<PAGE>

                                                                     Exhibit 4.1

               AMENDED AND RESTATED REGISTRATION RIGHTS AGREEMENT
               --------------------------------------------------

     THIS AMENDED AND RESTATED REGISTRATION RIGHTS AGREEMENT, dated as of
January 6, 2000 (this "Agreement"), is by and among Affiliated Research Centers,
                       ---------
Inc., a corporation organized under the laws of the State of Delaware (the
"Company"), and the undersigned (the "Investors").
 -------                              ---------

     WHEREAS, the Series A Investors (as defined below), the Series D Investors
(as defined below) and the Note Holders (as defined below) have previously
entered into a Registration Rights Agreement with the Company pursuant to which
the Company granted to such Series A Investors, Series D Investors and Note
Holders certain rights to registration of equity securities held by such Series
A Investors, Series D Investors and Note Holders; and

     WHEREAS, the Company contemplates entering into an Agreement and Plan of
Merger (the "Merger Agreement") with AmericasDoctor.com, Inc. ("AmDoc") and
             ----------------
ARC Merger Sub-1, Inc.; and

     WHEREAS, it is a condition to the execution of the Merger Agreement that
the Company, the Series A Investors, the Series D Investors, the Note Holders
and the AmDoc Investors (as defined below) enter into this agreement, which will
in all respects supercede and replace the Investors' previous agreements with
respect to registration of equity securities of the Company and AmDoc held by
them; and

     WHEREAS, the parties hereto desire to enter into this Agreement and the
mutual covenants of the parties relating thereto;

     NOW, THEREFORE, in consideration of the foregoing and the covenants of the
parties set forth herein and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, subject to the terms
and conditions set forth herein, the parties hereby agree as follows:

     Section 1.   Certain Definitions. In this Agreement the following terms
                  -------------------
shall have the following respective meanings:

       "Affiliate" means, when used with respect to a specified Person, another
        ---------
Person that directly, or indirectly through one or more intermediaries, controls
or is controlled by or is under common control with the Person specified.

       "Agreement" has the meaning ascribed to it in the preamble to this
        ---------
Agreement.

       "AmDoc" has the meaning ascribed to it in the recitals to this Agreement.
        -----

       "AmDoc Investors" means, collectively, Medical Advisory Systems, Inc.,
        ---------------
Premier Research Worldwide, Ltd., Tullis-Dickerson Capital Focus II, L.P., TD
Origen Capital Fund, L.P., TD Javelin Capital Fund, L.P. and GE Capital Equity
Investments, Inc.
<PAGE>

       "Class A Common Stock" means the Class A Common Stock, par value $.001
        --------------------
per share, of the Company.

       "Commission" means the Securities and Exchange Commission or any other
        ----------
federal agency at the time administering the Securities Act.

       "Company" has the meaning ascribed to it in the preamble to this
        -------
Agreement.

       "Exchange Act" means the Securities Exchange Act of 1934, as amended,
        ------------
and the rules and regulations of the Commission thereunder, all as the same
shall be in effect at the relevant time.

       "Holders" shall mean (i) the Investors and (ii) each Person holding
        -------
Registrable Stock as a result of a transfer or assignment to that Person of
Registrable Stock permitted pursuant to Section 9 of this Agreement.

       "Indemnified Party" has the meaning ascribed to it in Section 6(c) of
        -----------------
this Agreement.

       "Indemnifying Party" has the meaning ascribed to it in Section 6(c) of
        ------------------
this Agreement.

       "Initiating Holders" means Investors (or their assignees under Section
        ------------------
9 hereof) who, in the aggregate, are Holders of forty percent (40%) or more of
all Registrable Stock then issued and outstanding.

       "Investors" has the meaning ascribed to it in the preamble to this
        ---------
Agreement.

       "Merger Agreement" has the meaning ascribed to it in the recitals to
        ----------------
this Agreement.

       "majority in interest" means, as to any group of Holders, Holders of a
        --------------------
majority of the shares of Class A Common Stock (assuming conversion of all
shares of Preferred Stock held by such Holders) held by such Holders.

       "Note Holders" means, collectively, the Series A Investors, Hambrecht
        ------------
& Quist California, H&Q Affiliated Research Investors, L.P., and Hambrecht &
Quist Employee Venture Fund, L.P. II.

       "Note Warrants" means the warrants exercisable to purchase shares of
        -------------
Class A Common Stock at an exercise price per share equal to $0.01 held by the
Note Holders on the date hereof.

       "Person" means an individual, corporation, partnership, estate, trust,
        ------
association, private foundation, joint stock company or other entity.

       "Piggyback Notice" has the meaning ascribed to it in Section 3(a) of
        ----------------
this Agreement.


                                       2
<PAGE>

       "Piggyback Registration" has the meaning ascribed to it in Section 3(a)
        ----------------------
of this Agreement.

       "Preferred Stock" means, collectively, the Series A-1 Preferred Stock,
        ---------------
par value $.001 per share, of the Company, the Series A-2 Preferred Stock, par
value $.001 per share, of the Company, the Series A-3 Preferred Stock, par value
$.001 per share, of the Company, the Series A-4 Preferred Stock, par value $.001
per share, of the Company and the Series A-5 Preferred Stock, par value $.001
per share, of the Company and any other shares of the Preferred Stock, par value
$.001 per share, of the Company (whether now or hereafter authorized and
issued).

       The terms "Register," "Registered" and "Registration" refer to a
                  --------    ----------       ------------
registration effected by preparing and filing a registration statement in
compliance with the Securities Act providing for the sale by the Holders of
Registrable Stock in accordance with the method or methods of distribution
designated by the Holders, and the declaration or ordering of the effectiveness
of such registration statement by the Commission.

       "Registrable Stock" means (i) the shares of Preferred Stock or Class A
        -----------------
Common Stock held by the Investors immediately following the Effective Time (as
defined in the Merger Agreement) or any series of Preferred Stock purchased or
acquired (by exchange, conversion or otherwise) by the Investors after the date
hereof, and (ii) shares of Class A Common Stock issued or issuable pursuant to
the conversion of shares of Preferred Stock or upon exercise of the Warrants,
except that Registrable Stock shall cease to be Registrable Stock on the
earliest to occur of: (A) the date that such Registrable Stock has been disposed
of pursuant to an effective registration statement under the Securities Act, (B)
the date that such Registrable Stock may be distributed to the public pursuant
to Rule 144(k) or any similar provision then in force under the Securities Act,
(C) the date that such Registrable Stock has been transferred or disposed of,
new certificates therefor not bearing a legend restricting further transfer have
been delivered by the Company, and at such time, subsequent transfer or
disposition of such securities does not require registration or qualification of
such securities under the Securities Act or any similar state law then in force,
(D) the date that such Registrable Stock has ceased to be outstanding, or (E)
the date that is five (5) years after the closing of an initial public offering
of the Company's capital stock pursuant to a registration statement effective
under the Securities Act.

       "Registration Request" has the meaning ascribed to it in Section 2(a) of
        --------------------
this Agreement.

       "Rule 144" means Rule 144  (or any successor provision) promulgated by
        --------
the Commission under the Securities Act.

       "Securities Act" means the Securities Act of 1933, as amended, and the
        --------------
rules and regulations of the Commission thereunder, all as the same shall be in
effect at the relevant time.

       "Series A Investors" means, collectively, Galen Partners III, L.P.,
        ------------------
Galen Partners International III, L.P., Galen Employee Fund III, L.P., Delphi
Ventures III, L.P. and Delphi BioInvestments III, L.P.

       "Series D Investors" means, collectively, the Note Holders.
        ------------------

                                       3
<PAGE>

          "Warrants" means the Note Warrants and any warrants exercisable to
           --------
purchase shares of Preferred Stock or Class A Common Stock held by the AmDoc
Investors immediately prior to the Effective Time (as defined in the Merger
Agreement).

  Section 2.   Demand Registration.
               -------------------

          (a)  Upon receipt of a written request (a "Registration Request"),
                                                     --------------------
delivered not earlier than the earlier of (i) the second anniversary of the date
hereof or (ii) 180 days after the date of the closing of an initial public
offering by the Company of the Class A Common Stock (or equivalent security),
from Initiating Holders, the Company shall (A) promptly give written notice of
the Registration Request to all non-requesting Holders and (B) use its
reasonable best efforts to prepare and file with the Commission, as soon as
practicable after its receipt of such Registration Request, a registration
statement for the purpose of effecting a Registration of the sale of all
Registrable Stock by each of the Holders which submitted such Registration
Request and any other Holder who requests to have such Holder's Registrable
Stock included in such registration statement within 30 days after receipt of
notice by such Holder of the Registration Request; provided, however, that the
                                                   --------  -------
Company will not be required to effect the Registration of Registrable Stock (x)
in the case of a Registration to be effected on Form S-3 (or any similar short-
form registration statement adopted by the SEC from and after the date hereof),
unless the Registrable Stock covered thereby is offered at an aggregate offering
price of not less than $1,000,000 or (y) in the case of a Registration to be
effected on Form S-1 (or any similar long-form registration statement adopted by
the SEC from and after the date hereof), unless such Registration covers at
least 750,000 shares of Registrable Stock or (iii) if the Company shall at the
time have effective a registration statement pursuant to which the Initiating
Holders could effect the disposition of their Registrable Stock in the manner
requested.  The Company shall use its reasonable best efforts to effect such
Registration as soon as practicable (including, without limitation, the
execution of an undertaking to file post-effective amendments and appropriate
qualification under applicable state securities laws).  In the event such
Registration Request is delivered after the first anniversary of the closing of
the Company's  initial public offering of the Class A Common Stock and the
Company is eligible to register such Registrable Stock on a continuous basis
under Rule 415 of the Securities Act by filing a Registration Statement on Form
S-3, the Company shall, at the request of requesting Holders, effect such
Registration on a Registration Statement on Form S-3 pursuant to Rule 415.  The
Company shall use its reasonable best efforts to keep such Registration
continuously effective until the earlier of (i) the date on which all
Registrable Stock has been sold pursuant to such registration statement or Rule
144, (ii) the date on which all of the Registrable Stock held by such Holder may
be sold pursuant to Rule 144(k) or another available exemption under the
Securities Act and (iii) the date that is five (5) years following the closing
of the initial public offering by the Company of its Class A Common Stock (or
equivalent security); provided, however, that the Company shall not be obligated
                      --------  -------
to maintain the effectiveness of any Registration that is not effected under
Rule 415 for a period in excess of 180 days; provided, further, that the Company
                                             --------  -------
shall not be obligated to take any action to effect any such Registration,
qualification or compliance pursuant to this Section 2 in any particular
jurisdiction in which the Company would be required to (x) execute a general
consent to service of process in effecting such Registration, qualification or
compliance unless the Company is already subject to service in such jurisdiction
or (y) qualify as a foreign corporation in any jurisdiction in which the Company
is not then qualified.

                                       4
<PAGE>

          Notwithstanding the foregoing, the Company shall have the right (the
"Suspension Right") to defer such filing (or suspend sales under any filed
 ----------------
registration statement or defer the updating of any filed registration statement
and suspend sales thereunder) for a period of not more than 90 days during any
twelve-month period, if the Company furnishes to the Holders a certificate
signed by an executive officer or any director of the Company stating that, in
the good faith judgment of the Company, it would be detrimental to the Company
and its stockholders to file such registration statement or amendment thereto at
such time (or continue sales under a filed registration statement) and therefore
the Company has elected to defer the filing of such registration statement (or
suspend sales under a filed registration statement).

          (b)  Unless the Company is a registrant entitled to use Form S-3 (or
any similar short-form registration statement adopted by the SEC from and after
the date hereof) and the Registration requested by the Initiating Holders may be
accomplished on such form, the Company shall not be required to effect more than
two (2) Registrations pursuant to this Section 2.

          (c)  Notwithstanding anything to the contrary herein, the Company may
satisfy its obligations under this Section 2 by amending (to the extent
permitted by applicable law) within ten (10) business days after a written
request for Registration, any registration statement previously filed by the
Company under the Securities Act so that such registration statement (as
amended) shall permit the disposition (in accordance with the intended methods
of distribution specified as aforesaid) of all of the Registrable Stock for
which a demand registration has been made under this Section 2. If the Company
so amends a previously filed registration statement, it shall be deemed to have
effected a registration for purposes of this Section 2.

          (d)  If the Initiating Holders intend to distribute the Registrable
Stock covered by their request by means of an underwriting, they shall so advise
the Company as a part of their Registration Request, and the Company shall
include such information in the written notice referred to in Section 2(a)
above. The right of any Holder to registration under Section 2 shall be
conditioned upon such Holder's participation in such underwriting and the
inclusion of such Holder's Registrable Stock in the underwriting (unless
otherwise mutually agreed by a majority in interest of the Initiating Holders
and such Holder with respect to such participation and inclusion) to the extent
provided herein. A Holder may elect to include in such underwriting all or any
part of the Registrable Stock held by such Holder.

          The Company shall (together with all Holders proposing to distribute
their securities through such underwriting) enter into an underwriting agreement
in customary form with the underwriter(s) selected for such underwriting by a
majority in interest of the Initiating Holders and reasonably acceptable to the
Company. Notwithstanding any other provision of this Section 2, if the managing
underwriters advise the Company that the number of securities requested to be
included in such registration exceeds the number which can be sold in an orderly
manner within a price range acceptable to the Initiating Holders, the Company
will include in such registration (i) first, the Registrable Stock and other
securities requested to be included in the Registration by the Initiating
Holders, allocated, if necessary, pro rata among all such Initiating Holders on
the basis of the respective amounts of Registrable Stock and other securities
which they had requested to be included in such registration statement at the
time of filing of such registration statement, (ii) second, the Registrable
Stock and other securities requested to be

                                       5
<PAGE>

included by all other Holders, allocated, if necessary, pro rata among all such
other Holders on the basis of the respective amounts of Registrable Stock and
other securities which they had requested to be included in such registration
statement at the time of filing of such registration statement, and (iii) third,
the securities the Company proposes to sell, if any.

          If any Holder of Registrable Stock who has requested inclusion in such
Registration as provided above, disapproves of the terms of the underwriting,
such person may elect to withdraw therefrom by written notice to the Company,
the underwriter(s) and the Initiating Holders. The securities so withdrawn shall
be withdrawn from the Registration.

     Section 3.   Piggyback Registrations.
                  -----------------------

          (a)  At any time after 180 days after the date of the closing of an
initial public offering by the Company of its Class A Common Stock (or
equivalent security), if the Company proposes to register any of its common
equity securities or any securities convertible into its common equity
securities under the Securities Act (other than pursuant to (i) a registration
statement filed pursuant to Rule 415 under the Securities Act, (ii) a
registration on Form S-4 or any successor form, or (iii) an offering of
securities in connection with an employee benefit, share dividend, share
ownership or dividend reinvestment plan) and the registration form to be used
may be used for the registration of Registrable Stock, the Company will give
prompt written notice to all holders of Registrable Stock of its intention to
effect such a registration (each a "Piggyback Notice") and, subject to
                                    ----------------
subparagraph 3(c) below, the Company will include in such registration all
Registrable Stock with respect to which the Company has received written
requests for inclusion therein within 15 days after the date of receipt of the
Piggyback Notice (a "Piggyback Registration"), unless, if the Piggyback
                     ----------------------
Registration is not an underwritten offering, the Company in its reasonable
judgement determines that, or in the case of an underwritten Piggyback
Registration the managing underwriter(s) advise the Company that, the inclusion
of Registrable Stock would adversely interfere with such offering, affect the
Company's securities in the public markets or otherwise adversely affect the
Company. Nothing herein shall affect the right of the Company to withdraw any
such registration in its sole discretion.

          (b)  If a Piggyback Registration is a primary registration on behalf
of the Company and, if the Piggyback Registration is not an underwritten
offering, the Company in its reasonable judgement determines that, or in the
case of an underwritten Piggyback Registration, the managing underwriter(s)
advise the Company that the number of securities requested to be included in
such registration exceeds the number which can be sold in an orderly manner
within a price range acceptable to the Company, the Company will include in such
registration (i) first, the securities the Company proposes to sell (other than
securities held by officers and directors of the Company), (ii) second, the
Registrable Stock and any other securities requested to be included in such
registration, allocated pro rata among the Holders of Registrable Stock and the
holders of such other securities on the basis of the respective amounts of
Registrable Stock and other securities which they had requested to be included
in such registration at the time of filing such registration statement, and
(iii) third, securities held by officers and directors of the Company.

          (c)  If a Piggyback Registration is a secondary registration on behalf
of holders of the Company's securities other than the holders of Registrable
Stock, and, if the Piggyback

                                       6
<PAGE>

Registration is not an underwritten offering, the Company determines that, or in
the case of an underwritten Piggyback Registration, the managing underwriter(s)
advise the Company that the number of securities requested to be included in
such registration exceeds the number which can be sold in an orderly manner in
such offering within a price range acceptable to the holders initially
requesting such registration, the Company will include in such registration (i)
first, the securities requested to be included therein by the holders requesting
such registration and (ii) second, the Registrable Stock and other securities
requested to be included in such registration, allocated pro rata among the
Holders of Registrable Stock and the holders of such other securities requesting
such registration on the basis of the respective amounts of Registrable Stock
and other securities which they had requested to be included in such
registration at the time of filing such registration statement.

          (d)  If the Piggyback Registration of which the Company gives notice
is for a registered public offering involving an underwriting, the Company shall
so advise the Holders as a part of its Piggyback Notice. In such event, the
right of any Holder to registration pursuant to Section 3 shall be conditioned
upon such Holder's participation in such underwriting and the inclusion of such
Holder's Registrable Stock in the underwriting to the extent provided herein.
All Holders proposing to dispose of their securities through the underwriting
shall (together with the Company) enter into an underwriting agreement in
customary form with the underwriter(s) selected for the underwriting by the
Company. If any Holder disapproves of the terms of any such underwriting, such
Holder may elect to withdraw therefrom by written notice to the Company and the
underwriter(s). Any Registrable Stock or other securities excluded or withdrawn
from such underwriting shall be withdrawn from such registration.

     Section 4.   Registration Procedures.
                  -----------------------

          (a)  The Company shall promptly notify the Holders of the occurrence
of the following events:

               (i)    when any registration statement relating to the
          Registrable Stock or post-effective amendment thereto filed with the
          Commission has become effective;

               (ii)   the issuance by the Commission of any stop order
          suspending the effectiveness of any registration statement relating to
          the Registrable Stock;

               (iii)  the suspension of an effective registration statement by
          the Company in accordance with the last paragraph of Section 2(a)
          hereof;

               (iv)   the Company's receipt of any notification of the
          suspension of the qualification of any Registrable Stock covered by a
          registration statement for sale in any jurisdiction; and

               (v)    the existence of any event, fact or circumstance of which
          the Company has knowledge, that results in a registration statement or
          prospectus relating to Registrable Stock or any document incorporated
          therein by reference containing an untrue statement of material fact
          or omitting to state a material fact required to be stated therein or
          necessary to make the statements therein not misleading during the
          distribution of securities.

                                       7
<PAGE>

     The Company agrees to use its reasonable best efforts to obtain the
withdrawal of any order suspending the effectiveness of any such registration
statement or any state qualification as promptly as possible. The Investors
agree that upon receipt of any notice from the Company of the occurrence of any
event of the type described in Section 4(a)(ii), (iii), (iv) or (v) to
immediately discontinue their disposition of Registrable Stock pursuant to any
registration statement relating to such securities until the Investors' receipt
of written notice from the Company that such disposition may be made.

          (b)  The Company shall provide to the Holders, at no cost to the
Holders, a copy of the registration statement and any amendments thereto used to
effect the Registration of the Registrable Stock, each prospectus contained in
such registration statement or post-effective amendment and any amendment or
supplement thereto and such other documents as the requesting Holders may
reasonably request in order to facilitate the disposition of the Registrable
Stock covered by such registration statement. The Company consents to the use of
each such prospectus and any supplement thereto by the Holders in connection
with the offering and sale of the Registrable Stock covered by such registration
statement or any amendment thereto. If the Class A Common Stock (or equivalent
security) is listed on a "national securities exchange" as defined in Rule 153
under the Securities Act at any time during the period in which the Company is
obligated to keep the registration statement effective pursuant to Section 2(a),
the Company shall also file a sufficient number of copies of the prospectus and
any post-effective amendment or supplement thereto with such national securities
exchange (or, if the Class A Common Stock (or equivalent security) is no longer
listed thereon, with such other securities exchange or market on which the Class
A Common Stock (or equivalent security) is then listed) so as to enable the
Holders to have the benefits of the prospectus delivery provisions of Rule 153
under the Securities Act.

          (c)  The Company agrees to use its reasonable best efforts to cause
the Registrable Stock covered by a registration statement to be registered with
or approved by such state securities authorities as may be necessary to enable
the Holders to consummate the disposition of such stock pursuant to the plan of
distribution set forth in the registration statement; provided, however, that
                                                      --------  -------
the Company shall not be obligated to take any action to effect any such
Registration, qualification or compliance pursuant to this Section 4 in any
particular jurisdiction in which the Company would be required to execute a
general consent to service of process in effecting such Registration,
qualification or compliance unless the Company is already subject to service in
such jurisdiction or qualify as a foreign corporation in any jurisdiction in
which the Company is not then qualified.

          (d)  Subject to the Company's Suspension Right, if any event, fact or
circumstance requiring an amendment to a registration statement relating to the
Registrable Stock or supplement to a prospectus relating to the Registrable
Stock shall exist, as promptly as practicable upon becoming aware thereof the
Company agrees to notify the Holders and prepare and furnish to the Holders a
post-effective amendment to the registration statement or supplement to the
prospectus or any document incorporated therein by reference or file any other
required document so that, as thereafter delivered to the purchasers of the
Registrable Stock, the prospectus will not contain an untrue

                                       8
<PAGE>

statement of a material fact or omit to state any material fact required to be
stated therein or necessary to make the statements therein not misleading in the
light of circumstances under which such statements were made.

          (e)    The Company agrees to use its reasonable efforts (including the
payment of any listing fees) to obtain the listing of all Registrable Stock
covered by the registration statement on each securities exchange on which
securities of the same class or series are then listed.

          (f)    The Company agrees to use its reasonable efforts to comply with
the Securities Act and the Exchange Act in connection with the offer and sale of
Registrable Stock pursuant to a registration statement, and, as soon as
reasonably practicable following the end of any fiscal year during which a
registration statement effecting a Registration of the Registrable Stock shall
have been effective, to make available to its security holders an earnings
statement satisfying the provisions of Section 11(a) of the Securities Act.

          (g)    The Company agrees to cooperate with the selling Holders to
facilitate the timely preparation and delivery of certificates representing
Registrable Stock sold pursuant to a Registration and not bearing any Securities
Act legend and to enable certificates for such Registrable Stock to be issued
for such numbers of stock and registered in such names as the Holders may
reasonably request.

     Section 5.  Expenses of Registration.  All reasonable expenses, other
                 ------------------------
than underwriting discounts and commissions (which shall be borne by the Holders
of securities so registered and the Company pro rata, on the basis of the number
of shares so registered for the account of each of them), incurred in connection
with registrations, filings or qualifications pursuant to Sections 2, 3 and 4
hereof, including, without limitation, all registration, listing and
qualifications fees, printers and accounting fees, the fees and disbursements of
counsel for the Company and the reasonable fees and disbursements of one counsel
selected by the Holders to the extent related to the Registration and
Registrable Stock hereunder shall be borne by the Company; provided, however,
                                                           --------  -------
that the Company shall not be required to pay for any expenses of any
Registration begun pursuant to Section 2 hereof if the Registration Request is
subsequently withdrawn at the request of a majority in interest of the
Initiating Holders (in which case all Holders requesting withdrawal from such
Registration shall bear such expense and shall reimburse the Company for any
expenses incurred by it in connection with such Registration) unless such
withdrawal by such Initiating Holders is based upon material adverse information
relating to the Company that was not known to the Initiating Holders at the time
of such request.

     Section 6.  Indemnification.
                 ---------------

          (a)  The Company will indemnify each Holder, each Holder's officers,
directors and partners, and each person controlling such Holder within the
meaning of Section 15 of the Securities Act, and each underwriter, if any, and
each person controlling such underwriter within the meaning of Section 15 of the
Securities Act,

                                       9
<PAGE>

against all expenses, claims, losses, damages and liabilities (including
reasonable legal expenses), arising out of or based on any untrue statement (or
alleged untrue statement) of a material fact contained in any registration
statement or prospectus relating to the Registrable Stock, or any amendment or
supplement thereto, or based on any omission (or alleged omission) to state
therein a material fact required to be stated therein or necessary to make the
statements therein not misleading, provided, however, that the Company will not
                                   --------  -------
be liable in any such case (i) to the extent that any such claim, loss, damage,
liability or expense arises out of or is based on any untrue statement or
omission or alleged untrue statement or omission, made in reliance upon and in
conformity with information furnished in writing to the Company by such Holder
or underwriter for inclusion therein or (ii) to anyone who participates as an
underwriter in the offering or sale of Registrable Stock to any other person, if
any, who controls such underwriter, under the indemnity agreement in this
Section 6 for any such claim, loss, damage, liability or expense that arises out
of such person's failure to send or deliver a copy of the final prospectus to
the person asserting any untrue statement or alleged untrue statement or
omission or alleged omission at or prior to the written confirmation of the sale
of the Registrable Stock to such person if such statement or omission was
corrected in such final prospectus and the Company has previously furnished
copies thereof to such Holder or other person.

          (b)  Each Holder will indemnify the Company and each of its directors
and officers who signs the registration statement, each other Holder
participating in the Registration and each such Holder's officers, directors and
partners, each underwriter, if any, of the Company's securities covered by such
registration statement, and each person who controls the Company, such Holder or
such underwriter within the meaning of Section 15 of the Securities Act, against
all claims, losses, damages and liabilities (including reasonable legal fees and
expenses) arising out of or based on any untrue statement (or alleged untrue
statement) of a material fact contained in any such registration statement or
prospectus, or any amendment or supplement thereto, or based on any omission (or
alleged omission) to state therein a material fact required to be stated therein
or necessary to make the statements therein not misleading, in each case to the
extent, but only to the extent, that such untrue statement (or alleged untrue
statement) or omission (or alleged omission) is made in such registration
statement or prospectus, in reliance upon and in conformity with information
furnished in writing to the Company by such Holder for inclusion therein.

          (c)  Each party entitled to indemnification under this Section 6 (the
"Indemnified Party") shall give notice to the party required to provide
 -----------------
indemnification (the "Indemnifying Party") promptly after such Indemnified Party
                      ------------------
has actual knowledge of any claim as to which indemnity may be sought, but the
omission to so notify the Indemnifying Party shall not relieve it from any
liability which it may have to the Indemnified Party pursuant to the provisions
of this Section 6 except to the extent of the actual damages suffered by such
delay in notification. The Indemnifying Party shall assume the defense of such
action, including the employment of counsel to be chosen by the Indemnifying
Party to be reasonably satisfactory to the Indemnified Party, and payment of
expenses. The Indemnified Party shall have the right to employ its own counsel
in any such case, but the legal fees and expenses of such counsel shall be at
the

                                       10
<PAGE>

expense of the Indemnified Party, unless the employment of such counsel shall
have been authorized in writing by the Indemnifying Party in connection with the
defense of such action, or the Indemnifying Party shall not have employed
counsel to take charge of the defense of such action, in either of which events
such fees and expenses shall be borne by the Indemnifying Party. No Indemnifying
Party, in the defense of any such claim or litigation, shall, except with the
consent of each Indemnified Party, which consent shall not be unreasonably
withheld, consent to entry of any judgment or enter into any settlement which
does not include as an unconditional term thereof the giving by the claimant or
plaintiff to such Indemnified Party of a release from all liability in respect
to such claim or litigation. Each Indemnified Party shall furnish such
information regarding itself of the claim in question as the Indemnifying Party
may reasonably request in writing and as shall be reasonably required in
connection with the defense of such claim and litigation resulting therefrom.

          (d)    If the indemnification provided for in this Section 6 is
unavailable to a party that would have been an Indemnified Party under this
Section 6 in respect of any expenses, claims, losses, damages and liabilities
referred to herein, then each party that would have been an Indemnifying Party
hereunder shall, in lieu of indemnifying such Indemnified Party, contribute to
the amount paid or payable by such Indemnified Party as a result of such
expenses, claims, losses, damages and liabilities in such proportion as is
appropriate to reflect the relative fault of the Indemnifying Party on the one
hand and such Indemnified Party on the other in connection with the statement or
omission which resulted in such expenses, claims, losses, damages and
liabilities, as well as any other relevant equitable considerations. The
relative fault shall be determined by reference to, among other things, whether
the untrue or alleged untrue statement of a material fact or the omission or
alleged omission to state a material fact relates to information supplied by the
Indemnifying Party or such Indemnified Party and the parties' relative intent,
knowledge, access to information and opportunity to correct or prevent such
statement or omission. The Company and each Holder agree that it would not be
just and equitable if contribution pursuant to this Section were determined by
pro rata allocation or by any other method of allocation which does not take
account of the equitable considerations referred to above in this Section 6(d).

          (e)    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.

          (f)    In no event shall any Holder be liable for any expenses,
claims, losses, damages or liabilities pursuant to this Section 6 in excess of
the net proceeds to such Holder of any Registrable Stock sold by such Holder .

     Section 7.  Information to be Furnished by Holders. Each Holder shall
                 --------------------------------------
furnish to the Company such information as the Company may reasonably request
and as shall be required in connection with the Registration and related
proceedings referred to in Section 2 or Section 3 hereof. If any Holder fails to
provide the Company with such information within 10 days of receipt of the
Company's request, the Company's obligations under Section 2 or Section 3
hereof, as applicable, with respect to such

                                       11
<PAGE>

Holder or the Registrable Stock owned by such Holder shall be suspended until
such Holder provides such information.

     Section 8.  Rule 144 Sales.
                 --------------

          (a)    The Company covenants that it will at all times from and after
the date that is 90 days following the effective date of the first Registration
under the Securities Act filed by the Company for an offering of its securities
to the public, use its reasonable efforts to file the reports required to be
filed by the Company under the Exchange Act, so as to enable any Holder to sell
Registrable Stock pursuant to Rule 144 under the Securities Act.

          (b)    In connection with any sale, transfer or other disposition by
any Holder of any Registrable Stock pursuant to Rule 144 under the Securities
Act, the Company shall cooperate with such Holder to facilitate the timely
preparation and delivery of certificates representing Registrable Stock sold and
not bearing any Securities Act legend and to enable certificates for such
Registrable Stock to be issued for such number of shares and registered in such
names as the selling Holder may reasonably request.

     Section 9.  Assignment of Registration Rights. Subject to any transfer
                 ---------------------------------
restrictions otherwise applicable to the Registrable Stock, the rights of the
Holders hereunder, including the right to have the Company register Registrable
Stock pursuant to this Agreement, shall be assignable by each Holder to any
transferee of all or any portion of the Registrable Stock if: (i) the transfer
to such transferee is permitted under the Securities Act and applicable state
securities law or exemptions therefrom, (ii) the aggregate amount of Registrable
Stock that will be held by the transferee after giving effect to such transfer
is not less than 100,000 shares of Registrable Stock, (iii) 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 after such assignment, (iv) the
Company is 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, (v) following such
transfer or assignment, the further disposition of such securities by the
transferee or assignee is restricted under the Securities Act and applicable
state securities laws, and (vi) the transferee or assignee agrees in writing for
the benefit of the Company to be bound by all of the provisions contained
herein; provided, however, that each Holder may transfer or assign its
        --------  -------
registration rights hereunder together with a transfer of such Holder's shares
of Registrable Stock to not more than an aggregate of 50 of such Holder's
partners (including limited partners).

     Section 10. "Lock-up" Agreement. Each Holder agrees that it will enter into
                 -------------------
a written agreement (together with all executive officers, directors and
principal stockholders of the Company) containing customary terms and provisions
to the effect that, during the period of duration specified by the
underwriter(s), following the effective date of a registration statement of the
Company filed under the Securities Act, it will not, to the extent requested by
such underwriter(s), directly or indirectly sell, offer to sell, contract to
sell, grant any option to purchase or otherwise transfer or dispose of (other

                                       12
<PAGE>

than to donees who agree to be similarly bound) any securities of the Company
held by such Holder at any time during such period except securities included in
such registration; provided, however, that (a) the agreement referenced in this
                   --------  -------
Section 10 will be applicable only to the first such registration statement of
the Company which covers securities to be sold on its behalf to the public in an
underwritten offering, (b) the period of duration to be specified by the
underwriter(s) pursuant to this Section 10 does not exceed 180 days, and (c)
each officer and director of the Company and each other holder of 5% or more of
the outstanding equity securities of the Company enters into a similar
agreement. Each Holder further agrees not to transfer any securities of the
Company held by such Holder, either directly or indirectly, to any transferee
who does not agree to be bound by the terms of this Section 10. Each Holder (i)
agrees that any attempt to effect any transfer of securities of the Company not
in compliance with the provisions of this Section 10 will be void and of no
force and effect and (ii) agrees and consents to the entry of stop transfer
instructions with the Company's transfer agent against the transfer of any
securities of the Company held by such Holder except in compliance with this
Section 10.

     Section 11. Termination of Rights. The rights of the Holders to cause the
                 ---------------------
Company to register their Registrable Stock shall terminate five (5) years
following the initial underwritten public offering of the Company's Class A
Common Stock (or equivalent class of equity securities).

     Section 12. Limitations on Registration of Issues of Securities. From and
                 ---------------------------------------------------
after the date of this Agreement at any time at which at least 100,000 shares of
Registrable Stock are outstanding (as adjusted for any stock dividend, stock
split or combination of shares in connection with a recapitalization, merger,
consolidation, reorganization or otherwise), the Company shall not enter into
any agreement with any holder or prospective holder of any securities of the
Company giving such holder or prospective holder the right to require the
Company to initiate any registration of any securities of the Company without
the prior written consent of the Holders of at least a majority of the
Registrable Stock then outstanding.

     Section 13. Miscellaneous.
                 -------------

          (a)  Governing Law. This Agreement shall be governed by and construed
               -------------
in accordance with the laws of the State of Delaware applicable to contracts
made and to be performed in the State of Delaware, without regard to the
principles of conflicts thereof. The Company and Investors agree that service of
process upon the Company and Investors, respectively, mailed by first class
mail, shall be deemed in every respect effective service of process upon the
Company and Investors, respectively, in any such suit or proceeding. Nothing
herein shall affect the right of the Company or the right of any Investor to
serve process in any other manner permitted by law.

          (b)  Entire Agreement.  This Agreement constitutes the full and
               ----------------
entire understanding and agreement between the parties with regard to the
subject matter hereof.

                                       13
<PAGE>

          (c)  Amendment. No supplement, modification, waiver or termination of
               ---------
this Agreement shall be binding unless executed in writing by the Company and
the holders of at least 66 % of the Registrable Stock then outstanding.

          (d)  Notices, etc. Each notice, demand, request, request for approval,
               ------------
consent, approval, disapproval, designation or other communication (each of the
foregoing being referred to herein as a notice) required or desired to be given
or made under this Agreement shall be in writing (except as otherwise provided
in this Agreement), and shall be effective and deemed to have been received (i)
when delivered in person, (ii) when sent by fax with receipt acknowledged, (iii)
five (5) business days after having been mailed by certified or registered
United States mail, postage prepaid, return receipt requested, or (iv) the next
business day after having been sent by a nationally recognized overnight mail or
courier service, receipt requested. Notices shall be addressed as follows: (a)
if to an Investor, at the Investor's address or fax number set forth on Annex A
hereto, or at such other address or fax number as the Investor shall have
furnished to the Company in writing, or (b) if to any assignee or transferee of
an Investor, at such address or fax number as such assignee or transferee shall
have furnished the Company in writing, or (c) if to the Company, at the address
of its principal executive offices and addressed to the attention of the Chief
Executive Officer, or at such other address or fax number as the Company shall
have furnished to the Investors or any assignee or transferee. Any notice or
other communication required to be given hereunder to a Holder in connection
with a registration may instead be given to the designated representative of
such Holder.

          (e)  Counterparts. This Agreement may be executed in two or more
               ------------
counterparts, all of which shall be considered one and the same agreement and
shall become effective when counterparts have been signed by each party and
delivered to the other party. This Agreement, once executed by a party, may be
delivered to the other parties hereto by facsimile transmission of a copy of
this Agreement bearing the signature of the party so delivering this Agreement.
In the event any signature is delivered by facsimile transmission, the party
using such means of delivery shall cause the manually executed execution page(s)
to be physically delivered to the other party within five days of the execution
hereof.

          (f)  Severability. In the event that any provision of this Agreement
               ------------
becomes or is declared by a court of competent jurisdiction to be illegal,
unenforceable or void, this Agreement shall continue in full force and effect
without said provision.

          (g)  Section Titles. Section titles are for descriptive purposes only
               --------------
and shall not control or alter the meaning of this Agreement as set forth in the
text.

          (h)  Successors and Assigns. This Agreement shall be binding upon the
               ----------------------
parties hereto and their respective successors and assigns.

          (i)  Effective Time. This Agreement shall automatically become
               --------------
effective at the Effective Time (as defined in the Merger Agreement). If the
Effective

                                       14
<PAGE>

Time does not occur, this Agreement shall be null and void and shall
cease to be of any further force or effect.





                  [remainder of page intentionally left blank]

                                       15
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the date first above written.

                         AFFILIATED RESEARCH CENTERS, INC.



                         By:  /s/ Steven M. Rauscher
                              ---------------------------------------
                         Name:  Steven M. Rauscher
                                -------------------------------------
                         Title: Chief Executive Officer


                         GALEN PARTNERS III, L.P.
                         By:  Claudius L.L.C.


                         By:  /s/ Bruce F. Wesson
                              ---------------------------------------
                         Name:  Bruce F. Wesson
                                -------------------------------------
                         Title: President


                         GALEN PARTNERS INTERNATIONAL III, L.P.
                         By:  Claudius L.L.C.


                         By:  /s/ Bruce F. Wesson
                              ---------------------------------------
                         Name:  Bruce F. Wesson
                                -------------------------------------
                         Title: President


                         GALEN EMPLOYEE FUND III, L.P.
                         By:  Wesson Enterprises, Inc.


                         By:  /s/ Bruce F. Wesson
                              ---------------------------------------
                         Name:  Bruce F. Wesson
                                -------------------------------------
                         Title: President


                         DELPHI VENTURES III, L.P.
                         By:  Delphi Management Partners III, L.L.C.
                              General Partner


                         By:  /s/ Donald J. Lothrop
                              ---------------------------------------
                         Name:  Donald J. Lothrop
                                -------------------------------------
                         Title: Managing Member



<PAGE>

                         DELPHI BIOINVESTMENTS III, L.P.
                         By:  Delphi Management Partners III, L.L.C.
                              General Partner

                         By:  /s/ Donald J. Lothrop
                              ---------------------------------------
                         Name:  Donald J. Lothrop
                                -------------------------------------
                         Title: Managing Member


                         HAMBRECHT & QUIST CALIFORNIA

                         By:  /s/ Robert N. Savoie
                              ---------------------------------------
                         Name:  Robert N. Savoie
                                -------------------------------------
                         Title: Tax Director, Attorney-in-Fact
                                -------------------------------------


                         H&Q AFFILIATED RESEARCH INVESTORS, L.P.

                         By:  /s/ Chris McKay
                              ---------------------------------------
                         Name:  Chris McKay
                                -------------------------------------
                         Title: Attorney-in-fact
                                -------------------------------------


                         HAMBRECHT & QUIST EMPLOYEE VENTURE FUND, L.P. II

                         By:  /s/ Robert N. Savoie
                              ---------------------------------------
                         Name:  Robert N. Savoie
                                -------------------------------------
                         Title: Tax Director, Attorney-in-Fact
                                -------------------------------------


                         PREMIER RESEARCH WORLDWIDE, LTD.

                         By:  /s/ John Bauer
                              ---------------------------------------
                         Name:  John Bauer
                                -------------------------------------
                         Title: Chief Financial Officer
                                -------------------------------------


                         TULLIS-DICKERSON CAPITAL FOCUS II, L.P.
                         By:  Tullis Dickerson Partners II, L.L.C., its general
                              partner


                         By:  /s/ Thomas P. Dickerson
                              ---------------------------------------
                         Name:  Thomas P. Dickerson
                                -------------------------------------
                         Title: President
                                -------------------------------------


<PAGE>

                         TD ORIGEN CAPITAL FUND, L.P.
                         By:  TD II Regional Partners, Inc., its general partner


                         By:  /s/ Thomas P. Dickerson
                              ---------------------------------------
                         Name:  Thomas P. Dickerson
                                -------------------------------------
                         Title: President
                                -------------------------------------


                         TD JAVELIN CAPITAL FUND, L.P.
                         By:  JVP, L.P., its general partner

                         By:  JVP, Inc., its general partner


                         By:  /s/ Thomas P. Dickerson
                              ---------------------------------------
                         Name:  Thomas P. Dickerson
                                -------------------------------------
                         Title: President
                                -------------------------------------


                         GE CAPITAL EQUITY INVESTMENTS, INC.


                         By:  /s/ E.R. Frank-Bailey
                              ---------------------------------------
                         Name:  E.R. Frank Bailey
                                -------------------------------------
                         Title: Managing Director
                                -------------------------------------


                                       18
<PAGE>

                                                                         ANNEX A
Galen Partners III, L.P.
610 Fifth Avenue; 5/th/ Floor
Rockefeller Center
New York, New York 10020

Galen Partners International III, L.P.
610 Fifth Avenue; 5/th/ Floor
Rockefeller Center
New York, New York 10020

Galen Employee Fund III, L.P.
610 Fifth Avenue; 5/th/ Floor
Rockefeller Center
New York, New York 10020

Delphi Ventures III, L.P.
3000 Sand Hill Road
Building 1, Suite 135
Menlo Park, California 94025

Delphi BioInvestments III, L.P.
3000 Sand Hill Road
Building 1, Suite 135
Menlo Park, California 94025

Hambrecht & Quist California
One Bush Street
San Francisco, CA  94104

H&Q Affiliated Research Investors, L.P.
One Bush Street
San Francisco, CA  94104

Hambrecht & Quist Employee Venture Fund, L.P. II
One Bush Street
San Francisco, CA  94104

Medical Advisory Systems, Inc.
8050 Southern Maryland Boulevard
Owings, Maryland  20736

Premier Research Worldwide, Ltd.
124 South 15/th/  Street
Philadelphia, PA  19102


<PAGE>

Tullis-Dickerson Capital Focus II, L.P.
One Greenwich Plaza
Greenwich, CT 06830
Fax:  (203) 629-9293

     With a copy to:

     Law Offices of Gloria Skigen
     One Greenwich Plaza
     Third Floor
     Greenwich, CT 06830
     Fax:  (203) 861-2498

TD Origen Capital Fund, L.P.
150 Washington Avenue
Suite 201
Santa Fe, NM  87501
Fax:  (505) 982-7008
Attn:  J. Michael Schafer

     With a copy to:

     Law Offices of Gloria Skigen
     One Greenwich Plaza
     Third Floor
     Greenwich, CT 06830
     Fax:  (203) 861-2498

TD Javelin Capital Fund, L.P.
2850 Cahaba Road, Suite 240
Birmingham, AL  35223
Fax:  (205) 870-4822
Attn:  Lyle A. Hohnke

     With a copy to:

     Law Offices of Gloria Skigen
     One Greenwich Plaza
     Third Floor
     Greenwich, CT 06830
     Fax:  (203) 861-2498

GE Capital Equity Investments, Inc.
120 Long Ride Road
Stamford, CT  06297
Attn:  General Counsel