IGEN INTERNATIONAL, INC.
                                  EXHIBIT 2.1

                          AGREEMENT AND PLAN OF MERGER


     THIS AGREEMENT AND PLAN OF MERGER (the "Merger Agreement") is made as of
September 10, 1996, by and between IGEN, INC., a California corporation ("IGEN
California"), and IGEN INTERNATIONAL, INC., a Delaware corporation ("IGEN
Delaware").  IGEN California and IGEN Delaware are sometimes referred to as the
"Constituent Corporations."

     The authorized capital stock of IGEN California consists of 50,000,000
shares of Common Stock, $.001 par value, and 10,000,000 shares of Preferred
Stock, $.001 par value, of which 600,000 shares have been designated Series A
Junior Preferred Stock and none of which are issued and outstanding as of the
date hereof.  The authorized capital stock of IGEN Delaware, upon effectuation
of the transactions set forth in this Merger Agreement, will consist of
50,000,000 shares of Common Stock, $.001 par value, and 10,000,000 shares of
Preferred Stock, $.001 par value, of which 600,000 shares have been designated
Series A Junior Preferred Stock and none of which are issued and outstanding as
of the date hereof.

     The directors of the Constituent Corporations deem it advisable and to the
advantage of said corporations that IGEN California merge into IGEN Delaware
upon the terms and conditions herein provided.

     NOW, THEREFORE, the parties do hereby adopt the plan of reorganization
encompassed by this Merger Agreement and do hereby agree that IGEN California
shall merge into IGEN Delaware on the following terms, conditions and other
provisions:

1.   TERMS AND CONDITIONS.

     1.1.   Merger.  IGEN California shall be merged with and into IGEN Delaware
(the "Merger"), and IGEN Delaware shall be the surviving corporation (the
"Surviving Corporation") effective upon the date when this Merger Agreement is
filed with the Secretary of State of the State of Delaware (the "Effective
Date").

     1.2.   Succession.  On the Effective Date, IGEN Delaware shall continue its
corporate existence under the laws of the State of Delaware, and the separate
existence and corporate organization of IGEN California, except insofar as it
may be continued by operation of law, shall be terminated and cease.

     1.3.   Transfer of Assets and Liabilities.  On the Effective Date, the
rights, privileges, powers and franchises, both of a public as well as of a
private nature, of each of the Constituent Corporations shall be vested in and
possessed by the Surviving Corporation, subject to all of the disabilities,
duties and restrictions of or upon each of the Constituent Corporations; and all
and singular rights, privileges, powers and franchises of each of the
Constituent Corporations, and all property, real, personal and mixed, of each of
the Constituent Corporations, and all debts due to each of the Constituent
Corporations on whatever account, and all things in action or belonging to each
of the 


 
Constituent Corporations shall be transferred to and vested in the Surviving
Corporation; and all property, rights, privileges, powers and franchises, and
all and every other interest, shall be thereafter the property of the Surviving
Corporation as they were of the Constituent Corporations, and the title to any
real estate vested by deed or otherwise in either of the Constituent
Corporations shall not revert or be in any way impaired by reason of the Merger;
provided, however, that the liabilities of the Constituent Corporations and of
their shareholders, directors and officers shall not be affected and all rights
of creditors and all liens upon any property of either of the Constituent
Corporations shall be preserved unimpaired, and any claim existing or action or
proceeding pending by or against either of the Constituent Corporations may be
prosecuted to judgment as if the Merger had not taken place except as they may
be modified with the consent of such creditors and all debts, liabilities and
duties of or upon each of the Constituent Corporations shall attach to the
Surviving Corporation, and may be enforced against it to the same extent as if
such debts, liabilities and duties had been incurred or contracted by it.

     1.4.   Common Stock of IGEN California and IGEN Delaware.  On the Effective
Date, by virtue of the Merger and without any further action on the part of the
Constituent Corporations or their shareholders, (i) each share of Common Stock
of IGEN California issued and outstanding immediately prior thereto shall be
changed and converted into one fully paid and nonassessable share of the Common
Stock of IGEN Delaware, (ii) resulting fractional shares shall not be issued,
but holders of fractional shares shall be compensated, based upon the closing
price of the Common Stock on the Nasdaq National Market on the Effective Date,
and (iii) each share of Common Stock of IGEN Delaware issued and outstanding
immediately prior thereto shall be cancelled and returned to the status of
authorized but unissued shares.

     1.5.   Stock Certificates.  On and after the Effective Date, all of the
outstanding certificates which prior to that time represented shares of the
Common Stock of IGEN California shall be deemed for all purposes to evidence
ownership of and to represent the shares of IGEN Delaware into which the shares
of IGEN California represented by such certificates have been converted as
herein provided and shall be so registered on the books and records of the
Surviving Corporation or its transfer agent.  The registered owner of any such
outstanding stock certificate shall, until such certificate shall have been
surrendered for transfer or conversion or otherwise accounted for to the
Surviving Corporation or its transfer agent, have and been entitled to exercise
any voting and other rights with respect to and to receive any dividend and
other distributions upon the shares of IGEN Delaware evidenced by such
outstanding certificate as above provided.

     1.6.   Options.  On the Effective Date, the Surviving Corporation will
assume and continue all of IGEN California's stock option plans in existence on
the Effective Date, including without limitation the 1985 Stock Option Plan, the
401(k) Plan and the outstanding and unexercised portions of all options to
purchase Common Stock of IGEN California, including without limitation all
options outstanding under such stock option plans and any other outstanding
options, shall be combined, changed, and converted into options of IGEN Delaware
such that an option for one share of IGEN California shall be converted into an
option for one share of IGEN Delaware.  Effective on the Effective Date, IGEN
Delaware hereby assumes the outstanding and unexercised portions of such options
and the obligations of IGEN California with respect thereto.

                                      2.

 
     1.7. 1996 Preferred Share Purchase Plan. On the Effective Date, the 1996
Preferred Share Purchase Plan approved and adopted by IGEN California's Board of
Directors on December 14, 1995, will be terminated.

     1.8.   Employee Benefit Plans.  On the Effective Date, the Surviving
Corporation shall assume all obligations of IGEN California under any and all
employee benefit plans in effect as of such date with respect to which employee
rights or accrued benefits are outstanding as of such date; provided, however,
that one share of Common Stock of IGEN Delaware shall be substituted for each
share of Common Stock of IGEN California (if any) thereunder.  On the Effective
Date, the Surviving Corporation shall adopt and continue in effect all such
employee benefit plans upon the same terms and conditions as were in effect
immediately prior to the Merger and shall reserve that number of shares of IGEN
Delaware Common Stock with respect to each such employee benefit plan as is
proportional to that number of shares of IGEN California Common Stock (if any)
so reserved on the Effective Date.

2.  CHARTER DOCUMENTS.

     2.1.   Certificate of Incorporation.  The Certificate of Incorporation of
IGEN Delaware in effect on the Effective Date shall continue to be the
Certificate of Incorporation of the Surviving Corporation.

3.  MISCELLANEOUS.

     3.1.   Further Assurances.  From time to time, and when required by the
Surviving Corporation or by its successors and assigns, there shall be executed
and delivered on behalf of IGEN California such deeds and other instruments, and
there shall be taken or caused to be taken by it such further and other action,
as shall be appropriate or necessary in order to vest or perfect in or to
conform of record or otherwise, in the Surviving Corporation the title to and
possession of all the property, interests, assets, rights, privileges,
immunities, powers, franchises and authority of IGEN California and otherwise to
carry out the purposes of this Merger Agreement, and the officers and directors
of the Surviving Corporation are fully authorized in the name and on behalf of
IGEN California or otherwise to take any and all such action and to execute and
deliver any and all such deeds and other instruments.

     3.2.   Amendment.  At any time before or after approval by the shareholders
of IGEN California, this Merger Agreement may be amended in any manner (except
that, after the approval of the Merger Agreement by the shareholders of IGEN
California, the principal terms may not be 

                                      3.

 
amended without the further approval of the shareholders of IGEN California) as
may be determined in the judgment of the respective Board of Directors of IGEN
Delaware and IGEN California to be necessary, desirable, or expedient in order
to clarify the intention of the parties hereto or to effect or facilitate the
purpose and intent of this Merger Agreement.

     3.3.   Conditions to Merger. The obligation of the Constituent Corporations
to effect the transactions contemplated hereby is subject to satisfaction of the
following conditions (any or all of which may be waived by either of the
Constituent Corporations in its sole discretion to the extent permitted by law):

            (a)   the Merger shall have been approved by the shareholders of
IGEN California in accordance with applicable provisions of the General
Corporation Law of the State of California; and

            (b)   IGEN California, as sole stockholder of IGEN Delaware, shall
have approved the Merger in accordance with the General Corporation Law of the
State of Delaware; and

            (c)   any and all consents, permits, authorizations, approvals, and
orders deemed in the sole discretion of IGEN California to be material to
consummation of the Merger shall have been obtained.

     3.4.   Abandonment or Deferral. At any time before the Effective Date, this
Merger Agreement may be terminated and the Merger may be abandoned by the Board
of Directors of either IGEN California or IGEN Delaware or both, notwithstanding
the approval of this Merger Agreement by the shareholders of IGEN California or
IGEN Delaware, or the consummation of the Merger may be deferred for a
reasonable period of time if, in the opinion of the Board of Directors of IGEN
California and IGEN Delaware, such action would be in the best interest of such
corporations. In the event of termination of this Merger Agreement, this Merger
Agreement shall become void and of no effect and there shall be no liability on
the part of either Constituent Corporation or its Board of Directors or
shareholders with respect thereto, except that IGEN California shall pay all
expenses incurred in connection with the Merger or in respect of this Merger
Agreement or relating thereto.

     3.5.   Counterparts.  In order to facilitate the filing and recording of
this Merger Agreement, the same may be executed in any number of counterparts,
each of which shall be deemed to be an original.

                                      4.

 
     IN WITNESS WHEREOF, this Merger Agreement, having first been duly approved
by the Board of Directors of IGEN California and IGEN Delaware, is hereby
executed on behalf of each said corporation and attested by their respective
officers thereunto duly authorized.

                                      IGEN, Inc.
                                      a California corporation



                                      By /s/ Richard J. Massey
                                        ----------------------------------------
                                         Richard J. Massey
                                         President and Chief Operating Officer


ATTEST:



/s/ Andrei M. Manoliu
- ----------------------------------
Andrei M. Manoliu
Secretary


                                      IGEN International, Inc.
                                      a Delaware corporation



                                      By /s/ Richard J. Massey
                                        ----------------------------------------
                                         Richard J. Massey
                                         President and Chief Operating Officer  
                                           


ATTEST:



/s/ Andrei M. Manoliu
- ---------------------------------- 
Andrei M. Manoliu
Secretary


                                      5.

 
                            IGEN International, INC.
                  OFFICERS' CERTIFICATE OF APPROVAL OF MERGER


     The undersigned, Richard J. Massey and Andrei M. Manoliu, do hereby
certify that:

     1.   They are the President and Chief Operating Officer and Secretary,
respectively, of IGEN International, Inc., a Delaware corporation (the
"Corporation").

     2.   The Agreement and Plan of Merger attached to this Certificate
providing for the merger of IGEN, Inc., a California corporation, with and into
the Corporation was duly approved by the Board of Directors and by the
stockholder of the Corporation. 

     3.   The Corporation has two authorized classes of shares designated
Common Stock and Preferred Stock.  The number of shares of Common Stock
outstanding and entitled to vote upon the merger was One Thousand (1,000) shares
of Common Stock.  There are no shares of Preferred Stock outstanding. 

     4.   The terms of the Agreement and Plan of Merger were approved by the
Corporation by the vote of more than fifty percent (50%) of the outstanding
shares of Common Stock of the Corporation, which equaled or exceeded the vote
required. 

     IN WITNESS WHEREOF, the undersigned have executed this Certificate this
10th day of September, 1996.




                                      /s/ Richard J. Massey
                                      ----------------------------------------
                                      Richard J. Massey
                                      President and Chief Operating Officer



                                      /s/ Andrei M. Manoliu
                                      ----------------------------------------
                                      Andrei M. Manoliu
                                      Secretary


        

 
     Each of the undersigned declares under penalty of perjury that he has read
the foregoing Certificate and knows the contents thereof and that the same is
true of his own knowledge.

     Executed at Gaithersburg, Maryland on September 10, 1996.



                                          
                                         /s/ Richard J. Massey
                                      -----------------------------------------
                                         Richard J. Massey
                                         President and Chief Operating Officer



                                        /s/ Andrei M. Manoliu
                                      -----------------------------------------
                                         Andrei M. Manoliu
                                         Secretary


                                      2.

 
                                   IGEN, INC.
                  OFFICERS' CERTIFICATE OF APPROVAL OF MERGER


     The undersigned, Richard J. Massey and Andrei M. Manoliu, do hereby
certify that:

     1.   They are the President and Chief Operating Officer and Secretary,
respectively, of IGEN, Inc., a California corporation (the "Corporation").

     2.   The Agreement and Plan of Merger attached to this Certificate
providing for the merger of the Corporation, with and into IGEN International,
Inc., a Delaware Corporation, was duly approved by the Board of Directors and by
the shareholders of the Corporation. 

     3.  The Corporation has two authorized classes of shares, designated Common
Stock and Preferred Stock. The total number of outstanding shares of Common
Stock as of September 10, 1996 was 14,965,476. No shares of Preferred Stock
are outstanding. All outstanding shares of stock were entitled to vote on the
merger.

     4.  The terms of the Agreement and Plan of Merger were approved by the
Corporation by the vote of more than fifty percent (50%) of the outstanding
shares of Common Stock, which equaled or exceeded the vote required. 

     IN WITNESS WHEREOF, the undersigned have executed this Certificate this
10th day of September, 1996.



 
                                      /s/ Richard J. Massey
                                      ----------------------------------------
                                          Richard J. Massey
                                          President and Chief Operating Officer


 
                                      /s/ Andrei M. Manoliu
                                      ----------------------------------------
                                         Andrei M. Manoliu
                                         Secretary




 
     Each of the undersigned declares under penalty of perjury that he has read
the foregoing Certificate and knows the contents thereof and that the same is
true of his own knowledge.

     Executed at Gaithersburg, Maryland on September 10, 1996.



 
                                        /s/ Richard J. Massey
                                      -----------------------------------------
                                         Richard J. Massey
                                         President and Chief Operating Officer




                                        /s/ Andrei M. Manoliu
                                      ----------------------------------------- 
                                         Andrei M. Manoliu
                                         Secretary



                                      2.