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                                                                     EXHIBIT 4.1


                           CERTIFICATE OF DESIGNATION

                                       OF

                  SERIES A JUNIOR PARTICIPATING PREFERRED STOCK

                         (PURSUANT TO SECTION 151 OF THE

                        DELAWARE GENERAL CORPORATION LAW)

        MOLECULAR DEVICES CORPORATION, a corporation organized and existing
under the General Corporation Law of the State of Delaware (hereinafter called
the "Company"), hereby certifies that the following resolution was adopted by
the Board of Directors of the Corporation as required by Section 151 of the
General Corporation Law at a meeting duly called and held on October 25, 2001:

                RESOLVED, that pursuant to the authority granted to and vested
        in the Board of Directors of the Company in accordance with the
        provisions of its Amended and Restated Certificate of Incorporation, the
        Board of Directors hereby creates a series of Preferred Stock, par value
        $.001 per share, of the Company and hereby states the designation and
        number of shares, and fixes the relative designations and the powers,
        preferences and rights, and the qualifications, limitations and
        restrictions thereof (in addition to the provisions set forth in the
        Certificate of Incorporation of the Company, which are applicable to the
        Preferred Stock of all classes and series), as follows:

Series A Junior Participating Preferred Stock:

        SECTION 1. DESIGNATION AND AMOUNT. Six hundred thousand (600,000) shares
of Preferred Stock, $.001 par value, are designated "Series A Junior
Participating Preferred Stock" with the designations and the powers, preferences
and rights, and the qualifications, limitations and restrictions specified
herein (the "Junior Preferred Stock"). Such number of shares may be increased or
decreased by resolution of the Board of Directors; provided, that no decrease
shall reduce the number of shares of Junior Preferred Stock to a number less
than the number of shares then outstanding plus the number of shares reserved
for issuance upon the exercise of outstanding options, rights or warrants or
upon the conversion of any outstanding securities issued by the Company
convertible into Junior Preferred Stock.

        SECTION 2. DIVIDENDS AND DISTRIBUTIONS.

                (A) Subject to the rights of the holders of any shares of any
series of Preferred Stock (or any similar stock) ranking prior and superior to
the Junior Preferred Stock with respect to dividends, the holders of shares of
Junior Preferred Stock, in preference to the holders of



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Common Stock, par value $.001 per share (the "Common Stock"), of the Company,
and of any other junior stock, shall be entitled to receive, when, as and if
declared by the Board of Directors out of funds legally available for the
purpose, quarterly dividends payable in cash on the first day of April, July,
October and January in each year (each such date being referred to herein as a
"Quarterly Dividend Payment Date"), commencing on the first Quarterly Dividend
Payment Date after the first issuance of a share or fraction of a share of
Junior Preferred Stock, in an amount per share (rounded to the nearest cent)
equal to the greater of (a) $l.00 or (b) subject to the provision for adjustment
hereinafter set forth, 100 times the aggregate per share amount of all cash
dividends, and 100 times the aggregate per share amount (payable in kind) of all
non-cash dividends or other distributions, other than a dividend payable in
shares of Common Stock or a subdivision of the outstanding shares of Common
Stock (by reclassification or otherwise), declared on the Common Stock since the
immediately preceding Quarterly Dividend Payment Date or, with respect to the
first Quarterly Dividend Payment Date, since the first issuance of any share or
fraction of a share of Junior Preferred Stock. In the event the Company shall at
any time declare or pay any dividend on the Common Stock payable in shares of
Common Stock, or effect a subdivision or combination or consolidation of the
outstanding shares of Common Stock (by reclassification or otherwise than by
payment of a dividend in shares of Common Stock) into a greater or lesser number
of shares of Common Stock, then in each such case the amount to which holders of
shares of Junior Preferred Stock were entitled immediately prior to such event
under clause (b) of the preceding sentence shall be adjusted by multiplying such
amount by a fraction, the numerator of which is the number of shares of Common
Stock outstanding immediately after such event and the denominator of which is
the number of shares of Common Stock that were outstanding immediately prior to
such event.

                (B) The Company shall declare a dividend or distribution on the
Junior Preferred Stock as provided in paragraph (A) of this Section immediately
after it declares a dividend or distribution on the Common Stock (other than a
dividend payable in shares of Common Stock); provided, that in the event no
dividend or distribution shall have been declared on the Common Stock during the
period between any Quarterly Dividend Payment Date and the next subsequent
Quarterly Dividend Payment Date, a dividend of $1.00 per share on the Junior
Preferred Stock shall nevertheless be payable on such subsequent Quarterly
Dividend Payment Date.

                (C) Dividends shall begin to accrue and be cumulative on
outstanding shares of Junior Preferred Stock from the Quarterly Dividend Payment
Date next preceding the date of issue of such shares, unless the date of issue
of such shares is prior to the record date for the first Quarterly Dividend
Payment Date, in which case dividends on such shares shall begin to accrue from
the date of issue of such shares, or unless the date of issue is a Quarterly
Dividend Payment Date or is a date after the record date for the determination
of holders of shares of Junior Preferred Stock entitled to receive a quarterly
dividend and before such Quarterly Dividend Payment Date, in either of which
events such dividends shall begin to accrue and be cumulative from such
Quarterly Dividend Payment Date. Accrued but unpaid dividends shall not bear
interest. Dividends paid on the shares of Junior Preferred Stock in an amount
less than the total amount of such dividends at the time accrued and payable on
such shares shall be allocated pro rata on a share-by-share basis among all such
shares at the time outstanding. The Board of Directors may fix a record date for
the determination of holders of shares of Junior Preferred



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Stock entitled to receive payment of a dividend or distribution declared
thereon, which record date shall be not more than 60 days prior to the date
fixed for the payment thereof.

        SECTION 3. VOTING RIGHTS. The holders of shares of Junior Preferred
Stock shall have the following voting rights:

                (A) Subject to the provision for adjustment hereinafter set
forth, each share of Junior Preferred Stock shall entitle the holder thereof to
100 votes on all matters submitted to a vote of the stockholders of the Company.
In the event the Company shall at any time declare or pay any dividend on the
Common Stock payable in shares of Common Stock, or effect a subdivision or
combination or consolidation of the outstanding shares of Common Stock (by
reclassification or otherwise than by payment of a dividend in shares of Common
Stock) into a greater or lesser number of shares of Common Stock, then in each
such case the number of votes per share to which holders of shares of Junior
Preferred Stock were entitled immediately prior to such event shall be adjusted
by multiplying such number by a fraction, the numerator of which is the number
of shares of Common Stock outstanding immediately after such event and the
denominator of which is the number of shares of Common Stock that were
outstanding immediately prior to such event.

                (B) Except as otherwise provided herein, in any other
Certificate of Designation creating a series of Preferred Stock or any similar
stock, or by law, the holders of shares of Junior Preferred Stock and the
holders of shares of Common Stock and any other capital stock of the Company
having general voting rights shall vote together as one class on all matters
submitted to a vote of stockholders of the Company.

                (C) Except as set forth herein, or as otherwise provided by law,
holders of Junior Preferred Stock shall have no special voting rights and their
consent shall not be required (except to the extent they are entitled to vote
with holders of Common Stock as set forth herein) for taking any corporate
action.

        SECTION 4. CERTAIN RESTRICTIONS.

                (A) Whenever quarterly dividends or other dividends or
distributions payable on the Junior Preferred Stock as provided in Section 2 are
in arrears, thereafter and until all accrued and unpaid dividends and
distributions, whether or not declared, on shares of Junior Preferred Stock
outstanding shall have been paid in full, the Company shall not:

                        (i) declare or pay dividends, or make any other
distributions, on any shares of stock ranking junior (either as to dividends or
upon liquidation, dissolution or winding up) to the Junior Preferred Stock;

                        (ii) declare or pay dividends, or make any other
distributions, on any shares of stock ranking on a parity (either as to
dividends or upon liquidation, dissolution or winding up) with the Junior
Preferred Stock, except dividends paid ratably on the Junior Preferred Stock and
all such parity stock on which dividends are payable or in arrears in proportion
to the total amounts to which the holders of all such shares are then entitled;



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                        (iii) redeem or purchase or otherwise acquire for
consideration shares of any stock ranking junior (either as to dividends or upon
liquidation, dissolution or winding up) to the Junior Preferred Stock, provided
that the Company may at any time redeem, purchase or otherwise acquire shares of
any such junior stock in exchange for shares of any stock of the Company ranking
junior (either as to dividends or upon dissolution, liquidation or winding up)
to the Junior Preferred Stock; or

                        (iv) redeem or purchase or otherwise acquire for
consideration any shares of Junior Preferred Stock, or any shares of stock
ranking on a parity (either as to dividends or upon liquidation, dissolution or
winding up) with the Junior Preferred Stock, except in accordance with a
purchase offer made in writing or by publication (as determined by the Board of
Directors) to all holders of such shares upon such terms as the Board of
Directors, after consideration of the respective annual dividend rates and other
relative rights and preferences of the respective series and classes, shall
determine in good faith will result in fair and equitable treatment among the
respective series or classes.

                (B) The Company shall not permit any subsidiary of the Company
to purchase or otherwise acquire for consideration any shares of stock of the
Company unless the Company could, under paragraph (A) of this Section 4,
purchase or otherwise acquire such shares at such time and in such manner.

        SECTION 5. REACQUIRED SHARES. Any shares of Junior Preferred Stock
purchased or otherwise acquired by the Company in any manner whatsoever shall be
retired and cancelled promptly after the acquisition thereof. All such shares
shall upon their cancellation become authorized but unissued shares of Preferred
Stock and may be reissued as part of a new series of Preferred Stock subject to
the conditions and restrictions on issuance set forth herein, in the Amended and
Restated Certificate of Incorporation, or in any other Certificate of
Designation creating a series of Preferred Stock or any similar stock or as
otherwise required by law.

        SECTION 6. LIQUIDATION, DISSOLUTION OR WINDING UP. Upon any liquidation,
dissolution or winding up of the Company, no distribution shall be made (1) to
the holders of shares of stock ranking junior (either as to dividends or upon
liquidation, dissolution or winding up) to the Junior Preferred Stock unless,
prior thereto, the holders of shares of Junior Preferred Stock shall have
received $100 per share, plus an amount equal to accrued and unpaid dividends
and distributions thereon, whether or not declared, to the date of such payment,
provided that the holders of shares of Junior Preferred Stock shall be entitled
to receive an aggregate amount per share, subject to the provision for
adjustment hereinafter set forth, equal to 100 times the aggregate amount to be
distributed per share to holders of shares of Common Stock, or (2) to the
holders of shares of stock ranking on a parity (either as to dividends or upon
liquidation, dissolution or winding up) with the Junior Preferred Stock, except
distributions made ratably on the Junior Preferred Stock and all such parity
stock in proportion to the total amounts to which the holders of all such shares
are entitled upon such liquidation, dissolution or winding up. In the event the
Company shall at any time declare or pay any dividend on the Common Stock
payable in shares of Common Stock, or effect a subdivision or combination or
consolidation of the outstanding shares of Common Stock (by reclassification or
otherwise than by payment of a dividend in shares of Common Stock) into a
greater or lesser number of shares of Common Stock, then in each such case the
aggregate amount to which holders of shares of Junior



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Preferred Stock were entitled immediately prior to such event under the proviso
in clause (1) of the preceding sentence shall be adjusted by multiplying such
amount by a fraction the numerator of which is the number of shares of Common
Stock outstanding immediately after such event and the denominator of which is
the number of shares of Common Stock that were outstanding immediately prior to
such event.

        SECTION 7. CONSOLIDATION, MERGER, ETC. In case the Company shall enter
into any consolidation, merger, combination or other transaction in which the
shares of Common Stock are exchanged for or changed into other stock or
securities, cash and/or any other property, then in any such case each share of
Junior Preferred Stock shall at the same time be similarly exchanged or changed
into an amount per share, subject to the provision for adjustment hereinafter
set forth, equal to 100 times the aggregate amount of stock, securities, cash
and/or any other property (payable in kind), as the case may be, into which or
for which each share of Common Stock is changed or exchanged. In the event the
Company shall at any time declare or pay any dividend on the Common Stock
payable in shares of Common Stock, or effect a subdivision or combination or
consolidation of the outstanding shares of Common Stock (by reclassification or
otherwise than by payment of a dividend in shares of Common Stock) into a
greater or lesser number of shares of Common Stock, then in each such case the
amount set forth in the preceding sentence with respect to the exchange or
change of shares of Junior Preferred Stock shall be adjusted by multiplying such
amount by a fraction, the numerator of which is the number of shares of Common
Stock outstanding immediately after such event and the denominator of which is
the number of shares of Common Stock that were outstanding immediately prior to
such event.

        SECTION 8. NO REDEMPTION. The shares of Junior Preferred Stock shall not
be redeemable.

        SECTION 9. RANK. The Junior Preferred Stock shall rank, with respect to
the payment of dividends and the distribution of assets, junior to all series of
any other class of the Company's Preferred Stock.

        SECTION 10. AMENDMENT. The Amended and Restated Certificate of
Incorporation of the Company shall not be amended in any manner which would
materially alter or change the powers, preferences or special rights of the
Junior Preferred Stock so as to affect them adversely without the affirmative
vote of the holders of at least two-thirds of the outstanding shares of Junior
Preferred Stock, voting together as a single class.



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        IN WITNESS WHEREOF, the undersigned have executed this certificate as of
October 25, 2001.


                                      /s/ Timothy A. Harkness
                                      ------------------------------------------
                                      Timothy A. Harkness
                                      Vice President and Chief Financial Officer

                                      /s/ James C. Kitch
                                      ------------------------------------------
                                      James C. Kitch
                                      Secretary



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