QuickLinks -- Click here to rapidly navigate through this document



		OMB APPROVAL

		OMB Number:		3235-0145
		Expires:		December 31, 1997
		Estimated average burden
		hours per response		14.90

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

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. 1)*

MATRIX PHARMACEUTICAL

(Name of Issuer)
COMMON

(Title of Class of Securities)
		576844104
		(CUSIP Number)

Check the following box if a fee is being paid with this statement / /. (A fee is not required only if the filing person: (1) has a previous statement on file reporting beneficial ownership of more than five percent of the class of securities described in Item 1; and (2) has filed no amendment subsequent thereto reporting beneficial ownership of five percent or less of such class.) (See Rule 13d-7).

*The 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 the disclosures provided in a prior cover page.

The information required in 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).

CUSIP No. 576844104



(1)	Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only).
	PRIMECAP Management Company 95-3868081

(2)	Check the Appropriate Box if a Member	(a)	/ /
	of a Group (See Instructions)	(b)	/ /


(3)	Sec Use Only


(4)	Citizenship or Place of Organization
	225 South Lake Avenue #400, Pasadena, CA 91101

Number of Shares Beneficially Owned by Each Reporting Person With:		(5)	Sole Voting Power
			3,000

		(6)	Shared Voting Power
			-0-

		(7)	Sole Dispositive Power
			525,000

		(8)	Shared Dispositive Power
			-0-

(9)	Aggregate Amount Beneficially Owned by Each Reporting Person
	525,000

(10)	Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) / /


(11)	Percent of Class Represented by Amount in Row (9)
	2.01%

(12)	Type of Reporting Person (See Instructions)
	IA

*SEE INSTRUCTION BEFORE FILING OUT!
Page 2 of 8 pages

(c)	Number of shares as to which the person has:

	(i)	Sole power to vote or to direct the vote
	(ii)	Shared power to vote or to direct the vote
	(iii)	Sole power to dispose or to direct the disposition of
	(iv)	Shared power to dispose or to direct the disposition of

INSTRUCTION: For computations regarding securities which represent a right to acquire an underlying security see Rule 13d-3(d)(1).

ITEM 5. OWNERSHIP OF FIVE PERCENT OR LESS OF A CLASS

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following. /x/

INSTRUCTION: Dissolution of a group requires a response to this item.

ITEM 6. OWNERSHIP OF MORE THAN FIVE PERCENT ON BEHALF OF ANOTHER PERSON

If any other person is known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, such securities, a statement to that effect should be included in response to this item and, if such interest relates to more than five percent of the class, such person should be identified. A listing of the shareholders of an investment company registered under the Investment Company Act of 1940 or the beneficiaries of employee benefit plan, pension fund or endowment fund is not required.

ITEM 7.

IDENTIFICATION AND CLASSIFICATION OF THE SUBSIDIARY WHICH ACQUIRED THE SECURITY BEING REPORTED ON BY THE PARENT HOLDING COMPANY

If a parent holding company has filed this schedule, pursuant to Rule 13d-1(b)(ii)(G), so indicate under Item 3(g) and attach an exhibit stating the identity and the Item 3 classification of the relevant subsidiary. If a parent holding company has filed this schedule pursuant to Rule 13d-1(c), attach an exhibit stating the identification of the relevant subsidiary.

ITEM 8. IDENTIFICATION AND CLASSIFICATION OF MEMBERS OF THE GROUP

If a group has filed this schedule pursuant to Rule 13d-1(b)(ii)(H), so indicate under Item 3(h) and attach an exhibit stating the identity and Item 3 classification of each member of the group. If a group has filed this schedule pursuant to Rule 13d-1(c), attach an exhibit stating the identity of each member of the group.

ITEM 9. NOTICE OF DISSOLUTION OF GROUP

Notice of dissolution of a group may be furnished as an exhibit stating the date of the dissolution and that all further filings with respect to transactions in the security reported on will be filed, if required, by members of the group, in their individual capacity. See Item 5.

ITEM 10. CERTIFICATION

The following certification shall be included if the statement is filed pursuant to Rule 13d-1(b):

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired in the ordinary course of business and were not acquired for the purpose of and do not have the effect of changing or influencing the control of the issuer of such securities and were not acquired in connection with or as a participant in any transaction having that purposes or effect.

SIGNATURE

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.


		September 30, 2001
		Date

		/s/ THEO A. KOLOKOTRONES, PRESIDENT
		Signature

		Theo A. Kolokotrones, President
		Name/Title

QuickLinks

SIGNATURE

MATRIX PHARMACEUTICAL INC/DE SC 13G/A