
<PAGE>   1
             As filed with the Securities and Exchange Commission on
                      August 4, 1999 Registration No. 333-
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549



                                    FORM S-8
                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

                                ----------------

                                CERUS CORPORATION
             (Exact name of registrant as specified in its charter)

                                ----------------

        DELAWARE                                          68-0262011
(State of Incorporation)                    (I.R.S. Employer Identification No.)

                                ----------------

                         2525 STANWELL DRIVE, SUITE 300
                                CONCORD, CA 94520
                                 (925) 603-9071
          (Address and telephone number of principal executive offices)

                                ----------------

                           1999 EQUITY INCENTIVE PLAN
                            (Full title of the plans)

                                STEPHEN T. ISAACS
                      PRESIDENT AND CHIEF EXECUTIVE OFFICER
                                CERUS CORPORATION
                         2525 STANWELL DRIVE, SUITE 300
                                CONCORD, CA 94520
                                 (925) 603-9071
            (Name, address, including zip code, and telephone number,
                   including area code, of agent for service)

                                ----------------

                                   Copies to:
                            ALAN C. MENDELSON, ESQ.
                              ANDREA VACHSS, ESQ.
                               Cooley Godward LLP
                         One Maritime Plaza, 20th Floor
                             San Francisco, CA 94111
                                 (415) 693-2000

                                ----------------

                         CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
==============================================================================================================
                                             PROPOSED MAXIMUM     PROPOSED MAXIMUM
   TITLE OF SECURITIES      AMOUNT TO BE    OFFERING PRICE PER   AGGREGATE OFFERING        AMOUNT OF
    TO BE REGISTERED         REGISTERED          SHARE (1)            PRICE (1)        REGISTRATION FEE
--------------------------------------------------------------------------------------------------------------
<S>                          <C>              <C>                   <C>                   <C>
Stock Options and
Common Stock (par            580,000           $21.00 - $23.625     $13,505,625.00          $3,754.57
value $.001)
==============================================================================================================
</TABLE>


                                                                    Page 1 of 9
                                                        Exhibit Index at Page: 7
<PAGE>   2
<TABLE>
<CAPTION>
==============================================================================================================
    TYPE OF SHARES                 NUMBER OF SHARES    OFFERING PRICE PER SHARE    AGGREGATE OFFERING PRICE
--------------------------------------------------------------------------------------------------------------
<S>                                    <C>                   <C>                       <C>
Shares issuable pursuant to            75,000                $21.00(1)(a)               $1,575,000.00
outstanding options under
the 1999 Equity Incentive Plan
--------------------------------------------------------------------------------------------------------------
Shares reserved for future            505,000                $23.625(1)(b)              $11,930,625.00
grant pursuant to the
1999 Equity Incentive Plan
--------------------------------------------------------------------------------------------------------------
Proposed  Maximum                                                                       $13,505,625.00
Aggregate Offering Price
--------------------------------------------------------------------------------------------------------------
Registration Fee                                                                        $     3,754.57
==============================================================================================================
</TABLE>

(1)      Estimated solely for the purpose of calculating the amount of the
         registration fee pursuant to Rule 457(h). The offering price per share
         and aggregate offering price are based upon (a) the weighted average
         exercise price, for shares subject to outstanding options granted by
         Cerus Corporation ("Registrant" or "Company") under the Company's 1999
         Equity Incentive Plan (the "Plan"), or (b) the average of the high and
         low prices of the Company's Common Stock as reported on the Nasdaq
         National Market for July 29, 1999, for shares reserved for future grant
         pursuant to the Plan (pursuant to Rule 457(c) under the Act).

================================================================================

         Approximate date of commencement of proposed sale to the public: As
soon as practicable after this Registration Statement becomes effective.



                                       ii.
<PAGE>   3
ITEM 3.  INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

The following documents filed by the Company with the Securities and Exchange
Commission are incorporated by reference into this Registration Statement:

     1. the Company's Annual Report on Form 10-K for the fiscal year ended
        December 31, 1998;

     2. the Company's Quarterly Report on Form 10-Q for the quarter ended
        March 31, 1999; and

     3. the description of the Company's common stock set forth in it's
        Registration Statement on Form 8-A filed with the SEC on
        January 8, 1997.

         All reports and other documents subsequently filed by the Company
pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act prior to the
filing of a post-effective amendment which indicates that all securities offered
have been sold or which deregisters all securities then remaining unsold, shall
be deemed to be incorporated by reference herein and to be a part of this
Registration Statement from the date of the filing of such reports and
documents.

ITEM 4.  DESCRIPTION OF SECURITIES

Not Applicable.

ITEM 5.  INTERESTS OF NAMED EXPERTS AND COUNSEL

Not Applicable.

ITEM 6.  INDEMNIFICATION OF DIRECTORS AND OFFICERS

Under Section 145 of the Delaware General Corporation Law, the Registrant has
broad powers to indemnify its directors and officers against liabilities they
may incur in such capacities, including liabilities under the Securities Act of
1933, as amended ("Securities Act"). The Registrant's Bylaws also provide that
the Registrant will indemnify its directors and executive officers and may
indemnify its other officers, employees and other agents to the fullest extent
permitted by Delaware law.

The Registrant's Restated Certificate of Incorporation ("Restated Certificate")
provides that the liability of its directors for monetary damages shall be
eliminated to the fullest extent permissible under Delaware law. Pursuant to
Delaware law, this includes elimination of liability for monetary damages for
breach of the directors' fiduciary duty of care to the Registrant and its
stockholders. These provisions do not eliminate the directors' duty of care and,
in appropriate circumstances, equitable remedies such as injunctive or other
forms of non-monetary relief will remain available under Delaware law. In
addition, each director will continue to be subject to liability for breach of
the director's duty of loyalty to the Registrant, for acts or omissions not in
good faith or involving intentional misconduct, for knowing violations of law,
for any transaction from which the director derived an improper personal
benefit, and for payment of dividends or approval of stock repurchases or
redemptions that are unlawful under Delaware law. The provision also does not
affect a director's responsibilities under any other laws, such as the federal
securities laws or state or federal environmental laws.

The Registrant has entered into agreements with its directors and officers that
require Cerus to indemnify such persons to the fullest extent authorized or
permitted by the provisions of the Restated Certificate and Delaware law against
expenses, judgements, fines, settlements and other amounts actually and
responsibly incurred (including expenses of a derivative action) in connection
with any proceeding, whether actual or threatened, to which any such person may
be made a party by reason of the fact that such person is or was a director,
officer, employee or other agent of the Registrant or any of its affiliated
enterprise. Delaware law permits such indemnification, provided such person
acted in good faith and in a manner such person reasonably believed to be in or
not opposed to the best interest of the Registrant and, with respect to any
criminal proceeding, had no reasonable cause to believe his or her conduct was
unlawful. The indemnification agreements also set forth certain procedures that
will apply in the event of a claim for indemnification thereunder.

At present, there is no pending litigation or proceeding involving a director or
officer of the Registrant as to which indemnification is being sought nor is the
Registrant aware of any threatened litigation that may result in claims for
indemnification by any officer or director.

ITEM 7.  EXEMPTION FROM REGISTRATION CLAIMED

Not Applicable.




                                                                     Page 2 of 9
<PAGE>   4
                                    EXHIBITS

EXHIBIT
NUMBER

5.1            Opinion of Cooley Godward LLP.

23.1           Consent of Ernst & Young LLP, Independent Auditors.

23.2           Consent of Cooley Godward LLP.  Reference is made to Exhibit 5.1.

24.1           Power of Attorney.  Reference is made to the signature page.

4.1*           The Company's Bylaws.

4.2*           The Company's Amended and Restated Certificate of Incorporation.

4.3*           Specimen Stock Certificate.

99.1           The Company's 1999 Equity Incentive Plan.

         *     Documents incorporated by reference from the Company's
               Registration Statement on Form S-1 (File No. 333-11341) filed
               with the SEC on September 4, 1996.





                                                                     Page 3 of 9
<PAGE>   5
                                  UNDERTAKINGS

         1.       The undersigned registrant hereby undertakes:

                  (A) To file, during any period in which offers or sales are
being made, a post-effective amendment to this Registration Statement:

                        (I) To include any prospectus required by section
10(a)(3) of the Securities Act;

                        (II) To reflect in the prospectus any facts or events
arising after the effective date of the registration statement (or the most
recent post-effective amendment thereof) which, individually or in the
aggregate, represent a fundamental change in the information set forth in the
registration statement. Notwithstanding the foregoing, any increase or decrease
in volume of securities offered (if the total dollar value of securities offered
would not exceed that which was registered) and any deviation from the low or
high end of the estimated maximum offering range may be reflected in the form of
prospectus filed with the Commission pursuant to Rule 424(b) (230.424(b) of this
chapter) if, in the aggregate, the changes in volume and price represent no more
than a 20% change in the maximum aggregate offering price set forth in the
"Calculation of Registration Fee" table in the effective registration statement.

                        (III) To include any material information with respect
to the plan of distribution not previously disclosed in the registration
statement or any material change to such information in the registration
statement;

         Provided, however, that paragraphs (a)(i) and (a)(ii) do not apply if
the registration statement is on Form S-3 or Form S-8 and the information
required to be included in a post-effective amendment by those paragraphs is
contained in periodic reports filed by the issuer pursuant to section 13 or
section 15(d) of the Exchange Act that are incorporated by reference in the
registration statement.

                  (B) That, for the purpose of determining any liability under
the Securities Act, each such post-effective amendment shall be deemed to be a
new registration statement relating to the securities offered herein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

                  (C) To remove from registration by means of a post-effective
amendment any of the securities being registered which remain unsold at the
termination of the offering.

         2. The undersigned Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
Registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
Exchange Act (and, where applicable, each filing of an employee benefit plan's
annual report pursuant to section 15(d) of the Exchange Act) that is
incorporated by reference in the Registration Statement shall be deemed to be a
new registration statement relating to the securities offered herein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

         3. Insofar as indemnification for liabilities arising under the
Securities Act may be permitted to directors, officers and controlling persons
of the Registrant pursuant to the foregoing provisions, or otherwise, the
Registrant has been advised that in the opinion of the Securities and Exchange
Commission such indemnification is against public policy as expressed in the
Securities Act and is, therefore, unenforceable. In the event that a claim for
indemnification against such liabilities (other than the payment by the
Registrant of expenses incurred or paid by a director, officer or controlling
person of the Registrant in the successful defense of any action, suit or
proceeding) is asserted by such director, officer or controlling person in
connection with the securities being registered, the Registrant will, unless in
the opinion of its counsel the matter has been settled by controlling precedent,
submit to a court of appropriate jurisdiction the question whether such
indemnification by it is against public policy as expressed in the Securities
Act and will be governed by the final adjudication of such issue.




                                                                     Page 4 of 9

<PAGE>   6
                                   SIGNATURES

         THE REGISTRANT. Pursuant to the requirements of the Securities Act of
1933, the Registrant certifies that it has reasonable grounds to believe that it
meets all of the requirements for filing on Form S-8 and has duly caused this
Registration Statement to be signed on its behalf by the undersigned, thereunto
duly authorized, in the City of Concord, State of California on August 2, 1999.



                                   CERUS CORPORATION



                                   By /s/ Gregory W. Schafer
                                     ---------------------------------------
                                            Gregory W. Schafer
                                            Vice President, Finance and
                                            Chief Financial Officer



                                POWER OF ATTORNEY

         KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature
appears below constitutes and appoints Stephen T. Isaacs and Gregory W. Schafer
and each or any one of them, 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, to sign any and all amendments
(including post-effective amendments) to this Registration Statement on Form
S-8, 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 to be done
in connection therewith, as fully to all intents and purposes as he might or
could do in person, hereby ratifying and confirming all that said
attorneys-in-fact and agents, or any of them, or their or his substitutes or
substitute, may lawfully do or cause to be done by virtue hereof.

         Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the dates indicated.

<TABLE>
<CAPTION>
            SIGNATURE                                   TITLE                             DATE
            ---------                                   -----                             ----
<S>                                        <C>                                         <C>
     /s/ Stephen T. Isaacs
------------------------------------       President, Chief Executive Officer          August 2, 1999
         Stephen T. Isaacs                 and Director (Principal Executive
                                           Officer)

     /s/ Gregory W. Schafer
------------------------------------       Vice President, Finance and Chief           August 2, 1999
        Gregory W. Schafer                 Financial Officer (Principal
                                           Financial Officer and Principal
                                           Accounting Officer)

        /s/ B.J. Cassin
------------------------------------       Director                                    August 2, 1999
            B.J. Cassin
</TABLE>






                                                                     Page 5 of 9

<PAGE>   7

<TABLE>
<CAPTION>
            SIGNATURE                                   TITLE                             DATE
            ---------                                   -----                             ----
<S>                                        <C>                                         <C>

        /s/ John E. Hearst
------------------------------------       Director                                    August 2, 1999
            John E. Hearst


        /s/ Peter H. McNerney
------------------------------------       Director                                    August 2, 1999
             Peter H. McNerney


        /s/ Dale A. Smith
------------------------------------       Director                                    August 2, 1999
         Dale A. Smith


        /s/ Henry E. Stickney
------------------------------------       Director                                    August 2, 1999
         Henry E. Stickney
</TABLE>




                                                                     Page 6 of 9

<PAGE>   8
                                  EXHIBIT INDEX

EXHIBIT
NUMBER

5.1            Opinion of Cooley Godward LLP.

23.1           Consent of Ernst & Young LLP, Independent Auditors.

23.2           Consent of Cooley Godward LLP.  Reference is made to Exhibit 5.1.

24.1           Power of Attorney.  Reference is made to the signature page.

4.1*           The Company's Bylaws.

4.2*           The Company's Amended and Restated Certificate of Incorporation.

4.3*           Specimen Stock Certificate.

99.1           The Company's 1999 Equity Incentive Plan.

         *     Documents incorporated by reference from the Company's
               Registration Statement on Form S-1 (File No. 333-11341) filed
               with the SEC on September 4, 1996.






                                                                     Page 7 of 9

