Exhibit 10.41


                          REGISTRATION RIGHTS AGREEMENT

     This Registration Rights Agreement is made and entered into as of March 26,
2002 (this  "Agreement"),  by and between IGEN  International,  Inc., a Delaware
corporation (the "Company"),  Citadel Equity Fund Ltd., a Cayman Islands company
(the "Purchaser").

     This  Agreement is being entered into  pursuant to the Purchase  Agreement,
dated as of the date hereof,  by and between the Company and the Purchaser  (the
"Purchase Agreement").

     The Company and the Purchaser hereby agree as follows:

     1. Definitions.

     Capitalized  terms used and not  otherwise  defined  herein  shall have the
meanings given such terms in the Purchase Agreement.  As used in this Agreement,
the following terms shall have the following meanings:

     "Affiliate"  means,  with  respect to any  Person,  any other  Person  that
directly or indirectly controls or is controlled by or under common control with
such Person.  For the  purposes of this  definition,  "control,"  when used with
respect to any Person, means the possession, direct or indirect, of the power to
direct or cause the  direction  of the  management  and policies of such Person,
whether  through the ownership of voting  securities,  by contract or otherwise;
and  the  terms  "affiliated,"  "controlling"  and  "controlled"  have  meanings
correlative to the foregoing.

     "Blackout Period" shall have the meaning set forth in Section 3(o).

     "Board" shall have the meaning set forth in Section 3(o).

     "Business  Day"  means any day  except  Saturday,  Sunday and any day which
shall be a legal holiday or a day on which banking  institutions in the state of
New York generally are authorized or required by law or other government actions
to close.

     "Closing Date" shall have the meaning set forth in the Purchase Agreement.

     "Commission" means the Securities and Exchange Commission.

     "Common Stock" means the Company's Common Stock, $.001 par value.

     "Effectiveness  Date" means with respect to the Registration  Statement the
earliest of (x) the 105th day  following  the  Closing  Date,  before  which the
Company will use its commercially  reasonable  efforts to cause the registration
statement  to  become  effective,  and (y) the date  which is  within  seven (7)
Business Days of the date on which the  Commission  informs




<PAGE>

the Company that the Commission (i) will not review the  Registration  Statement
or (ii) that the Company may request the  acceleration of the  effectiveness  of
the Registration Statement.

     "Effectiveness Period" shall have the meaning set forth in Section 2.

     "Event" shall have the meaning set forth in Section 7(d).

     "Exchange Act" means the Securities Exchange Act of 1934, as amended.

     "Filing  Date"  means the date the  Registration  Statement  is filed which
date,  using the  Company's  commercially  reasonable  efforts,  shall be within
twenty-one (21) days following the Closing Date.

     "Holder" or "Holders" means the holder or holders, as the case may be, from
time to time of Registrable Securities including,  including without limitation,
the Purchaser and its assignees.

     "Indemnified Party" shall have the meaning set forth in Section 5(c).

     "Indemnifying Party" shall have the meaning set forth in Section 5(c).

     "Liquidated Damages" shall have the meaning set forth in Section 7(d).

     "Losses" shall have the meaning set forth in Section 5(a).

     "Person"  means  an  individual  or  a  corporation,   partnership,  trust,
incorporated or  unincorporated  association,  joint venture,  limited liability
company, joint stock company,  government (or an agency or political subdivision
thereof) or other entity of any kind.

     "Proceeding"  means an action,  claim,  suit,  investigation  or proceeding
(including,  without limitation, an investigation or partial proceeding, such as
a deposition), whether commenced or threatened.

     "Prospectus"  means the prospectus  included in the Registration  Statement
(including,  without  limitation,  a prospectus  that  includes any  information
previously omitted from a prospectus filed as part of an effective  registration
statement in reliance upon Rule 430A  promulgated  under the Securities Act), as
amended or supplemented by any prospectus supplement,  with respect to the terms
of the  offering of any  portion of the  Registrable  Securities  covered by the
Registration  Statement,  and  all  other  amendments  and  supplements  to  the
Prospectus,  including post-effective  amendments, and all material incorporated
by reference in such Prospectus.

     "Registrable  Securities"  means the shares of Common Stock issued pursuant
to the Purchase  Agreement (the "Common Shares") and upon any stock split, stock
dividend, recapitalization or similar event with respect to such Common Shares.


<PAGE>

     "Registration   Statement"  means  the   registration   statement  and  any
additional registration statements contemplated by Section 2, including (in each
case) the Prospectus,  amendments and supplements to such registration statement
or  Prospectus,  including  pre- and  post-effective  amendments,  all  exhibits
thereto,  and all  material  incorporated  by  reference  in  such  registration
statement.

     "Rule 144" means Rule 144  promulgated  by the  Commission  pursuant to the
Securities  Act, as such Rule may be amended  from time to time,  or any similar
rule or regulation  hereafter adopted by the Commission having substantially the
same effect as such Rule.

     "Rule 158" means Rule 158  promulgated  by the  Commission  pursuant to the
Securities  Act, as such Rule may be amended  from time to time,  or any similar
rule or regulation  hereafter adopted by the Commission having substantially the
same effect as such Rule.

     "Rule 415" means Rule 415  promulgated  by the  Commission  pursuant to the
Securities  Act, as such Rule may be amended  from time to time,  or any similar
rule or regulation  hereafter adopted by the Commission having substantially the
same effect as such Rule.

     "Securities Act" means the Securities Act of 1933, as amended.

     "Special Counsel" means any special counsel to the Holders.

     2.  Registration.  On or prior to the Filing Date the Company shall prepare
and file with the  Commission  a "shelf"  Registration  Statement  covering  all
Registrable Securities for an offering to be made on a continuous basis pursuant
to Rule 415.  The  Registration  Statement  shall be on Form S-3  (except if the
Company is not then eligible to register for resale the  Registrable  Securities
on Form S-3,  in which case such  registration  shall be on another  appropriate
form in accordance  herewith).  The Company shall (i) not permit any  securities
other  than  the  Registrable  Securities  to be  included  in the  Registration
Statement,   (ii)  use  its  commercially   reasonable   efforts  to  cause  the
Registration  Statement  to be  declared  effective  under  the  Securities  Act
(including   filing  with  the   Commission  a  request  for   acceleration   of
effectiveness in accordance with Rule 12dl-2  promulgated under the Exchange Act
within seven (7) Business Days of the date that the Company is notified  (orally
or in  writing,  whichever  is earlier) by the  Commission  that a  Registration
Statement will not be  "reviewed," or not be subject to further  review) as soon
as  practicable  after  the  filing  thereof,  but in  any  event  prior  to the
Effectiveness  Date,  and (subject to the  provisions of Section 3(o) hereof) to
keep such Registration Statement continuously effective under the Securities Act
until  such  date  as is the  earlier  of (x)  the  date  when  all  Registrable
Securities covered by such Registration Statement have been sold or (y) the date
on which the Registrable Securities may be sold without any restriction pursuant
to Rule 144(k) as determined by the counsel to the Company pursuant to a written
letter,   addressed  to  the  Company's  transfer  agent  to  such  effect  (the
"Effectiveness Period").


<PAGE>

     3. Registration Procedures.

     In connection with the Company's registration  obligations  hereunder,  the
Company shall:

     (a) Prepare and file with the Commission as soon as practicable  and in any
event on or prior to the Filing Date, a  Registration  Statement on Form S-3 (or
if the  Company is not then  eligible  to  register  for resale the  Registrable
Securities on Form S-3 such registration shall be on another appropriate form in
accordance  herewith) in accordance  with the method or methods of  distribution
thereof as specified by the Holder (except if subsequently otherwise directed by
the  Holder),  and  use  its  commercially   reasonable  efforts  to  cause  the
Registration  Statement  to become  effective  and remain  effective as provided
herein;  provided,  however, that not less than three (3) Business Days prior to
the  filing of the  Registration  Statement  or any  related  Prospectus  or any
amendment  or  supplement   thereto   (including  any  document  that  would  be
incorporated therein by reference),  the Company shall furnish to the Holder and
any Special Counsel,  copies of all such documents  proposed to be filed,  which
documents  (other than those  incorporated  by reference) will be subject to the
timely  review  of  such  Special  Counsel.  The  Company  shall  not  file  the
Registration  Statement or any such  Prospectus or any amendments or supplements
thereto  relating to the Purchaser or the  transactions  contemplated  hereby to
which the  Holder or any  Special  Counsel  shall  reasonably  object in writing
within three (3) Business Days of their receipt thereof; provided, however, that
if Special  Counsel does not respond within such three-day  period,  the Company
shall not incur any liquidated  damages  pursuant to Section  7(d)(i) hereof for
the time period of such delay.

     (b) (i) Prepare and file with the  Commission  such  amendments,  including
post-effective  amendments, to the Registration Statement as may be necessary to
keep the Registration  Statement continuously effective (subject to Section 3(o)
hereof) as to the applicable Registrable Securities for the Effectiveness Period
and prepare and file with the Commission such additional Registration Statements
in order to register for resale under the Securities Act all of the  Registrable
Securities;  (ii) cause the related  Prospectus to be amended or supplemented by
any required  Prospectus  supplement,  and as so  supplemented  or amended to be
filed pursuant to Rule 424 (or any similar provisions then in force) promulgated
under the Securities Act; (iii) respond  promptly to any comments  received from
the  Commission  with respect to the  Registration  Statement  or any  amendment
thereto  and  promptly  provide  the  Holders  true and  complete  copies of all
correspondence from the Commission relating to the Registration  Statement;  and
(iv) comply in all material  respects with the  provisions of the Securities Act
and  the  Exchange  Act  with  respect  to the  disposition  of all  Registrable
Securities covered by the Registration Statement during the applicable period in
accordance  with the intended  methods of disposition by the Holders thereof set
forth in the  Registration  Statement as so amended or in such  Prospectus as so
supplemented.

     (c) Notify the Holders of Registrable Securities to be sold and any Special
Counsel  promptly  (and,  in the case of (i)(A)  below,  not less than three (3)
Business  Days  prior to such  filing)  and (if  requested  by any such  Person)
confirm such notice in writing no later than three (3) Business  Days  following
the day (i)(A) when a Prospectus or any Prospectus  supplement or post-effective
amendment to the  Registration  Statement is proposed to be filed;




<PAGE>

(B) when the Commission notifies the Company whether there will be a "review" of
such Registration  Statement and whenever the Commission  comments in writing on
such Registration  Statement and (C) with respect to the Registration  Statement
or any  post-effective  amendment  relating to the Purchaser or the transactions
contemplated hereby, when the same has become effective; provided, however, that
if Special  Counsel does not respond within such time period,  the Company shall
not incur any liquidated damages pursuant to Section 7(d)(i) hereof for the time
period of such delay; (ii) of any request by the Commission or any other Federal
or  state   governmental   authority  for   amendments  or  supplements  to  the
Registration Statement or Prospectus or for additional information; (iii) of the
issuance by the Commission of any stop order suspending the effectiveness of the
Registration  Statement covering any or all of the Registrable Securities or the
initiation  of any  Proceedings  for that  purpose;  (iv) of the  receipt by the
Company of any notification  with respect to the suspension of the qualification
or exemption from qualification of any of the Registrable Securities for sale in
any  jurisdiction,  or the  initiation or threatening of any Proceeding for such
purpose; and (v) of the occurrence of any event that makes any statement made in
the Registration  Statement or Prospectus or any document incorporated or deemed
to be incorporated  therein by reference  untrue in any material respect or that
requires  any  revisions  to the  Registration  Statement,  Prospectus  or other
documents so that, in the case of the Registration  Statement or the Prospectus,
as the case may be, it will not contain any untrue  statement of a material fact
or omit to state any material fact required to be stated therein or necessary to
make the statements  therein, in the light of the circumstances under which they
were made, not misleading.

     The Company shall promptly furnish to Special Counsel,  without charge, any
correspondence  from the Commission or the Commission's  staff to the Company or
its representatives relating to any Registration Statement.

     (d) Use its commercially  reasonable  efforts to avoid the issuance of, or,
if issued,  obtain the withdrawal of, (i) any order suspending the effectiveness
of the Registration  Statement or (ii) any suspension of the  qualification  (or
exemption from  qualification) of any of the Registrable  Securities for sale in
any jurisdiction, at the earliest practicable moment.

     (e) If requested by the Holder of a majority in interest of the Registrable
Securities,   (i)   promptly   incorporate   in  a  Prospectus   supplement   or
post-effective  amendment to the Registration  Statement such information as the
Company  reasonably agrees should be included therein and (ii) make all required
filings of such Prospectus  supplement or such post-effective  amendment as soon
as practicable after the Company has received  notification of the matters to be
incorporated in such Prospectus supplement or post-effective amendment.

     (f) Furnish to the Holder and any Special Counsel, without charge, at least
one conformed copy of each  Registration  Statement and each amendment  thereto,
including  financial  statements and schedules,  all documents  incorporated  or
deemed to be incorporated  therein by reference,  and all exhibits to the extent
requested by such Person  (including those previously  furnished or incorporated
by reference) promptly after the filing of such documents with the Commission.


<PAGE>

     (g) Promptly deliver to the Holder and any Special Counsel, without charge,
as  many  copies  of the  Registration  Statement,  Prospectus  or  Prospectuses
(including each form of prospectus) and each amendment or supplement  thereto as
such Persons may reasonably request;  and the Company hereby consents to the use
of such  Prospectus  and each  amendment  or  supplement  thereto by each of the
selling  Holders in  connection  with the offering  and sale of the  Registrable
Securities covered by such Prospectus and any amendment or supplement thereto.

     (h)  Prior  to any  public  offering  of  Registrable  Securities,  use its
commercially  reasonable  efforts to register or qualify or  cooperate  with the
selling Holders and any Special  Counsel in connection with the  registration or
qualification  (or exemption from such  registration or  qualification)  of such
Registrable  Securities for offer and sale under the securities or Blue Sky laws
of such jurisdictions within the United States as any Holder reasonably requests
in  writing,  to keep each such  registration  or  qualification  (or  exemption
therefrom)  effective during the  Effectiveness  Period (subject to Section 3(o)
hereof) and to do any and all other acts or things  necessary  or  advisable  to
enable the  disposition  in such  jurisdictions  of the  Registrable  Securities
covered by a Registration Statement;  provided,  however, that the Company shall
not be required to qualify generally to do business in any jurisdiction where it
is not then so qualified or to take any action that would  subject it to general
service of process in any such  jurisdiction  where it is not then so subject or
subject the Company to any material tax in any such jurisdiction where it is not
then so subject.

     (i) Cooperate  with the Holders to facilitate  the timely  preparation  and
delivery of certificates or via the Deposit Withdrawal Agent Commission ("DWAC")
representing  Registrable  Securities  to be  sold  pursuant  to a  Registration
Statement and to enable such Registrable  Securities to be in such denominations
and  registered  in such  names as the  Holders  may  request  at least  two (2)
Business Days prior to any sale of Registrable Securities.

     (j) Upon the occurrence of any event  contemplated by Section  3(c)(v),  as
promptly  as  possible,   prepare  a  supplement  or   amendment,   including  a
post-effective  amendment,  to the Registration Statement or a supplement to the
related  Prospectus or any document  incorporated  or deemed to be  incorporated
therein  by  reference,  and file  any  other  required  document  so  that,  as
thereafter  delivered,  neither the  Registration  Statement nor such Prospectus
will contain an untrue  statement of a material fact or omit to state a material
fact required to be stated therein or necessary to make the statements  therein,
in the light of the circumstances under which they were made, not misleading.

     (k) Use its  commercially  reasonable  efforts  to  cause  all  Registrable
Securities  relating to such  Registration  Statement to be listed on the Nasdaq
National Market and any other securities exchange,  quotation system,  market or
over-the-counter  bulletin board, if any, on which similar  securities issued by
the  Company  are then  listed as and when  required  pursuant  to the  Purchase
Agreement.

     (l)  Comply  in  all  material  respects  with  all  applicable  rules  and
regulations  of the  Commission  and make  generally  available  to its security
holders  earning  statements  satisfying  the provisions of Section 11(a) of the
Securities Act and Rule 158 not later than forty-five (45) days after the end of
any twelve  (12)-month  period (or ninety  (90) days after the end of any twelve
(12)-month  period if such period is a fiscal year)  commencing on the first day
of the first  fiscal  quarter of the  Company  after the  effective  date of the
Registration  Statement,  which statement  shall conform to the  requirements of
Rule 158.


<PAGE>

     (m) If the Registration Statement refers to any Holder by name or otherwise
as the holder of any securities of the Company,  then such Holder shall have the
right to require (if such  reference  to such Holder by name or otherwise is not
required by the Securities Act or any similar federal statute then in force) the
deletion of the  reference to such Holder in any  amendment or supplement to the
Registration  Statement  filed or  prepared  subsequent  to the time  that  such
reference ceases to be required.

     (n)  Each  Holder  covenants  and  agrees  that  (i) it will  not  sell any
Registrable  Securities under the  Registration  Statement until it has received
copies of the  Prospectus as then amended or  supplemented  as  contemplated  in
Section 3(o) and notice from the Company that such  Registration  Statement  and
any  post-effective  amendments thereto have become effective as contemplated by
Section 3(c),  (ii) it and its officers,  directors or Affiliates,  if any, will
comply  with the  prospectus  delivery  requirements  of the  Securities  Act as
applicable to them in connection with sales of Registrable  Securities  pursuant
to the  Registration  Statement,  (iii) it shall comply with federal  securities
laws and (iv) it shall furnish to the Company information  regarding such Holder
and the distribution of such Registrable  Securities as is required by law to be
disclosed in the Registration  Statement,  and the Company may exclude from such
registration the Registrable  Securities of any such Holder who fails to furnish
such  information  within  a  reasonable  time  prior  to  the  filing  of  each
Registration  Statement,  supplemented  Prospectus  and/or amended  Registration
Statement.

     Each Holder agrees by its acquisition of such Registrable  Securities that,
upon receipt of a notice from the Company of the  occurrence of any event of the
kind described in Section 3(c)(ii),  3(c)(iii),  3(c)(iv),  3(c)(v), 3(c)(vi) or
3(o),  such Holder will forthwith  discontinue  disposition of such  Registrable
Securities under the  Registration  Statement until such Holder's receipt of the
copies of the  supplemented  Prospectus  and/or amended  Registration  Statement
contemplated  by Section 3(j), or until it is advised in writing (the  "Advice")
by the Company that the use of the applicable Prospectus may be resumed, and, in
either case, has received copies of any additional or supplemental  filings that
are incorporated or deemed to be incorporated by reference in such Prospectus or
Registration Statement.

     (o) If (i) there is material non-public  information  regarding the Company
which the Company's Board of Directors (the "Board")  reasonably  determines not
to be in the  Company's  best  interest to disclose and which the Company is not
otherwise  required  to  disclose,  or  (ii)  there  is a  significant  business
opportunity  (including,  but not limited to, the  acquisition or disposition of
assets  (other  than  in  the  ordinary  course  of  business)  or  any  merger,
consolidation,  tender  offer or other  similar  transaction)  available  to the
Company which the Board  reasonably  determines  not to be in the Company's best
interest to disclose and which the Company  would be required to disclose  under
the  Registration  Statement,  then the Company may suspend  effectiveness  of a
registration  statement and suspend the sale of Registrable  Securities  under a
Registration  Statement for a period not to exceed twenty (20) consecutive days,
provided  that the  Company  may not  suspend  its  obligation  pursuant to this
Section  3(o) for more than  forty-five


<PAGE>

(45) days in the  aggregate  during  any  twelve  (12)  month  period  (each,  a
"Blackout  Period");  provided,  however,  that  no  such  suspension  shall  be
permitted for consecutive  twenty (20) day periods,  arising out of the same set
of facts, circumstances or transactions.

     (p) Within two (2) Business  Days after the  Registration  Statement  which
includes the Registrable Securities is ordered effective by the Commission,  the
Company shall deliver, and shall cause legal counsel for the Company to deliver,
to the  transfer  agent  for such  Registrable  Securities  (with  copies to the
Holders  whose   Registrable   Securities  are  included  in  such  Registration
Statement)  confirmation  that the  Registration  Statement  has  been  declared
effective by the Commission in the form attached hereto as Exhibit A.

     4. Registration Expenses.

     All  reasonable  fees  and  expenses  incident  to  the  performance  of or
compliance  with this  Agreement  by the  Company  shall be borne by the Company
whether or not the  Registration  Statement  is filed or becomes  effective  and
whether or not any Registrable  Securities are sold pursuant to the Registration
Statement.  The fees and expenses  referred to in the foregoing  sentence  shall
include,  without limitation the following: (i) all registration and filing fees
(including,  without  limitation,  fees and expenses (A) with respect to filings
required to be made with the Nasdaq  National  Market and each other  securities
exchange or market on which Registrable  Securities are required hereunder to be
listed, (B) with respect to filings required to be made with the Commission, and
(C) in  compliance  with  state  securities  or Blue Sky  laws),  (ii)  printing
expenses (including,  without limitation,  expenses of printing certificates for
Registrable   Securities  and  of  printing  prospectuses  if  the  printing  of
prospectuses  is  requested  by the  holders  of a majority  of the  Registrable
Securities included in the Registration Statement),  (iii) messenger,  telephone
and delivery  expenses,  (iv) fees and  disbursements of counsel for the Company
and (v) fees and  expenses  of all other  Persons  retained  by the  Company  in
connection  with  the  consummation  of the  transactions  contemplated  by this
Agreement,  including,  without  limitation,  the Company's  independent  public
accountants  (including the expenses of any comfort letters or costs  associated
with the  delivery by  independent  public  accountants  of a comfort  letter or
comfort letters).  In addition,  the Company shall be responsible for all of its
internal   expenses   incurred  in  connection  with  the  consummation  of  the
transactions contemplated by this Agreement (including,  without limitation, all
salaries  and  expenses  of its  officers  and  employees  performing  legal  or
accounting  duties),  the  expense of any annual  audit,  the fees and  expenses
incurred in  connection  with the listing of the  Registrable  Securities on any
securities exchange as required hereunder.  The Company shall not be responsible
for any underwriting discounts and commissions, transfer taxes or the legal fees
of Special Counsel.

     5. Indemnification

     (a) Indemnification by the Company. The Company shall,  notwithstanding any
termination of this  Agreement,  indemnify and hold harmless the Purchaser,  its
permitted  assignees  and each of its  directors,  fund  managers,  officers and
employees,  and each Person who controls any such Purchaser  (within the meaning
of Section 15 of the  Securities  Act or Section 20 of the  Exchange  Act),  its
permitted assignees and the directors, fund managers,  officers and employees of
each such controlling Person, and the respective successors, assigns, estate and
<PAGE>

personal  representatives  of  each  of the  foregoing,  to the  fullest  extent
permitted  by  applicable  law,  from and against  any and all  claims,  losses,
damages,   liabilities,   penalties,   judgments,   costs  (including,   without
limitation, costs of investigation) and expenses (including, without limitation,
reasonable attorneys' fees and expenses) (collectively,  "Losses"),  arising out
of or  relating to any untrue or alleged  untrue  statement  of a material  fact
contained in the Registration Statement,  the final Prospectus,  as supplemented
or amended,  if  applicable,  or arising  out of or relating to any  omission or
alleged  omission of a material fact required to be stated  therein or necessary
to make  the  statements  therein  (in the  case  of any  Prospectus  or form of
prospectus or supplement  thereto, in the light of the circumstances under which
they were made) not  misleading,  except to the extent,  but only to the extent,
that such untrue  statements  or  omissions  are based  solely upon  information
regarding  such  Holder  furnished  in  writing to the  Company  by such  Holder
expressly for use therein, which information was reviewed and expressly approved
in  writing  by  such  Holder  and  Special  Counsel  expressly  for  use in the
Registration  Statement,  such  Prospectus  or such form of Prospectus or in any
amendment or  supplement  thereto or as a result of the failure of the Holder to
deliver a Prospectus,  as amended or supplemented,  to a purchaser in connection
with an offer or sale.  The Company  shall  notify the  Holders  promptly of the
institution, threat or assertion of any Proceeding of which the Company is aware
in  connection  with  the  transactions  contemplated  by this  Agreement.  Such
indemnity shall remain in full force and effect  regardless of any investigation
made by or on behalf of an Indemnified Party (as defined in Section 5(c) hereof)
and shall survive the transfer of the Registrable Securities by the Holders.

     (b)  Indemnification  by Purchaser.  Purchaser and its permitted  assignees
shall, severally and not jointly,  indemnify and hold harmless the Company, each
of its  directors,  officers  and  employees  and each Person who  controls  the
Company  (within the meaning of Section 15 of the  Securities Act and Section 20
of the  Exchange  Act),  and  the  directors,  officers  and  employees  of such
controlling Persons, and the respective successors, assigns, estate and personal
representatives  of each of the foregoing,  to the fullest  extent  permitted by
applicable law, from and against all Losses, as incurred,  arising solely out of
or based solely upon failure of the  Purchaser  to deliver the  prospectus  when
required  or  any  untrue   statement  of  a  material  fact  contained  in  the
Registration Statement,  any Prospectus,  or any form of prospectus,  or arising
solely out of or based solely upon any omission of a material  fact  required to
be stated  therein or necessary to make the  statements  therein (in the case of
any Prospectus or form of prospectus or supplement  thereto, in the light of the
circumstances  under which they were made) not  misleading,  to the extent,  but
only to the extent,  that such untrue  statement  or omission is contained in or
omitted  from any  information  so  furnished  in writing by such  Holder or the
Special Counsel to the Company  specifically  for inclusion in the  Registration
Statement or such  Prospectus and that such  information  was reasonably  relied
upon by the Company for use in the  Registration  Statement,  such Prospectus or
such form of prospectus or to the extent that such  information  relates to such
Holder  or  such  Holder's   proposed  method  of  distribution  of  Registrable
Securities  and was  reviewed and  expressly  approved in writing by such Holder
expressly for use in the Registration Statement, such Prospectus or such form of
Prospectus  Supplement.  Notwithstanding  anything  to  the  contrary  contained
herein,  the Holder shall be liable under this Section 5(b) for only that amount
as does not exceed the net  proceeds  to such  Holder as a result of the sale of
Registrable Securities pursuant to such Registration Statement.
<PAGE>

     (c) Conduct of  Indemnification  Proceedings.  If any  Proceeding  shall be
brought or asserted  against  any Person  entitled to  indemnity  hereunder  (an
"Indemnified  Party"),  such Indemnified  Party promptly shall notify the Person
from whom  indemnity  is sought (the  "Indemnifying  Party) in writing,  and the
Indemnifying Party shall assume the defense thereof, including the employment of
counsel reasonably  satisfactory to the Indemnified Party and the payment of all
fees and expenses  incurred in connection with defense thereof;  provided,  that
the failure of any  Indemnified  Party to give such notice shall not relieve the
Indemnifying Party of its obligations or liabilities pursuant to this Agreement,
except (and only) to the extent that it shall be finally  determined  by a court
of  competent  jurisdiction  (which  determination  is not  subject to appeal or
further review) that such failure shall have materially and adversely prejudiced
the Indemnifying Party.

     An Indemnified Party shall have the right to employ separate counsel in any
such  Proceeding  and to participate  in the defense  thereof,  but the fees and
expenses of such counsel  shall be at the expense of such  Indemnified  Party or
Parties  unless:  (1) the  Indemnifying  Party has agreed in writing to pay such
fees and expenses;  or (2) the Indemnifying  Party shall have failed promptly to
assume  the  defense  of  such  Proceeding  and  to  employ  counsel  reasonably
satisfactory to such Indemnified Party in any such Proceeding;  or (3) the named
parties to any such Proceeding  (including any impleaded  parties)  include both
such Indemnified  Party and the Indemnifying  Party, and such Indemnified  Party
shall have been  advised by counsel  that a conflict  of  interest  is likely to
exist if the same  counsel  were to  represent  such  Indemnified  Party and the
Indemnifying  Party (in which  case,  if such  Indemnified  Party  notifies  the
Indemnifying  Party in writing that it elects to employ separate  counsel at the
expense of the Indemnifying  Party,  the  Indemnifying  Party shall not have the
right to assume the defense  thereof and such counsel shall be at the expense of
the  Indemnifying  Party).  The  Indemnifying  Party shall not be liable for any
settlement of any such Proceeding  effected without its written  consent,  which
consent shall not be unreasonably withheld. No Indemnifying Party shall, without
the prior written consent of the Indemnified Party, effect any settlement of any
pending Proceeding in respect of which any Indemnified Party is a party,  unless
such settlement includes an unconditional release of such Indemnified Party from
all liability on claims that are the subject matter of such Proceeding.

     All  reasonable  fees and  expenses  of the  Indemnified  Party  (including
reasonable  fees  and  expenses  to  the  extent  incurred  in  connection  with
investigating   or  preparing  to  defend  such   Proceeding  in  a  manner  not
inconsistent  with this  Section)  shall be paid to the  Indemnified  Party,  as
incurred,  within  twenty (20) Business  Days of written  notice  thereof to the
Indemnifying  Party  (regardless of whether it is ultimately  determined that an
Indemnified Party is not entitled to indemnification  hereunder;  provided, that
the  Indemnifying  Party may require  such  Indemnified  Party to  undertake  to
reimburse  all such fees and  expenses  to the extent it is  finally  judicially
determined  that  such  Indemnified  Party is not  entitled  to  indemnification
hereunder or pursuant to applicable law).

     (d) Contribution. If a claim for indemnification under Section 5(a) or 5(b)
is  unavailable  to an  Indemnified  Party  because of a failure or refusal of a
governmental  authority to enforce such  indemnification  in accordance with its
terms (by reason of public policy or otherwise),  then each Indemnifying  Party,
in lieu of indemnifying such Indemnified  Party,  shall


<PAGE>

contribute to the amount paid or payable by such  Indemnified  Party as a result
of such Losses,  in such  proportion as is  appropriate  to reflect the relative
fault of the  Indemnifying  Party and  Indemnified  Party in connection with the
actions,  statements  or omissions  that  resulted in such Losses as well as any
other relevant equitable considerations. The relative fault of such Indemnifying
Party and  Indemnified  Party shall be  determined  by reference to, among other
things,  whether any action in question,  including any untrue or alleged untrue
statement of a material fact or omission or alleged omission of a material fact,
has been  taken  or made  by,  or  relates  to  information  supplied  by,  such
Indemnifying  Party or  Indemnified  Party,  and the parties'  relative  intent,
knowledge,  access to  information  and  opportunity  to correct or prevent such
action, statement or omission. The amount paid or payable by a party as a result
of any Losses shall be deemed to include,  subject to the  limitations set forth
in Section 5(c), any reasonable  attorneys' or other reasonable fees or expenses
incurred  by such party in  connection  with any  Proceeding  to the extent such
party   would  have  been   indemnified   for  such  fees  or  expenses  if  the
indemnification  provided  for in this  Section was  available  to such party in
accordance with its terms.  Notwithstanding  anything to the contrary  contained
herein,  the Holder shall be liable or required to contribute under this Section
5(c) for only that amount as does not exceed the net  proceeds to such Holder as
a result of the sale of  Registrable  Securities  pursuant to such  Registration
Statement.

     The  parties  hereto  agree  that it  would  not be just and  equitable  if
contribution  pursuant  to  this  Section  5(d)  were  determined  by  pro  rata
allocation or by any other method of allocation  that does not take into account
the equitable considerations referred to in the immediately preceding paragraph.
No Person guilty of fraudulent  misrepresentation (within the meaning of Section
11(f) of the Securities Act) shall be entitled to  contribution  from any Person
who was not guilty of such fraudulent misrepresentation.

     The indemnity and contribution  agreements contained in this Section are in
addition  to any  liability  that  the  Indemnifying  Parties  may  have  to the
Indemnified Parties.

     6. Rule 144.

     As long as any Holder owns Registrable Securities, the Company covenants to
timely  file (or  obtain  extensions  in  respect  thereof  and file  within the
applicable  grace period) all reports  required to be filed by the Company after
the date hereof pursuant to Section 13(a) or 15(d) of the Exchange Act and (only
if such  filings are not  available  via EDGAR) to promptly  furnish the Holders
with true and complete  copies of all such  filings.  As long as any Holder owns
Registrable Securities,  if the Company is not required to file reports pursuant
to Section  13(a) or 15(d) of the  Exchange  Act, it will prepare and furnish to
the  Holders  and  make  publicly  available  in  accordance  with  Rule  144(c)
promulgated under the Securities Act annual and quarterly financial  statements,
together with a discussion and analysis of such financial statements in form and
substance  substantially similar to those that would otherwise be required to be
included in reports  required by Section  13(a) or 15(d) of the Exchange Act, as
well as any other  information  required  thereby,  in the time period that such
filings  would have been  required to have been made under the Exchange Act. The
Company  further  covenants  that it will take such further action as any Holder
may reasonably  request,  all to the extent required from time to time to enable
such  Person  to sell  Registrable  Securities  without  registration  under the
Securities


<PAGE>

Act within the  limitation of the  exemptions  provided by Rule 144  promulgated
under the Securities Act,  including  providing any legal opinions of counsel to
the  Company  referred  to in the  Purchase  Agreement.  Upon the request of any
Holder,  the Company shall deliver to such Holder a written  certification  of a
duly  authorized  officer as to whether it has complied with such  requirements.
This Agreement shall  terminate on the date on which the Registrable  Securities
have been sold  pursuant  to a  registration  statement  or may be sold  without
restriction pursuant to Rule 144(k) of the Securities Act.

     7. Miscellaneous.

     (a)  Remedies.  In the event of a breach by the Company or by a Holder,  of
any of their  obligations under this Agreement,  each Holder or the Company,  as
the case may be, in addition to being entitled to exercise all rights granted by
law and under this Agreement, including recovery of damages, will be entitled to
specific  performance of its rights under this  Agreement.  The Company and each
Holder agree that monetary damages would not provide  adequate  compensation for
any losses incurred by reason of a breach by it of any of the provisions of this
Agreement  and  hereby  further  agrees  that,  in the event of any  action  for
specific  performance in respect of such breach, it shall waive the defense that
a remedy at law would be adequate.

     (b)  No  Inconsistent  Agreements.  Neither  the  Company  nor  any  of its
subsidiaries  has, as of the date hereof  entered into and  currently in effect,
nor shall the  Company  or any of its  Affiliates,  on or after the date of this
Agreement,  enter into any  agreement  with  respect to its  securities  that is
inconsistent  with the  rights  granted  to the  Holders  in this  Agreement  or
otherwise conflicts with the provisions hereof.  Without limiting the generality
of the  foregoing,  without the written  consent of the Holders of a majority of
the then outstanding Registrable Securities,  the Company shall not grant to any
Person the right to  request  the  Company to  register  any  securities  of the
Company under the Securities Act if the rights so granted are inconsistent  with
the rights granted to Holders set forth herein, or otherwise prevent the Company
with complying with all of its obligations hereunder.

     (c) No  Piggyback  on  Registrations.  Neither  the  Company nor any of its
security  holders (other than the Holders in such capacity  pursuant hereto) may
include securities of the Company in the Registration Statement.

     (d) Failure to File  Registration  Statement and Other Events.  The Company
and the Holders agree that the Holders will suffer  damages if the  Registration
Statement is not filed on or prior to the Filing Date and not declared effective
by the  Commission on or prior to the  Effectiveness  Date and maintained in the
manner contemplated  herein during the Effectiveness  Period or if certain other
events  occur.  The Company and the Holders  further  agree that it would not be
feasible to ascertain the extent of such damages with precision. Accordingly, if
(i) the  Registration  Statement is not filed on or prior to the Filing Date, or
is not declared  effective by the  Commission  on or prior to the  Effectiveness
Date,  or (ii) the  Company  fails to file with the  Commission  a  request  for
acceleration in accordance with Rule 12dl-2  promulgated  under the Exchange Act
within seven (7) Business Days of the date that the Company is notified  (orally
or in  writing,  whichever  is earlier) by the  Commission  that a


<PAGE>

Registration Statement will not be "reviewed," or not subject to further review,
or (iii) the Registration  Statement is filed with and declared effective by the
Commission  but  thereafter  ceases  to  be  effective  as  to  all  Registrable
Securities at any time during the Effectiveness Period,  without being succeeded
within a reasonable period by a subsequent Registration Statement filed with and
declared  effective by the Commission,  or (iv) the Company has breached Section
3(o) of this  Agreement  (any such  failure or breach  being  referred  to as an
"Event"),  the Company shall pay as liquidated  damages for such failure and not
as a penalty  (the  "Liquidated  Damages") to each Holder an amount equal to one
percent  (1%) for the first  calendar  month and two percent  (2%) per  calendar
month  thereafter of such Holder's pro rata share of the purchase  price paid by
all  Holders  for all shares of Common  Shares  purchased  and then  outstanding
pursuant to the Purchase  Agreement for each thirty (30) day period,  or portion
thereof, following the Event until the applicable Event has been cured. Payments
to be made  pursuant to this Section  7(d) shall be due and payable  immediately
upon  demand at the option of the Holders in cash.  The  parties  agree that the
Liquidated  Damages represent a reasonable  estimate on the part of the parties,
as of the date of this Agreement,  of the amount of damages that may be incurred
by the  Holders if the  Registration  Statement  is not filed on or prior to the
Filing Date or has not been declared  effective by the Commission on or prior to
the Effectiveness Date and maintained in the manner  contemplated  herein during
the Effectiveness Period or if any other Event as described herein has occurred.

     (e) Specific Enforcement, Consent to Jurisdiction.

     (i) The  Company  and the Holders  acknowledge  and agree that  irreparable
damage would occur in the event that any of the  provisions of this Agreement or
the Purchase  Agreement  were not  performed in accordance  with their  specific
terms or were  otherwise  breached.  It is  accordingly  agreed that the parties
shall be entitled to an injunction or injunctions to prevent or cure breaches of
the  provisions  of this  Agreement  or the  Purchase  Agreement  and to enforce
specifically the terms and provisions hereof or thereof,  this being in addition
to any other remedy to which any of them may be entitled by law or equity.

     (ii) Both the Company and the  Purchaser (i) hereby  irrevocably  submit to
the  jurisdiction  of the state courts of the State of Delaware for the purposes
of any suit,  action or proceeding  arising out of or relating to this Agreement
and (ii)  hereby  waive,  and agree not to  assert in any such  suit,  action or
proceeding,  any claim that it is not personally  subject to the jurisdiction of
such court,  that the suit,  action or proceeding is brought in an  inconvenient
forum or that the venue of the suit, action or proceeding is improper.  Both the
Company  and the  Purchaser  consent to process  being  served in any such suit,
action or  proceeding  by mailing a copy thereof to such party at the address in
effect for notices to it under this Agreement and agrees that such service shall
constitute good and sufficient service of process and notice thereof. Nothing in
this Section 7(e) shall affect or limit any right to serve  process in any other
manner permitted by law.

     (f) Amendments and Waivers. The provisions of this Agreement, including the
provisions of this sentence, may not be amended,  modified or supplemented,  and
waivers or consents to departures  from the provisions  hereof may not be given,
unless the same shall be in writing  and signed by the Company and a majority of
the Holders.
<PAGE>

     (g)  Notices.  Any and all notices or other  communications  or  deliveries
required or permitted to be provided  hereunder shall be in writing and shall be
deemed given and  effective on the earlier of (i) the date of  transmission,  if
such  notice or  communication  is  delivered  via  facsimile  at the  facsimile
telephone number specified for notice prior to 5:00 p.m., eastern standard time,
on a Business Day, (ii) the Business Day after the date of transmission, if such
notice or  communication  is delivered via facsimile at the facsimile  telephone
number specified for notice later than 5:00 p.m.,  eastern time, on any date and
earlier  than 11:59 p.m.,  eastern  time,  on such date,  (iii) the Business Day
following  the  date of  mailing,  if sent by  nationally  recognized  overnight
courier  service  or (iv)  actual  receipt  by the party to whom such  notice is
required to be given.

                           (x) if to the Company:

                                    IGEN International, Inc.
                                    16020 Industrial Drive
                                    Gaithersburg, MD 20877
                                    Tel. No.:  (301) 869-9800
                                    Fax No.:  (301) 208-3799
                                    Attention: Richard Massey

                                    with a copy to:

                                    Kirkpatrick & Lockhart LLP
                                    1800 Massachusetts Avenue, N.W.
                                    Suite 200
                                    Washington, D.C. 20036
                                    Tel. No.:  (202) 778-9076
                                    Fax No.:  (202) 778-9100
                                    Attention: Thomas F. Cooney, III

                           (y) if to the Purchaser:

                                    Citadel Equity Fund Ltd.
                                    c/o Citadel Investment Group, L.L.C.
                                    225 West Washington Street
                                    Chicago, Illinois  60606
                                    Attention:  Kenneth A. Simpler
                                    Tel. No.: (312) 696-2100
                                    Fax No.: (312) 338-0780
<PAGE>

                                    with a copy to:

                                    Katten Muchin Zavis
                                    525 West Monroe, Suite 1600
                                    Chicago, Illinois  60661-3693
                                    Attention:  Robert J. Brantman
                                    Tel. No.: (312) 902-5200
                                    Fax No.: (312) 902-1061


or to such other address or addresses or facsimile number or numbers as any such
party may most recently have designated in writing to the other parties hereto
by such notice. Any notice sent by facsimile shall be confirmed by other means
pursuant to this Section 7(g).

     (h) Successors and Assigns.  This Agreement shall be binding upon and inure
to the benefit of the parties and their  successors  and  permitted  assigns and
shall inure to the benefit of each Holder and its  successors  and assigns.  The
Company  may not  assign  this  Agreement  or any of its  rights or  obligations
hereunder without the prior written consent of a majority of Holders.

     (i) Assignment of Registration Rights. The rights of each Holder hereunder,
including  the  right  to have  the  Company  register  for  resale  Registrable
Securities in accordance with the terms of this  Agreement,  shall be assignable
by each  Holder to any  transferee  of such  Holder  of all or a portion  of the
shares of  Registrable  Securities if: (i) the Holder agrees in writing with the
transferee  or assignee to assign such rights,  and a copy of such  agreement is
furnished to the Company within a reasonable  time after such  assignment,  (ii)
the Company is,  within a  reasonable  time after such  transfer or  assignment,
furnished with written notice of (a) the name and address of such  transferee or
assignee,  and (b) the securities with respect to which such registration rights
are being  transferred or assigned,  (iii) following such transfer or assignment
the further  disposition  of such  securities by the  transferee or assignees is
restricted  under the Securities Act and applicable  state securities laws, (iv)
at or before the time the Company  receives the written notice  contemplated  by
clause (ii) of this Section,  the transferee or assignee  agrees in writing with
the Company to be bound by all of the provisions of this Agreement, and (v) such
transfer shall have been made in accordance with the applicable  requirements of
the  Purchase  Agreement  and shall be for no less  than 50% of the  Registrable
Securities.  In addition,  each Holder shall have the right to assign its rights
hereunder to any other Person so long as such assignment is for no less than 30%
of the  Registrable  Securities  with the prior written  consent of the Company,
which consent shall not be unreasonably withheld. The rights to assignment shall
apply to the Holders (and to subsequent) successors and assigns.

     (j)  Counterparts.  This  Agreement  may  be  executed  in  any  number  of
counterparts,  each of which when so executed  shall be deemed to be an original
and, all of which taken together shall constitute one and the same Agreement. In
the event that any  signature  is  delivered  by  facsimile  transmission,  such
signature shall create a valid binding  obligation of the party executing (or on
whose behalf such signature is executed) the same with the same force and effect
as if such facsimile signature were the original thereof.
<PAGE>

     (k)  Governing  Law. This  Agreement  shall be governed by and construed in
accordance with the laws of the State of Delaware,  without regard to principles
of  conflicts  of law  thereof.  This  Agreement  shall  not be  interpreted  or
construed  with any  presumption  against the party causing this Agreement to be
drafted.

     (l) Cumulative  Remedies.  The remedies  provided herein are cumulative and
not exclusive of any remedies provided by law.

     (m) Severability.  If any term, provision,  covenant or restriction of this
Agreement is held to be invalid,  illegal, void or unenforceable in any respect,
the remainder of the terms,  provisions,  covenants and  restrictions  set forth
herein  shall  remain in full force and effect and shall in no way be  affected,
impaired or  invalidated,  and the  parties  hereto  shall use their  reasonable
efforts  to find  and  employ  an  alternative  means  to  achieve  the  same or
substantially  the same  result as that  contemplated  by such term,  provision,
covenant  or  restriction.  It is  hereby  stipulated  and  declared  to be  the
intention of the parties  that they would have  executed  the  remaining  terms,
provisions, covenants and restrictions without including any of such that may be
hereafter declared invalid, illegal, void or unenforceable.

     (n)  Headings.  The  headings  herein  are  for  convenience  only,  do not
constitute a part of this  Agreement  and shall not be deemed to limit or affect
any of the provisions hereof.

     (o) Shares Held by the Company and its Affiliates.  Whenever the consent or
approval of Holders of a  specified  percentage  of  Registrable  Securities  is
required hereunder, Registrable Securities held by the Company or its Affiliates
(other than any Holder or transferees  or successors or assigns  thereof if such
Holder is deemed to be an  Affiliate  solely by reason of its  holdings  of such
Registrable Securities) shall not be counted in determining whether such consent
or approval was given by the Holders of such required  percentage  and shall not
be counted as outstanding.

     (p) Force Majeure.  If either party is prevented from performing any of its
obligations under this Agreement due to any cause beyond the party's  reasonable
control,  including,  without  limitation,  an act of God, an act of  terrorism,
fire, flood, explosion,  war, strike, embargo,  government regulation,  civil or
military  authority,  the time for that party's performance will be extended for
the period of the delay or inability to perform due to such occurrence.


                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

<PAGE>


     IN WITNESS WHEREOF, the parties hereto have caused this Registration Rights
Agreement to be duly executed by their respective  authorized  persons as of the
date first indicated above.


                    IGEN INTERNATIONAL, INC.

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


                    CITADEL EQUITY FUND LTD.


                    By:      /s/Kenneth Simpler
                             ------------------
                             Name: Kenneth Simpler
                             Title: Vice President