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                                                                     Exhibit 2.3


                              AMENDED AND RESTATED
                                 PROMISSORY NOTE

$43,475,000.00                                                 February 28, 2002


1.   Promise To Pay.
     --------------

     FOR VALUE RECEIVED, QIAGEN Sciences, Inc., a Maryland corporation having an
address of 19825 Executive Park Circle, Germantown, Maryland 20874 ("Borrower")
promises to pay to the order of QIAGEN North American Holdings, Inc., a
California corporation having an address at 28159 Stanford Avenue, Valencia,
California 91355 ("Lender"), the principal sum of Forty Three Million Four
Hundred Seventy Five Thousand and No/100 Dollars ($43,475,000.00).

     This Amended and Restated Note is made by Borrower in full substitution of
a note made by Borrower in favor of Lender in the original principal amount of
Fifty Million Euros (EUR 50.000.000), dated as of November 5, 2001 (the
"Original Note"). This Amended and Restated Note constitutes a modification of
the amount and currency of the existing indebtedness evidenced by the Original
Note, but renews and does not cancel such existing indebtedness.

2.   Payment; Default.
     ----------------

     Reference is made to that Kreditvertrag dated May 28, 2001, as amended and
restated, among QIAGEN GmbH as borrower, Deutsche Bank AG as consortium lender,
and others (the "Deutsche Loan Agreement"). Interest shall accrue on the
outstanding principal balance of this Note at an interest rate of per annum
equal to the interest rate under the Deutsche Loan Agreement, plus one half
percent (0.5%), plus one half percent (0.5%). Interest and principal shall be
due and payable under this Amended and Restated Note on the same dates that
interest and principal are due and payable under the Deutsche Loan Agreement. On
May 27, 2003, all outstanding principal shall be due and payable. The Borrower
may voluntarily prepay this Amended and Restated Note in whole or in part at any
time and from time to time without penalty, together with interest accrued on
the amount prepaid through the date of prepayment. If Borrower fails to make any
payment hereunder within ten (10) days of the due date, the Lender may declare
the principal balance of this Amended and Restated Note to be immediately due
and payable and the Lender may exercise any rights and remedies under this
Amended and Restated Note, any other documents or applicable law as Lender may
elect.

3.   Certain Waivers, Consents and Agreements.
     ----------------------------------------

     Each and every party liable hereon or for the indebtedness evidenced hereby
whether as maker, endorser, guarantor, surety or otherwise hereby: (a) waives
presentment, demand, protest, suretyship defenses and defenses in the nature
thereof; (b) waives any defenses based upon and specifically assents to any and
all extensions and postponements of the time for payment, changes in terms and
conditions and all other indulgences and forbearances which may be granted by
the holder to any party now or hereafter liable hereunder or for the
indebtedness evidenced hereby; (c) agrees to any substitution, exchange,
release, surrender or other delivery of

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any security or collateral now or hereafter held hereunder, and to the addition
or release of any other party or person primarily or secondarily liable; (d)
agrees that if any security or collateral given to secure this Amended and
Restated Note or the indebtedness evidenced hereby shall be found to be
unenforceable in full or to any extent, or if Lender or any other party shall
fail to duly perfect or protect such collateral, the same shall not relieve or
release any party liable hereon or thereon nor vitiate any other security or
collateral given for any obligations evidenced hereby or thereby; (e) agrees to
pay all costs and expenses incurred by Lender or any other holder of this
Amended and Restated Note in connection with the indebtedness evidenced hereby,
including, without limitation, all attorneys' fees and costs, the collection of
the indebtedness evidenced hereby and the enforcement of rights and remedies
hereunder whether or not suit is instituted; and (f) consents to all of the
terms and conditions contained in this Amended and Restated Note and all other
instruments now or hereafter executed evidencing or governing all or any portion
of the security or collateral for this Amended and Restated Note.

4.   Delay Not A Bar.
     ---------------

     No delay or omission on the part of the holder in exercising any right
hereunder or any right under any instrument or agreement now or hereafter
executed in connection herewith, or any agreement or instrument which is given
or may be given to secure the indebtedness evidenced hereby, or any other
agreement now or hereafter executed in connection herewith shall operate as a
waiver of any such right or of any other right of such holder, nor shall any
delay, omission or waiver on any one occasion be deemed to be a bar to or waiver
of the same or of any other right on any future occasion.

5.   Partial Invalidity.
     ------------------

     The invalidity or unenforceability of any provision hereof, or of any other
instrument, agreement or document now or hereafter executed in connection
herewith shall not impair or vitiate any other provision of any of such
instruments, agreements and documents, all of which provisions shall be
enforceable to the fullest extent now or hereafter permitted by law.

6.   Compliance With Usury Laws.
     --------------------------

     All agreements between Borrower and Lender are hereby expressly limited so
that in no contingency or event whatsoever, whether by reason of acceleration of
maturity of the indebtedness evidenced hereby or otherwise, shall the amount
paid or agreed to be paid to Lender for the use or the forbearance of the
indebtedness evidenced hereby exceed the maximum permissible under applicable
law. As used herein, the term "applicable law" shall mean the law in effect as
of the date hereof, provided, however, that in the event there is a change in
the law which results in a higher permissible rate of interest, then this
Amended and Restated Note shall be governed by such new law as of its effective
date. In this regard, it is expressly agreed that it is the intent of Borrower
and Lender in the execution, delivery and acceptance of this Amended and
Restated Note to contract in strict compliance with the laws of the State of
Maryland from time to time in effect. If, under or from any circumstances
whatsoever, fulfillment of any provision hereof at the time performance of such
provision shall be due, shall involve transcending the limit of validity
prescribed by applicable law, then the obligation to be fulfilled shall
automatically be reduced to the limit of such validity, and if under or from any

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circumstances whatsoever Lender should ever receive as interest an amount which
would exceed the highest lawful rate, such amount which would be excessive
interest shall be applied to the reduction of the principal balance evidenced
hereby and not to the payment of interest. This provision shall control every
other provision of all agreements between Borrower and Lender.

7.   Security.
     --------

     [Reserved.]

8.   Notices.
     -------

     Any notices given with respect to this Amended and Restated Note shall be
in writing and shall be sent by hand delivery, reputable overnight courier, or
by registered or certified mail, return receipt requested, postage prepaid, or
by facsimile (with a copy simultaneously sent by one of the previously listed
methods), addressed to Borrower at the address set forth above (or to such other
address or addresses as may from time to time hereafter be designated by
Borrower, by like notice.

9.   Governing Law and Consent to Jurisdiction.
     -----------------------------------------

     9.1. Substantial Relationship. It is understood and agreed that this
     Amended and Restated Note was negotiated, executed and delivered in the
     State of Maryland, which State the parties agree has a substantial
     relationship to the parties and to the underlying transactions embodied
     hereby.

     9.2. Place of Delivery. Borrower agrees to furnish to Lender at Lender's
     office at 28159 Stanford Avenue, Valencia, California 91355 all further
     instruments, certifications and documents to be furnished hereunder.

     9.3. Governing Law. This Amended and Restated Note shall in all respects be
     governed, construed, applied and enforced in accordance with the internal
     laws of the State of Maryland without regard to principles of conflicts of
     law.

10.  Waiver of Jury Trial.
     --------------------

     BORROWER HEREBY KNOWINGLY, VOLUNTARILY AND INTENTIONALLY WAIVES THE RIGHT
TO A TRIAL BY JURY IN RESPECT OF ANY LITIGATION BASED HEREON, ARISING OUT OF,
UNDER OR IN CONNECTION WITH THIS NOTE OR ANY COURSE OF CONDUCT, COURSE OF
DEALINGS, STATEMENTS (WHETHER VERBAL OR WRITTEN) OR ACTIONS IN CONNECTION WITH
THIS NOTE. THIS WAIVER CONSTITUTES A MATERIAL INDUCEMENT FOR LENDER TO ACCEPT
THIS NOTE AND MAKE THE LOAN CONTEMPLATED HEREBY.

11.  No Oral Change.
     --------------

     This Amended and Restated Note may be further amended, terminated, extended
or otherwise modified only by a writing signed by the party against which
enforcement is sought. In no event shall any oral agreements, promises, actions,
inactions, knowledge, course of

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conduct, course of dealing, or the like be effective to further amend,
terminate, extend or otherwise modify this Amended and Restated Note.

12.  Rights of the Holder.
     --------------------

     This Amended and Restated Note and the rights and remedies provided for
herein may be enforced by Lender or any subsequent holder hereof. Wherever the
context permits each reference to the term "holder" herein shall mean and refer
to Lender or the then subsequent holder of this Amended and Restated Note.

     IN WITNESS WHEREOF, Borrower has caused this Amended and Restated Note to
be duly executed as of the date set forth above as a sealed instrument.

Witnesses:                                           BORROWER:

                                                     QIAGEN SCIENCES, INC.


/s/ William E. Dove, Jr.                             By:   /s/ Michael Burgett
--------------------------                                 -------------------
Name: William E. Dove, Jr.                           Name: Michael Burgett
                                                     Title: Vice President

/s/ David B. Johnson
--------------------------
Name: David B. Johnson



                              ACCEPTANCE BY LENDER:

     The foregoing Amended and Restated Note is hereby accepted by Lender in
full substitution for the Original Note (as defined herein). The Original Note
(but not the indebtedness evidenced by the Original Note) is hereby canceled.

Executed as of February 28, 2002

QIAGEN NORTH AMERICAN HOLDINGS, INC.

By: /s/ Peer Schatz
   -------------------------
   Peer Schatz
   Treasurer and Secretary

By: /s/ Metin Colpan
   -------------------------
   Metin Colpan
   President