
14. TYPE OF REPORTING PERSON

    PN
-----------------------------------------------------------------------------
*See Item 3
**See Item 2 and 5
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!
             INCLUDE BOTH SIDES OF THE COVER PAGE, RESPONSES TO ITEMS 1-7
               (INCLUDING EXHIBITS) OF THE SCHEDULE, AND THE SIGNATURE
                                ATTESTATION.

                                SCHEDULE 13D
--------------------------                          -------------------------
CUSIP NO. 170040109                                              Page 4 of 12
----------------------------------------------------------------------------
1. NAME OF REPORTING PERSON/S.S. OR I.R.S. INDENTIFICATION NO. OF ABOVE
   PERSON (entities only)

   VA Partners, L.L.C.
-----------------------------------------------------------------------------
2. CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*                  (a) [X]
                                                                      (b) [ ]
-----------------------------------------------------------------------------
3. SEC USE ONLY

-----------------------------------------------------------------------------
4. SOURCE OF FUNDS*

   WC*
-----------------------------------------------------------------------------
5. CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED
   PURSUANT TO ITEMS 2(d) or 2(e)                                         [ ]
-----------------------------------------------------------------------------
6. CITIZENSHIP OR PLACE OF ORGANIZATION

   Delaware
-----------------------------------------------------------------------------
                    7.  SOLE VOTING POWER
                        0
   NUMBER OF       ----------------------------------------------------------
   SHARES           8.  SHARED VOTING POWER
   BENEFICIALLY         9,800,000**
   OWNED BY EACH   ----------------------------------------------------------
   PERSON WITH      9.  SOLE DISPOSITIVE POWER
                        0
                   ----------------------------------------------------------
                   10.  SHARED DISPOSITIVE POWER
                        9,800,000**
-----------------------------------------------------------------------------
11. AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

    9,800,000**
-----------------------------------------------------------------------------
12. CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
    CERTAIN SHARES                                                        [ ]
-----------------------------------------------------------------------------
13. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

    5.2%
-----------------------------------------------------------------------------
14. TYPE OF REPORTING PERSON

    00 (LLC)
-----------------------------------------------------------------------------
*See Item 3
**See Item 2 and 5
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!
             INCLUDE BOTH SIDES OF THE COVER PAGE, RESPONSES TO ITEMS 1-7
               (INCLUDING EXHIBITS) OF THE SCHEDULE, AND THE SIGNATURE
                                ATTESTATION.


                                SCHEDULE 13D
--------------------------                          -------------------------
CUSIP NO. 170040109                                             Page 5 of 12
-----------------------------------------------------------------------------
1. NAME OF REPORTING PERSON/S.S. OR I.R.S. INDENTIFICATION NO. OF ABOVE
   PERSON (entities only)

   Jeffrey W. Ubben
---------------------------------------------------------------------------
2. CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*                  (a) [X]
                                                                      (b) [ ]
-----------------------------------------------------------------------------
3. SEC USE ONLY

---------------------------------------------------------------------------
4. SOURCE OF FUNDS*

   00*
-----------------------------------------------------------------------------
5. CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED
   PURSUANT TO ITEMS 2(d) or 2(e)                                         [ ]
-----------------------------------------------------------------------------
6. CITIZENSHIP OR PLACE OF ORGANIZATION

   United States
-----------------------------------------------------------------------------
                    7.  SOLE VOTING POWER
                        0
   NUMBER OF       ----------------------------------------------------------
   SHARES           8.  SHARED VOTING POWER
   BENEFICIALLY         9,800,000**
   OWNED BY EACH   ----------------------------------------------------------
   PERSON WITH      9.  SOLE DISPOSITIVE POWER
                        0
                   ----------------------------------------------------------
                   10.  SHARED DISPOSITIVE POWER
                        9,800,000**
-----------------------------------------------------------------------------
11. AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

    9,800,000**
-----------------------------------------------------------------------------
12. CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
    CERTAIN SHARES                                                        [ ]
-----------------------------------------------------------------------------
13. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

    5.2%
-----------------------------------------------------------------------------
14. TYPE OF REPORTING PERSON

    IN
-----------------------------------------------------------------------------
*See Item 3
**See Item 2 and 5
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!
             INCLUDE BOTH SIDES OF THE COVER PAGE, RESPONSES TO ITEMS 1-7
               (INCLUDING EXHIBITS) OF THE SCHEDULE, AND THE SIGNATURE
                                ATTESTATION.


                                SCHEDULE 13D
--------------------------                          -------------------------
CUSIP NO. 170040109                                              Page 6 of 12
-----------------------------------------------------------------------------
1. NAME OF REPORTING PERSON/S.S. OR I.R.S. INDENTIFICATION NO. OF ABOVE
   PERSON (entities only)

   George F. Hamel, Jr.
---------------------------------------------------------------------------
2. CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*                  (a) [X]
                                                                      (b) [ ]
-----------------------------------------------------------------------------
3. SEC USE ONLY

-----------------------------------------------------------------------------
4. SOURCE OF FUNDS*

   00*
-----------------------------------------------------------------------------
5. CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED
   PURSUANT TO ITEMS 2(d) or 2(e)                                        [ ]
-----------------------------------------------------------------------------
6. CITIZENSHIP OR PLACE OF ORGANIZATION

   United States
-----------------------------------------------------------------------------
                   7.  SOLE VOTING POWER
                        0
   NUMBER OF       ----------------------------------------------------------
   SHARES          8.  SHARED VOTING POWER
   BENEFICIALLY         9,800,000**
   OWNED BY EACH   ----------------------------------------------------------
   PERSON WITH     9.  SOLE DISPOSITIVE POWER
                        0
                   ----------------------------------------------------------
                   10.  SHARED DISPOSITIVE POWER
                        9,800,000**
-----------------------------------------------------------------------------
11. AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

    9,800,000**
-----------------------------------------------------------------------------
12. CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
    CERTAIN SHARES                                                        [ ]
-----------------------------------------------------------------------------
13. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

    5.2%
-----------------------------------------------------------------------------
14. TYPE OF REPORTING PERSON

    IN
-----------------------------------------------------------------------------
*See Item 3
**See Item 2 and 5
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!
             INCLUDE BOTH SIDES OF THE COVER PAGE, RESPONSES TO ITEMS 1-7
               (INCLUDING EXHIBITS) OF THE SCHEDULE, AND THE SIGNATURE
                                ATTESTATION.


                                SCHEDULE 13D
--------------------------                          -------------------------
CUSIP NO. 170040109                                             Page 7 of 12
-----------------------------------------------------------------------------
1. NAME OF REPORTING PERSON/S.S. OR I.R.S. INDENTIFICATION NO. OF ABOVE
   PERSON (entities only)

   Peter H. Kamin
-----------------------------------------------------------------------------
2. CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*                  (a) [X]
                                                                      (b) [ ]
---------------------------------------------------------------------------
3. SEC USE ONLY

-----------------------------------------------------------------------------
4. SOURCE OF FUNDS*

   00*
-----------------------------------------------------------------------------
5. CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED
   PURSUANT TO ITEMS 2(d) or 2(e)                                        [ ]
-----------------------------------------------------------------------------
6. CITIZENSHIP OR PLACE OF ORGANIZATION

   United States
-----------------------------------------------------------------------------
                    7.  SOLE VOTING POWER
                        0
   NUMBER OF       ----------------------------------------------------------
   SHARES           8.  SHARED VOTING POWER
   BENEFICIALLY         9,800,000**
   OWNED BY EACH   ----------------------------------------------------------
   PERSON WITH      9.  SOLE DISPOSITIVE POWER
                        0
                   ----------------------------------------------------------
                   10.  SHARED DISPOSITIVE POWER
                        9,800,000**
-----------------------------------------------------------------------------
11. AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

    9,800,000**
-----------------------------------------------------------------------------
12. CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
    CERTAIN SHARES                                                        [ ]
-----------------------------------------------------------------------------
13. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

    5.2%
-----------------------------------------------------------------------------
14. TYPE OF REPORTING PERSON

    IN
-----------------------------------------------------------------------------
*See Item 3
**See Item 2 and 5
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!
             INCLUDE BOTH SIDES OF THE COVER PAGE, RESPONSES TO ITEMS 1-7
               (INCLUDING EXHIBITS) OF THE SCHEDULE, AND THE SIGNATURE
                                ATTESTATION.



      THE PURPOSE OF THIS AMENDMENT NO. 3 TO SCHEDULE 13D IS TO AMEND THE
PURPOSE OF TRANSACTION SECTION OF REPORTS FILED BY THE REPORTING PERSONS.
THE INFORMATION BELOW SUPPLEMENTS THE INFORMATION PREVIOUSLY REPORTED.


ITEM 4.  PURPOSE OF TRANSACTION.


On April 3, 2006 Chiron Corporation announced that it had entered into an
amendment to its previously announced merger agreement with Novartis AG
providing for Novartis to increase to $48.00 per Chiron share the price it
will pay for the approximately 56 percent of Chiron shares that Novartis
does not already own. The Reporting Persons informed Chiron and Novartis
that they, and their affiliated entities, intend to vote in favor of the
transaction at this revised price, subject to the fiduciary and legal
obligations applicable to them.




                               Page 8 of 12





ITEM 6.  CONTRACTS, ARRANGEMENTS, UNDERSTANDINGS OR RELATIONSHIPS WITH
RESPECT TO SECURITIES OF THE ISSUER

Other than as described elsewhere in this Report, the Reporting Persons have
no understandings, arrangements, relationships or contracts relating to the
Common Stock which are required to be described hereunder.

ITEM 7.  MATERIAL TO BE FILED AS EXHIBITS

A)  Joint Filing Agreement







                               Page 9 of 12




                                  SIGNATURES

     After reasonable inquiry and to the best of my knowledge and belief, I
certify that the information set forth in this statement is true, complete
and correct.

                             POWER OF ATTORNEY

KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears
below on this Schedule 13D hereby constitutes and appoints Jeffrey W. Ubben,
George F. Hamel, Jr. and Peter H. Kamin, and each of them, with full power to
act without the other, his true and lawful attorney-in-fact and agent, with
full power of substitution and resubstitution, for him and in his name, place
and stead, in any and all capacities (until revoked in writing) to sign any
and all amendments to this Schedule 13D, and to file the same, with all
exhibits thereto and other documents in connection therewith, with the
Securities and Exchange Commission, granting unto said attorneys-in-fact and
agents, and each of them, full power and authority to do and perform each and
every act and thing requisite and necessary fully to all intents and purposes
as he might or could do in person, thereby ratifying and confirming all that
said attorneys-in-fact and agents or any of them, or their or his substitute
or substitutes, may lawfully do or cause to be done by virtue hereof.

                              ValueAct Capital Master Fund L.P., by
                              VA Partners, L.L.C., its General Partner

                              By:     /s/  George F. Hamel, Jr.
                                --------------------------------------
Dated:  April 3, 2006         George F. Hamel, Jr., Managing Member


                          ValueAct Capital Partners Co-Investors L.P., by
                          VA Partners, L.L.C., its General Partner

                              By:     /s/  George F. Hamel, Jr.
                                --------------------------------------
Dated:  April 3, 2006         George F. Hamel, Jr., Managing Member


                              VA Partners, L.L.C.

                              By:    /s/  George F. Hamel, Jr.
                                --------------------------------------
Dated:  April 3, 2006         George F. Hamel, Jr., Managing Member


                                  Page 10 of 12



                              By:    /s/  Jeffrey W. Ubben
                                --------------------------------------
Dated:  April 3, 2006         Jeffrey W. Ubben, Managing Member


                              By:    /s/  George F. Hamel, Jr.
                                --------------------------------------
Dated:  April 3, 2006         George F. Hamel, Jr., Managing Member


                              By:    /s/  Peter H. Kamin
                                --------------------------------------
Dated:  April 3, 2006         Peter H. Kamin, Managing Member



                                  Page 11 of 12



                                   Exhibit A
                           JOINT FILING UNDERTAKING

The undersigned parties hereby agree that the Schedule 13D filed herewith
(and any amendments thereto) relating to the Common Stock of Chiron
Corporation is being filed jointly on behalf of each of them with the
Securities and Exchange Commission pursuant to Section 13(d) of the
Securities Exchange Act of 1934, as amended.


                              ValueAct Capital Master Fund L.P., by
                              VA Partners, L.L.C., its General Partner


                              By:     /s/  George F. Hamel, Jr.
                                --------------------------------------
Dated:  April 3, 2006         George F. Hamel, Jr., Managing Member


                          ValueAct Capital Partners Co-Investors L.P., by
                          VA Partners, L.L.C., its General Partner

                              By:     /s/  George F. Hamel, Jr.
                                --------------------------------------
Dated:  April 3, 2006         George F. Hamel, Jr., Managing Member


                              VA Partners, L.L.C.

                              By:    /s/  George F. Hamel, Jr.
                                --------------------------------------
Dated:  April 3, 2006         George F. Hamel, Jr., Managing Member


                              By:    /s/  Jeffrey W. Ubben
                                --------------------------------------
Dated:  April 3, 2006         Jeffrey W. Ubben, Managing Member


                              By:    /s/  George F. Hamel, Jr.
                                --------------------------------------
Dated:  April 3, 2006         George F. Hamel, Jr., Managing Member


                              By:    /s/  Peter H. Kamin
                                --------------------------------------
Dated:  April 3, 2006         Peter H. Kamin, Managing Member



                               Page 12 of 12