<PAGE>

                                                                    Exhibit 4.12

                              EMPLOYMENT AGREEMENT

                                 by and between

                                     QIAGEN,
                           Auf dem Wolf 39, 4052 Basel

                             - hereinafter QIAGEN -

                                       and

                               Mr. Peer M. Schatz

                            - hereinafter Employee -

1.   Introduction

     Mr. Schatz is the Financial Controller of QIAGEN GmbH in Dusseldorf. Mr.
     Schatz is Director and Financial Controller of QIAGEN AG in Basel as well.
     The following employment agreement relates exclusively to this activity.

2.   Position and Function

     Mr. Schatz is directly responsible to and reports to the Board of
     Directors. His main tasks are administration and finance.

3.   Commencement, Duration and Conclusion of Employment Relationship

3.1  Commencement

     Mr. Schatz has been employed by QIAGEN AG since September 15, 1994.

     3.2. Duration and Conclusion

     The employment agreement is concluded for an indeterminate time period. The
     employment agreement may be terminated at the end of any calendar month,

<PAGE>

     with three months notice communicated by registered letter. The Company
     reserves the right to terminate Employee at any time if serious cause
     exists.

4.   Obligations of Mr. Schatz

     4.1 Fiduciary and Secrecy Obligations

     Mr. Schatz is obligated to perform all tasks in conjunction with the
     aforementioned activities conscientiously and to be best of his abilities,
     to safeguard DIAGEN's interests in every respect, and to maintain secrecy
     on all confidential matters and activities. This secrecy obligation extends
     in full measure wherever may be applicable to the time subsequent to
     commencing employment.

     4.2 Work Time

     Mr. Schatz' schedule shall be determined by the scope and urgency of
     contingent activities, on average requiring one day per week.

     5.  QIAGEN AG Obligations

     5.1 Salary

         The Employee shall receive a gross monthly salary of FR 5,000.--(in
         words five thousand francs). The Employee is not entitled to receive a
         thirteenth salary. The gross salary includes all legally required
         insurance contributions such as AHV, ALV, IV, EO and deductible
         withholding tax. Since Mr. Schatz shall perform his activities in
         Germany and is sufficiently insured by BVG against all risks, he has no
         additional obligation under BVG regulations.

     5.2 Dependents Allowance

         Mr. Schatz' entitlement to claim a Dependents Allowance is expressly
         regulated in his agreement with QIAGEN GmbH. According to the
         aforementioned agreement, he shall not be entitled to receive a
         Dependents Allowance.

     5.3 Payment of Salary During Illness or Accident and Fulfillment of Legal
         Obligations

         The scope of salary payment during illness, accident and fulfillment of
         legal obligations is also determined by the regulations governing the
         forementioned

<PAGE>

          employment relationship contained within the agreement between QIAGEN
          GmbH and Mr. Schatz.

     5.4  Expenses

          Necessary expenditures and expenses in the interests of the firm as
          well as compensation for the use of private auto for business purposes
          shall be reimbursed according to the provisions relating to
          compensation of expenses.

     6.   Final Provisions

     6.1. Amendments and Supplements

          Any future amendments of and/or supplements to the agreement shall be
          in writing in order to acquire legal effect. For questions which have
          not been addressed by either the present agreement or that between Mr.
          Schatz and QIAGEN GmbH, the provisions of Swiss Code of Obligations
          (OR).

     6.2  Disputes

          In the case that disputes arise in conjunction with this agreement,
          the contractual partners shall attempt to remedy these in an amicable
          fashion. If this is not possible, the applicable court of jurisdiction
          shall be the city of Basel.

     6.3  Integrated Component of Employment Agreement

          Provisions on compensation of expenses and holidays form an integral
          component of this employment agreement.

     Basel, May 29, 1998                               QIAGEN A.G.

      /s/ Peer M. Schatz                               /s/ Vincent Sigrist
     -------------------                               ----------------------
     Peer M. Schatz                                    General Manager


     ___________________________                       /s/ Sabine Buhrer
                                                       ----------------------
                                                       Finance & Administration
                                                       Manager