February 9, 2001


Acqua Wellington North American Equities Fund, Ltd.
C/o Fortis Fund Services Ltd.
Montague Sterling Centre
East Bay Street, P.O. Box SS-6238
Nassau, Bahamas


Dear Sirs:

         This letter sets forth the agreement  ("Agreement") of Acqua Wellington
North American  Equities Fund, Ltd. (the  "Purchaser")  and IGEN  International,
Inc. (the "Company") regarding the purchase by the Purchaser from the Company of
the Company's common stock (the "Common Stock") on the date hereof.  The parties
agree as follows:

1.            This  Agreement  relates  to  the  purchase  by  the  Purchaser of
         211,900  shares  of  the  Company's  Common  Stock  ("Shares")  for  an
         aggregate purchase price of $3,000,000, which purchase is being settled
         by the parties on February 12, 2001.

2.            The Company is a corporation  duly incorporated, validly  existing
         and in good  standing  under the laws of Delaware.  The Company has the
         requisite  corporate power and authority to enter into and perform this
         Agreement and to issue and sell the Shares in accordance with the terms
         hereof.  The execution,  delivery and  performance of this Agreement by
         the Company and the consummation by it of the transactions contemplated
         hereby have been duly and validly authorized by all necessary corporate
         action.  A copy  of the  duly  executed  resolutions  of the  Board  of
         Directors  of the  Company  is  attached  hereto as Exhibit  "B".  This
         Agreement has been duly executed and delivered on behalf of the Company
         by a duly  authorized  officer.  A copy of a duly  executed  incumbency
         certificate  of the  Company's  duly  authorized  officers  is attached
         hereto as Exhibit "C". This Agreement constitutes,  or shall constitute
         when  executed and  delivered,  a valid and binding  obligation  of the
         Company  enforceable  against the Company in accordance with its terms,
         except  as  may  be  limited  by  applicable  bankruptcy,   insolvency,
         reorganization, moratorium, liquidation, conservatorship,  receivership
         or similar laws relating to, or affecting  generally the enforcement of
         creditors'  rights and  remedies or by other  equitable  principles  of
         general application.

3.            The  Shares  to  be  issued  under  this  Agreement have been duly
         authorized  by all  necessary  corporate  action and, when paid for and
         issued in accordance with the terms hereof, the Shares shall be validly
         issued and outstanding, fully paid and nonassessable, and the Purchaser
         shall be entitled to all rights accorded to a holder of Common Stock of
         the Company.

4.            The  Company  represents  and  warrants  that (a) the Shares  have
         been  registered  under the  Securities  Act of 1933,  as amended  (the
         "Securities  Act"),  pursuant to a registration  statement on Form S-3,
         Commission File Number 333-53086 (the  "Registration  Statement");  and
         (b) the Company  shall  contemporaneously  with the  execution  of this

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         purchase   agreement   file  a   prospectus   supplement   ("Prospectus
         Supplement")  to the  Registration  Statement in  connection  with this
         transaction. Copies of the Registration Statement as filed and declared
         effective by the Securities and Exchange Commission ("Commission"), and
         the Prospectus  Supplement in the form proposed to be filed are annexed
         hereto as Exhibits "D" and "E", respectively.

5.            The  Company has  taken or will  take all action  necessary on its
         part to list  the  Shares  for  trading  on the  NASDAQ  system  or any
         relevant market or system, if applicable.

6.            The  Company  will  continue  to  take  all  action  necessary  to
         continue  to be  registered  under  Sections  12(b)  or  12(g)  of  the
         Securities  Exchange Act of 1934 ("Exchange Act") and will continue the
         listing or trading of its Common Stock on the NASDAQ National Market or
         any relevant  market or system,  if applicable,  and will comply in all
         material  respects with the Company's  reporting,  listing  (including,
         without  limitation,  the  listing  of  the  Shares  purchased  by  the
         Purchaser) or other  obligations under the rules of the NASDAQ National
         Market or any relevant market or system.

7.            The  Company  has  delivered or  made  available to  the Purchaser
         true and complete  copies of the filings filed by the Company under the
         Exchange   Act  with  the   Commission   since   September   30,   2000
         (collectively,  the "Commission Filings"). The Company has not provided
         to the Purchaser any  information  which,  according to applicable law,
         rule or regulation,  should have been disclosed publicly by the Company
         but which has not been so  disclosed,  other  than with  respect to the
         transactions  contemplated  by this Agreement.  As of their  respective
         dates, each of the Commission Filings complied in all material respects
         with the requirements of the Exchange Act and the rules and regulations
         of the Commission  promulgated  thereunder and other federal, state and
         local laws, rules and regulations applicable to such documents, and, as
         of their respective dates,  none of the Commission  Filings referred to
         above  contained any untrue  statement of a material fact or omitted to
         state a material  fact  required to be stated  therein or  necessary in
         order to make the  statements  therein,  in light of the  circumstances
         under which they were made, not misleading. The financial statements of
         the Company included in the Commission Filings comply as to form in all
         material  respects  with  applicable  accounting  requirements  and the
         published rules and  regulations of the Commission or other  applicable
         rules and regulations  with respect  thereto.  No event or circumstance
         has occurred or exists with respect to the Company or its  subsidiaries
         or their respective businesses,  properties,  prospects,  operations or
         financial  condition,  which, under applicable law, rule or regulation,
         requires public disclosure or announcement by the Company but which has
         not been so publicly announced or disclosed.

8.            The Company  will  promptly  notify the  Purchaser of (a) any stop
         order or other  suspension  of the  effectiveness  of the  Registration
         Statement  and (b) the  happening of any event as a result of which the
         prospectus  included in the Registration  Statement  includes an untrue
         statement of a material fact or omits to state a material fact required
         to be stated therein,  or necessary to make the statements  therein, in
         light of the circumstances under which they were made, not misleading.

9.            The  Purchaser is  a limited  liability  company  duly  organized,
         validly   existing  and  in  good  standing   under  the  laws  of  the
         Commonwealth of The Bahamas.  The Purchaser has the requisite corporate
         authority to enter into and to perform this  Agreement  and to purchase
         the Shares in accordance with the terms hereof. The execution, delivery
         and performance of this Agreement by Purchaser and the  consummation of
         the transactions  contemplated  hereby have been duly authorized by all
         necessary  corporate action, and no further consent or authorization of
         the Purchaser, its Board of Directors or stockholders is required. This


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         Agreement constitutes, or shall constitute when executed and delivered,
         a valid and binding obligation of the Purchaser enforceable against the
         Purchaser  in  accordance  with its terms,  except as may be limited by
         applicable   bankruptcy,   insolvency,   reorganization,    moratorium,
         liquidation, conservatorship, receivership or similar laws relating to,
         or  affecting  generally  the  enforcement  of  creditors'  rights  and
         remedies  or by other  equitable  principles  of  general  application.
         Purchaser  shall comply with all applicable  federal  securities  laws,
         rules  and  regulations  in  connection  with  the  sale of the  Shares
         purchased by the Purchaser  hereunder.  The Purchaser shall confirm the
         accuracy  of  the   information   regarding   the   Purchaser  and  the
         distribution  of  the  Shares  as  is  disclosed  in  the  Registration
         Statement,  the Prospectus or any Prospectus Supplement.  The Purchaser
         shall sell the Shares  purchased  hereunder in accordance with the Plan
         of  Distribution   described  in  the  Registration  Statement  or  the
         Prospectus or any applicable Prospectus Supplement.

10.           The  Company may not issue  a press  release or  otherwise  make a
         public statement or announcement  with respect to the completion of the
         transaction  contemplated  hereby  without  the  prior  consent  of the
         Purchaser,  which will not be  unreasonably  withheld,  conditioned  or
         delayed.  The  parties  agree that the form of press  release  attached
         hereto as Exhibit F is approved for release by the Company.

11.           Each party will  indemnify  the other party as provided in Exhibit
         "A" attached hereto against  liability with respect to the Registration
         Statement (including,  without limitation,  the Prospectus  Supplement)
         relating  to the  Shares  which are being  sold by the  Company  to the
         Purchaser hereunder.  For purposes of said Exhibit A, capitalized terms
         used therein without definition shall have the same meanings therein as
         are ascribed to said terms in this Agreement.

12.           This Agreement and the legal relations  between the parties hereto
         with respect to any purchase of Shares by the Purchaser hereunder shall
         be governed and construed in accordance  with the  substantive  laws of
         the State of New York  without  giving  effect to the  conflicts of law
         principles thereunder.

13.           This Agreement  contains the entire  understanding  of the parties
         with respect to the matters  covered hereby and, except as specifically
         set forth  herein,  neither  the Company  nor the  Purchaser  makes any
         representation,  warranty, covenant or undertaking with respect to such
         matters.



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<PAGE>

         Delivery of an executed copy of a signature  page to this  Agreement by
facsimile  transmission  shall be effective  as delivery of a manually  executed
copy of this Agreement and shall be effective and enforceable as the original.

         Please  execute  a copy of this  letter  which,  when  executed  by the
Purchaser, will constitute an Agreement between the Company and the Purchaser.

                                     Very truly yours,

                                     IGEN INTERNATIONAL, INC.



                                     By:  /s/ Samuel J. Wohlstadter
                                          --------------------------
                                     Name:  Samuel J. Wohlstadter
                                     Title: Chairman and Chief Executive Officer

AGREED TO:

PURCHASER:

ACQUA WELLINGTON NORTH
AMERICAN EQUITIES FUND, LTD.


By:    /s/ Anthony L.M. Inder Rieden
       ----------------------------
Name:  Anthony L.M. Inder Rieden
Title: Director











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<PAGE>


                                   Exhibit 'A'

                            Terms of Indemnification

(a)      INDEMNIFICATION  BY THE COMPANY.  The Company  will  indemnify and hold
         harmless  the  Purchaser  and each  person,  if any,  who  controls the
         Purchaser  within the  meaning of Section 15 of the  Securities  Act of
         1933,  as amended  (the  "Securities  Act"),  or  Section  20(a) of the
         Securities  Exchange Act of 1934, as amended (the "Exchange Act"), from
         and  against  any  losses,  claims,  damages,  liabilities,  costs  and
         expenses  (including,  without limitation,  reasonable costs of defense
         and  investigation  and all attorneys'  fees and expenses) to which the
         Purchaser  and each person,  if any, who  controls  the  Purchaser  may
         become subject, under the Securities Act or otherwise,  insofar as such
         losses,  claims,  damages,  liabilities  and  expenses  (or  actions in
         respect  thereof)  arise  out of,  or are  based  upon  (i) any  untrue
         statement or alleged untrue statement of a material fact contained,  or
         incorporated by reference,  in the Registration  Statement  relating to
         Shares being sold to the  Purchaser  (including  the  prospectus  dated
         January 10, 2001,  the  prospectus  supplement  dated February 12, 2001
         (the  "Prospectus  Supplement")  which are or will become a part of the
         Registration  Statement),   or  any  amendment  or  supplement  to  the
         Registration  Statement,  or (ii) the  omission or alleged  omission to
         state in that  Registration  Statement or any document  incorporated by
         reference in the Registration Statement, a material fact required to be
         stated  therein  or  necessary  to  make  the  statements  therein  not
         misleading (an "Indemnifiable Matter").

         The Company will  reimburse  the  Purchaser  and each such  controlling
         person  promptly  upon  demand for any legal or other costs or expenses
         reasonably  incurred  by the  Purchaser  or the  controlling  person in
         investigating,  defending  against,  or preparing to defend against any
         claim relating to an Indemnifiable Matter, except that the Company will
         not be  liable to the  extent  such  claim,  suit or  proceeding  which
         results in a loss, claim,  damage,  liability or expense arises out of,
         or is based  upon,  an  untrue  statement,  alleged  untrue  statement,
         omission or alleged omission,  included in any Prospectus Supplement or
         any amendment or supplement  to the  Prospectus  Supplement in reliance
         upon,  and in conformity  with,  written  information  furnished by the
         Purchaser to the Company for inclusion in the Prospectus Supplement.

(b)      INDEMNIFICATION BY THE PURCHASER.  The  Purchaser  will  indemnify  and
         hold harmless the Company, each of its directors and officers, and each
         person,  if any, who controls the Company within the meaning of Section
         15 of the  Securities Act or Section 20(a) of the Exchange Act from and
         against  any  losses,   claims,   damages,   liabilities  and  expenses
         (including  reasonable  costs  of  defense  and  investigation  and all
         attorneys'  fees) to which the Company and each  director,  officer and
         person, if any, who controls the Company may become subject,  under the
         Securities Act or otherwise,  insofar as such losses, claims,  damages,
         liabilities  and expenses (or actions in respect  thereof) arise out of
         or are based upon (i) any untrue  statement or alleged untrue statement
         of a material  fact  contained  in the  Registration  Statement  or any
         Prospectus or  Prospectus  Supplement or any amendment or supplement to
         it  or  (ii)  the  omission  or  alleged   omission  to  state  in  the
         Registration  Statement or any  Prospectus or Prospectus  Supplement or
         any amendment or supplement to it a material fact required to be stated
         therein or necessary to make the statements therein not misleading,  to
         the  extent,  but only to the  extent,  the untrue  statement,  alleged
         untrue  statement,  omission or alleged  omission  was made in reliance
         upon,  and in conformity  with,  written  information  furnished by the
         Purchaser to the Company for inclusion in the  Registration  Statement,



<PAGE>

         the Prospectus or Prospectus  Supplement or any amendment or supplement
         thereto,  and the  Purchaser  will  reimburse the Company and each such
         director,  officer or controlling  person  promptly upon demand for any
         legal or other costs or expense  reasonably  incurred by the Company or
         the other person in investigating,  defending against,  or preparing to
         defend against any such claim, action, suit or proceeding.

(c)      INDEMNIFICATION  PROCEDURES.  Promptly after a  person  receives notice
         of a claim or the  commencement  of an action,  suit or proceeding  for
         which the person intends to seek indemnification under Paragraph (a) or
         (b),  the person will notify the  indemnifying  party in writing of the
         claim or commencement of the action, suit or proceeding, but failure to
         notify the indemnifying  party will not relieve the indemnifying  party
         from  liability  under  Paragraph (a) or (b),  except to the extent the
         indemnifying  party has been  materially  prejudiced  by the failure to
         give notice.  The indemnifying party will be entitled to participate in
         the  defense  of any  claim,  action,  suit or  proceeding  as to which
         indemnification  is being sought,  and the indemnifying  party may (but
         will not be required to) assume the defense against the claim,  action,
         suit or proceeding with legal counsel chosen by the indemnifying party.
         After an  indemnifying  party  notifies an  indemnified  party that the
         indemnifying  party  wishes to assume the  defense of a claim,  action,
         suit or proceeding  the  indemnifying  party will not be liable for any
         legal or other expenses incurred by the indemnified party in connection
         with the defense against the claim, action, suit or proceeding,  except
         that if, in the opinion of legal counsel to the indemnifying party, one
         or more of the indemnified parties should be separately  represented in
         connection with a claim,  action,  suit or proceeding the  indemnifying
         party will pay the fees and  expenses of one  separate  counsel for the
         indemnified  parties.  Each indemnified party, as a condition precedent
         to receiving  indemnification as provided in Paragraph (a) or (b), will
         cooperate in all reasonable respects with the indemnifying party in the
         defense  of  the  claim,   action,  suit  or  proceeding  as  to  which
         indemnification is sought. No indemnifying party will be liable for any
         settlement of any claim,  action,  suit or proceeding  effected without
         its prior written  consent.  No  indemnifying  party will,  without the
         prior written consent of the indemnified  party,  effect any settlement
         of a pending or threatened  claim,  action or  proceeding  with respect
         which an  indemnified  party is, or is informed  that it may be, made a
         party and for which it would be entitled to indemnification, unless the
         settlement  includes an unconditional  release of the indemnified party
         from all  liability  and  claims  which are the  subject  matter of the
         pending or threatened action.

(d)      CONTRIBUTION.  If for  any  reason  the  indemnification  provided  for
         in this  Agreement is not  available  to, or is not  sufficient to hold
         harmless,  an indemnified party in respect of any loss, claim,  damage,
         liability,  cost or expense  referred to in Paragraph  (a) or (b), each
         indemnifying party will, in lieu of indemnifying the indemnified party,
         contribute  to the amount  paid or payable  by the  indemnified  party,
         contribute to the amount paid or payable by the indemnified  party as a
         result of the loss, claim,  damage,  liability,  cost or expense (i) in
         the proportion  which is  appropriate to reflect the relative  benefits
         received  by the  indemnifying  party,  on  the  one  hand,  and by the
         indemnified  party,  on the other hand, from the sale of stock which is
         the subject of the claim,  action, suit or proceeding which resulted in
         the loss, claim, liability,  cost or expense or (ii) if that allocation
         is  not  permitted  by  applicable   law,  in  such  proportion  as  is
         appropriate  to reflect not only the  relative  benefits of the sale of
         stock,  but also the relative fault of the  indemnifying  party and the
         indemnified party with respect to the statements or omissions which are
         the subject of the claim,  action,  suit or proceeding that resulted in
         the loss,  claim,  damage,  liability,  cost or  expense as well as any
         other relevant equitable considerations.