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                                                                   Exhibit 10.28

                         REGISTRATION RIGHTS AGREEMENT



                                 by and among


                            VIROPHARMA INCORPORATED

                                  as Issuer,


                                      and


                       MORGAN STANLEY & CO. INCORPORATED

                                      and

                        U.S. BANCORP PIPER JAFFRAY INC.

                             as Initial Purchasers



                           Dated as of March 1, 2000
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     THIS REGISTRATION RIGHTS AGREEMENT is made and entered into as of March 1,
2000, by and among ViroPharma Incorporated, a Delaware corporation (the
"Company"), and Morgan Stanley & Co. Incorporated and U.S. Bancorp Piper Jaffray
Inc. (together, the "Initial Purchasers") pursuant to the Purchase Agreement,
dated February 24, 2000 (the "Purchase Agreement") among the Company and the
Initial Purchasers. In order to induce the Initial Purchasers to enter into the
Purchase Agreement, the Company has agreed to provide the registration rights
set forth in this Agreement. The execution of this Agreement is a condition to
the closing under the Purchase Agreement.

     The Company agrees with the Initial Purchasers, (i) for their benefit as
Initial Purchasers and (ii) for the benefit of the beneficial owners (including
the Initial Purchasers) from time to time of the Notes (as defined herein) and
the beneficial owners from time to time of the Underlying Common Stock (as
defined herein) issued upon conversion of the Notes (each of the foregoing a
"Holder" and together the "Holders"), as follows:

     Section 1.  Definitions.  Capitalized terms used herein without definition
shall have their respective meanings set forth in the Purchase Agreement. As
used in this Agreement, the following terms shall have the following meanings:

     "Affiliate" means with respect to any specified person, an "affiliate," as
defined in Rule 144, of such person.

     "Amendment Effectiveness Deadline Date" has the meaning set forth in
Section 2(d) hereof.

     "Applicable Conversion Price"  as of any date of determination means the
Conversion Price in effect as of such date of determination or, if no Notes are
then outstanding, the Conversion Price that would be in effect were Notes then
outstanding.

     "Business Day" means each Monday, Tuesday, Wednesday, Thursday and Friday
that is not a day on which banking institutions in The City of New York are
authorized or obligated by law or executive order to close.

     "Common Stock" means the shares of common stock, par value $.002 per share,
of the Company and any other shares of common stock as may constitute "Common
Stock" for purposes of the Indenture, including the Underlying Common Stock.

     "Conversion Price" has the meaning assigned such term in the Indenture.

     "Damages Accrual Period" has the meaning set forth in Section 2(e) hereof.

     "Damages Payment Date" means each interest payment date under the Indenture
in the case of Notes, and each March 1 and September 1 in the case of the
Underlying Common Stock.

     "Deferral Notice" has the meaning set forth in Section 3(i) hereof.

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     "Deferral Period" has the meaning set forth in Section 3(i) hereof.

     "Effectiveness Deadline Date" has the meaning set forth in Section 2(a)
hereof.

     "Effectiveness Period" means the period commencing on the date hereof and
ending on the date that all Registrable Securities have ceased to be Registrable
Securities.

     "Event" has the meaning set forth in Section 2(e) hereof.

     "Event Date" has the meaning set forth in Section 2(e) hereof.

     "Event Termination Date" has the meaning set forth in Section 2(e) hereof.

     "Exchange Act" means the Securities Exchange Act of 1934, as amended, and
the rules and regulations of the SEC promulgated thereunder.

     "Filing Deadline Date" has the meaning set forth in Section 2(a) hereof.

     "Holder" has the meaning set forth in the second paragraph of this
Agreement.

     "Indenture" means the Indenture, dated as of March 1, 2000, between the
Company and Summit Bank, as trustee, pursuant to which the Notes are being
issued.

     "Initial Purchasers" means Morgan Stanley & Co. Incorporated and U.S.
Bancorp Piper Jaffray Inc.

     "Initial Shelf Registration Statement" has the meaning set forth in Section
2(a) hereof.

     "Issue Date" means March 1, 2000.

     "Liquidated Damages Amount" has the meaning set forth in Section 2(e)
hereof.

     "Losses" has the meaning set forth in Section 6 hereof.

     "Material Event" has the meaning set forth in Section 3(i) hereof.

     "Notes" means the 6% Convertible Subordinated Notes due 2007 of the Company
to be purchased pursuant to the Purchase Agreement.

     "Notice and Questionnaire" means a written notice delivered to the Company
containing substantially the information called for by the Selling
Securityholder Notice and Questionnaire attached as Annex B to the Offering
Memorandum of the Company issued February 24, 2000 relating to the Notes.

     "Notice Holder" means, on any date, any Holder that has delivered a Notice
and Questionnaire to the Company on or prior to such date.

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     "Purchase Agreement" has the meaning set forth in the preamble hereof.

     "Prospectus" means the prospectus included in any Registration Statement
(including, without limitation, a prospectus that discloses 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 amendment or prospectus supplement, including
post-effective amendments, and all materials incorporated by reference or
explicitly deemed to be incorporated by reference in such Prospectus.

     "Record Holder" means (i) with respect to any Damages Payment Date relating
to any Notes as to which any such Liquidated Damages Amount has accrued, the
holder of record of such Note on the record date with respect to the interest
payment date under the Indenture on which such Damages Payment Date shall occur
and (ii) with respect to any Damages Payment Date relating to the Underlying
Common Stock as to which any such Liquidated Damages Amount has accrued, the
registered holder of such Underlying Common Stock on the preceding February 15
in the case of a March 1 Damages Payment Date and on the preceding August 15 in
the case of a September 1 Damages Payment Date.

     "Registrable Securities" means the Notes until such Notes have been
converted into or exchanged for the Underlying Common Stock and, at all times
subsequent to any such conversion or exchange the Underlying Common Stock and
any securities into or for which such Underlying Common Stock has been converted
or exchanged, and any security issued with respect thereto upon any stock
dividend, split or similar event until, in the case of any such security, (A)
the earliest of (i) its effective registration under the Securities Act and
resale in accordance with the Registration Statement covering it, (ii)
expiration of the holding period that would be applicable thereto under Rule
144(k) were it not held by an Affiliate of the Company or (iii) its sale to the
public pursuant to Rule 144 (or any similar provision then in force, but not
Rule 144A) under the Securities Act, and (B) as a result of the event or
circumstance described in any of the foregoing clauses (i) through (iii), the
legend with respect to transfer restrictions required under the Indenture are
removed or removable in accordance with the terms of the Indenture or such
legend, as the case may be.

     "Registration Expenses" has the meaning set forth in Section 5 hereof.

     "Registration Statement" means any registration statement of the Company
that covers any of the Registrable Securities pursuant to the provisions of this
Agreement including the Prospectus, amendments and supplements to such
registration statement, including post-effective amendments, all exhibits, and
all materials incorporated by reference or explicitly deemed to be incorporated
by reference in such registration statement.

     "Restricted Securities" means "Restricted Securities" as defined in Rule
144.

     "Rule 144" means Rule 144 under the Securities Act, as such Rule may be
amended from time to time, or any similar rule or regulation hereafter adopted
by the SEC.

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     "Rule 144A" means Rule 144A under the Securities Act, as such Rule may be
amended from time to time, or any similar rule or regulation hereafter adopted
by the SEC.

     "SEC" means the Securities and Exchange Commission.

     "Securities Act" means the Securities Act of 1933, as amended, and the
rules and regulations promulgated by the SEC thereunder.

     "Shelf Registration Statement" has the meaning set forth in Section 2(a)
hereof.

     "Special Counsel" means Ropes & Gray or such other single successor counsel
as shall be specified by the Holders of a majority of the Registrable
Securities, but which may, with the written consent of the Initial Purchasers
(which shall not be unreasonably withheld), be another nationally recognized law
firm experienced in securities law matters designated by the Company, the
reasonable fees and expenses of which will be paid by the Company pursuant to
Section 5 hereof.

     "Subsequent Shelf Registration Statement" has the meaning set forth in
Section 2(b) hereof.

     "TIA" means the Trust Indenture Act of 1939, as amended.

     "Trustee" means Summit Bank, the Trustee under the Indenture.

     "Underlying Common Stock" means the Common Stock into which the Notes are
convertible or issued upon any such conversion.

     Section 2.  Shelf Registration.  (a) The Company shall use its best efforts
to prepare and file or cause to be prepared and filed with the SEC, as soon as
practicable but in any event by the date (the "Filing Deadline Date") ninety
(90) days after the Issue Date, a Registration Statement for an offering to be
made on a delayed or continuous basis pursuant to Rule 415 of the Securities Act
(a "Shelf Registration Statement") registering the resale from time to time by
Holders thereof of all of the Registrable Securities (the "Initial Shelf
Registration Statement"). The Initial Shelf Registration Statement shall be on
Form S-3 or another appropriate form permitting registration of such Registrable
Securities for resale by such Holders in accordance with the methods of
distribution elected by the Holders and set forth in the Initial Shelf
Registration Statement. The Company shall use its reasonable efforts to cause
the Initial Shelf Registration Statement to be declared effective under the
Securities Act as promptly as is practicable but in any event by the date (the
"Effectiveness Deadline Date") that is one hundred eighty (180) days after the
Issue Date, and to keep the Initial Shelf Registration Statement (or any
Subsequent Shelf Registration Statement) continuously effective under the
Securities Act until the expiration of the Effectiveness Period. At the time the
Initial Shelf Registration Statement is declared effective, each Holder that
became a Notice Holder on or prior to the date ten (10) Business Days prior to
such time of effectiveness shall be named as a selling securityholder in the
Initial Shelf Registration Statement and the related Prospectus in such a manner
as to permit

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such Holder to deliver such Prospectus to purchasers of Registrable Securities
in accordance with applicable law. None of the Company's security holders (other
than the Holders of Registrable Securities) shall have the right to include any
of the Company's securities in the Shelf Registration Statement. The Holders
acknowledge that the filing of a separate registration statement for an offering
to be made on a continuous basis pursuant to Rule 415 of the Securities Act
registering the resale of shares of Common Stock held by, or issuable upon
conversion of outstanding securities held by, PSV, LP (the "PSV Shelf
Registration Statement") will not be deemed to be a breach of this Agreement.

       (b)  If the Initial Shelf Registration Statement or any Subsequent Shelf
Registration Statement ceases to be effective for any reason at any time during
the Effectiveness Period (other than because all Registrable Securities
registered thereunder shall have been resold pursuant thereto or shall have
otherwise ceased to be Registrable Securities), the Company shall use its
reasonable efforts to obtain the prompt withdrawal of any order suspending the
effectiveness thereof, and in any event shall within thirty (30) days of such
cessation of effectiveness in a manner reasonably expected to obtain the
withdrawal of the order suspending the effectiveness thereof, or file either an
amendment to the Shelf Registration Statement or an additional Shelf
Registration Statement covering all of the securities that as of the date of
such filing are Registrable Securities (a "Subsequent Shelf Registration
Statement"). If a Subsequent Shelf Registration Statement is filed, the Company
shall use its reasonable efforts to cause the Subsequent Shelf Registration
Statement to become effective as promptly as is practicable after such filing
and to keep such Registration Statement (or subsequent Shelf Registration
Statement) continuously effective until the end of the Effectiveness Period.

       (c)  The Company shall supplement and amend the Shelf Registration
Statement if required by the rules, regulations or instructions applicable to
the registration form used by the Company for such Shelf Registration Statement,
if required by the Securities Act or as reasonably requested by the Initial
Purchasers or by the Trustee on behalf of the Holders of the Registrable
Securities covered by such Shelf Registration Statement.

       (d)  Each Holder of Registrable Securities agrees that if such Holder
wishes to sell Registrable Securities pursuant to a Shelf Registration Statement
and related Prospectus, it will do so only in accordance with this Section 2(d)
and Section 3(i). Each Holder of Registrable Securities wishing to sell
Registrable Securities pursuant to a Shelf Registration Statement and related
Prospectus agrees to deliver a Notice and Questionnaire to the Company at least
three (3) Business Days prior to any intended distribution of Registrable
Securities under the Shelf Registration Statement. From and after the date the
Initial Shelf Registration Statement is declared effective, the Company shall,
as promptly as practicable after the date a Notice and Questionnaire is
delivered, and in any event upon the later of (x) five (5) Business Days after
such date or (y) five (5) Business Days after the expiration of any Deferral
Period in effect when the Notice and Questionnaire is delivered, or put into
effect within five (5) Business Days of such delivery date, (i) if required by
applicable law, file with the SEC a post-effective amendment to the Shelf
Registration Statement or prepare and, if required by applicable law, file a
supplement to the related Prospectus or a supplement or amendment to any
document incorporated therein by

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reference or file any other required document so that the Holder delivering such
Notice and Questionnaire is named as a selling securityholder in the Shelf
Registration Statement and the related Prospectus in such a manner as to permit
such Holder to deliver such Prospectus to purchasers of the Registrable
Securities in accordance with applicable law and, if the Company shall file a
post-effective amendment to the Shelf Registration Statement, use its reasonable
efforts to cause such post-effective amendment to be declared effective under
the Securities Act as promptly as is practicable, but in any event by the date
(the "Amendment Effectiveness Deadline Date") that is forty-five (45) days after
the date such post-effective amendment is required by this clause to be filed;
(ii) provide such Holder copies of any documents filed pursuant to Section
2(d)(i); and (iii) notify such Holder as promptly as practicable after the
effectiveness under the Securities Act of any post-effective amendment filed
pursuant to Section 2(d)(i); provided, that if such Notice and Questionnaire is
delivered during a Deferral Period, the Company shall so inform the Holder
delivering such Notice and Questionnaire and shall take the actions set forth in
clauses (i), (ii) and (iii) above upon expiration of the Deferral Period in
accordance with Section 3(i). Notwithstanding anything contained herein to the
contrary, (i) the Company shall be under no obligation to name any Holder that
is not a Notice Holder as a selling securityholder in any Registration Statement
or related Prospectus and (ii) the Amendment Effectiveness Deadline Date shall
be extended by up to ten (10) Business Days from the expiration of a Deferral
Period (and the Company shall incur no obligation to pay Liquidated Damages
during such extension) if such Deferral Period shall be in effect on the
Amendment Effectiveness Deadline Date.

       (e)  The parties hereto agree that the Holders of Registrable Securities
will suffer damages, and that it would not be feasible to ascertain the extent
of such damages with precision, if (i) the Initial Shelf Registration Statement
has not been filed on or prior to the Filing Deadline Date, (ii) the Initial
Shelf Registration Statement has not been declared effective under the
Securities Act on or prior to the Effectiveness Deadline Date, (iii) the Company
has failed to perform its obligations set forth in Section 2(d) within the time
period required therein, (iv) the aggregate duration of Deferral Periods in any
period exceeds the number of days permitted in respect of such period pursuant
to Section 3(i) hereof or (v) the number of Deferral Periods in any period
exceeds the number permitted in respect of such period pursuant to Section 3(i)
hereof (each of the events of a type described in any of the foregoing clauses
(i) through (v) are individually referred to herein as an "Event," and the
Filing Deadline Date in the case of clause (i), the Effectiveness Deadline Date
in the case of clause (ii), the date by which the Company is required to perform
its obligations set forth in Section 2(d) in the case of clause (iii) (including
the filing of any post-effective amendment prior to the Amendment Effectiveness
Deadline Date), the date on which the aggregate duration of Deferral Periods in
any period exceeds the number of days permitted by Section 3(i) hereof in the
case of clause (iv), and the date of the commencement of a Deferral Period that
causes the limit on the number of Deferral Periods in any period under Section
3(i) hereof to be exceeded in the case of clause (v), being referred to herein
as an "Event Date"). Events shall be deemed to continue until the "Event
Termination Date," which shall be the following dates with respect to the
respective types of Events: the date the Initial Shelf Registration Statement is
filed in the case of an Event of the type described in

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clause (i); the date the Initial Shelf Registration Statement is declared
effective under the Securities Act in the case of an Event of the type described
in clause (ii); the date the Company performs its obligations set forth in
Section 2(d) in the case of an Event of the type described in clause (iii)
(including, without limitation, the date the relevant post-effective amendment
to the Shelf Registration Statement is declared effective under the Securities
Act); termination of the Deferral Period that caused the limit on the aggregate
duration of Deferral Periods in a period set forth in Section 3(i) to be
exceeded in the case of the commencement of an Event of the type described in
clause (iv) and termination of the Deferral Period the commencement of which
caused the number of Deferral Periods in a period permitted by Section 3(i) to
be exceeded in the case of an Event of the type described in clause (v).

     Accordingly, commencing on (and including) any Event Date and ending on
(but excluding) the next date on which there are no Events that have occurred
and are continuing (a "Damages Accrual Period"), the Company agrees to pay, as
liquidated damages and not as a penalty, an amount (the "Liquidated Damages
Amount"), payable on the Damages Payment Dates to Record Holders of Notes that
are Registrable Securities and of shares of Underlying Common Stock issued upon
conversion of Notes that are Registrable Securities, as the case may be,
accruing, for each portion of such Damages Accrual Period beginning on and
including a Damages Payment Date (or, in respect of the first time that the
Liquidated Damages Amount is to be paid to Holders on a Damages Payment Date as
a result of the occurrence of any particular Event, from the Event Date) and
ending on but excluding the first to occur of (A) the date of the end of the
Damages Accrual Period or (B) the next Damages Payment Date, at a rate per annum
equal to one-half of one percent (0.5 %) of the aggregate principal amount of
such Notes or, in the case of Notes that have been converted into or exchanged
for Underlying Common Stock, the Applicable Conversion Price of such shares of
Underlying Common Stock, as the case may be, in each case determined as of the
Business Day immediately preceding the next Damages Payment Date; provided, that
in the case of a Damages Accrual Period that is in effect solely as a result of
an Event of the type described in clause (iii) of the immediately preceding
paragraph, such Liquidated Damages Amount shall be paid only to the Holders that
have delivered Notice and Questionnaires that caused the Company to incur the
obligations set forth in Section 2(d) the non-performance of which is the basis
of such Event, provided further, that any Liquidated Damages Amount accrued with
respect to any Note or portion thereof called for redemption on a redemption
date or converted into Underlying Common Stock on a conversion date prior to the
Damages Payment Date, shall, in any such event, be paid instead to the Holder
who submitted such Note or portion thereof for redemption or conversion on the
applicable redemption date or conversion date, as the case may be, on such date
(or promptly following the conversion date, in the case of conversion).
Notwithstanding the foregoing, no Liquidated Damages Amounts shall accrue as to
any Registrable Security from and after the earlier of (x) the date such
security is no longer a Registrable Security and (y) expiration of the
Effectiveness Period. The rate of accrual of the Liquidated Damages Amount with
respect to any period shall not exceed the rate provided for in this paragraph
notwithstanding the occurrence of multiple concurrent Events. The Liquidated
Damages Amount shall be computed on the basis of a 360-day year comprised of
twelve (12) 30-day months. Following the cure of all Events requiring the
payment by the

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Company of Liquidated Damages Amounts to the Holders of Registrable Securities
pursuant to this Section, the Damages Accrual Period shall end and the accrual
of Liquidated Damages Amounts will cease (without in any way limiting the effect
of any subsequent Event requiring the payment of Liquidated Damages Amount by
the Company).

     The Trustee shall be entitled, on behalf of Holders of Notes or Underlying
Common Stock, to seek any available remedy for the enforcement of this
Agreement, including for the payment of any Liquidated Damages Amount.
Notwithstanding the foregoing, the parties agree that the sole damages payable
for a violation (i) of the terms of this Agreement and (ii) Section 6(k) of the
Purchase Agreement (insofar as it relates to the Company's Obligations under
this Agreement), in each case with respect to which liquidated damages are
expressly provided, shall be such liquidated damages. Nothing shall preclude a
Notice Holder or Holder of Registrable Securities from pursuing or obtaining
specific performance or other equitable relief with respect to this Agreement.

     All of the Company's obligations set forth in this Section 2(e) that are
outstanding with respect to any Registrable Security at the time such security
ceases to be a Registrable Security shall survive until such time as all such
obligations with respect to such security have been satisfied in full
(notwithstanding termination of this Agreement pursuant to Section 8(k)).

     The parties hereto agree that the liquidated damages provided for in this
Section 2(e) constitute a reasonable estimate of the damages that may be
incurred by Holders of Registrable Securities by reason of the failure of the
Shelf Registration Statement to be filed or declared effective or available for
effecting resales of Registrable Securities in accordance with the provisions
hereof.

     Section 3.  Registration Procedures.  In connection with the registration
obligations of the Company under Section 2 hereof, the Company shall:

     (a)  Prepare and file with the SEC a Registration Statement or Registration
Statements on any appropriate form under the Securities Act available for the
sale of the Registrable Securities by the Holders thereof in accordance with the
intended method or methods of distribution thereof, and use its reasonable
efforts to cause each such Registration Statement to become effective and remain
effective as provided herein; provided, that before filing any Registration
Statement or Prospectus or any amendments or supplements thereto with the SEC,
the Company shall furnish to the Initial Purchasers and the Special Counsel
copies of all such documents proposed to be filed and use its reasonable efforts
to reflect in each such document when so filed with the SEC such comments as the
Special Counsel reasonably shall propose to the Company within three (3)
Business Days after the date on which the Company delivered such copies to the
Initial Purchasers and the Special Counsel.

     (b)  Prepare and file with the SEC such amendments and post-effective
amendments to each Registration Statement as may be necessary to keep such
Registration Statement continuously effective until the expiration of the
Effectiveness Period; cause the related

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Prospectus to be supplemented by any required Prospectus supplement, and as so
supplemented to be filed pursuant to Rule 424 (or any similar provisions then in
force) under the Securities Act; and use its reasonable efforts to comply with
the provisions of the Securities Act applicable to it with respect to the
disposition of all securities covered by such Registration Statement during the
Effectiveness Period in accordance with the intended methods of disposition by
the sellers thereof set forth in such Registration Statement as so amended or
such Prospectus as so supplemented.

     (c)  As promptly as practicable give notice to the Notice Holders, the
Initial Purchasers and the Special Counsel (i) when any Prospectus, Prospectus
supplement, Registration Statement or post-effective amendment to a Registration
Statement has been filed with the SEC and, with respect to a Registration
Statement or any post-effective amendment, when the same has been declared
effective, (ii) of any request, following the effectiveness of the Initial Shelf
Registration Statement under the Securities Act, by the SEC or any other federal
or state governmental authority for amendments or supplements to any
Registration Statement or related Prospectus or for additional information,
(iii) of the issuance by the SEC or any other federal or state governmental
authority of any stop order suspending the effectiveness of any Registration
Statement or the initiation or threatening 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, (v) of the occurrence of (but
not the nature of or details concerning) a Material Event and (vi) of the
determination by the Company that a post-effective amendment to a Registration
Statement will be filed with the SEC, which notice may, at the discretion of the
Company (or as required pursuant to Section 3 (i)), state that it constitutes a
Deferral Notice, in which event the provisions of Section 3(i) shall apply.

     (d)  Use reasonable efforts to obtain the withdrawal of any order
suspending the effectiveness of a Registration Statement or the lifting of any
suspension of the qualification (or exemption from qualification) of any of the
Registrable Securities for sale in any jurisdiction in which they have been
qualified for sale, in either case at the earliest possible moment, and provide
prompt notice to each Notice Holder and the Initial Purchasers of the withdrawal
of any such order.

     (e)  If reasonably requested by the Initial Purchasers or any Notice
Holder, as promptly as practicable incorporate in a Prospectus supplement or
post-effective amendment to a Registration Statement such information as the
Initial Purchasers, the Special Counsel or such Notice Holder shall, on the
basis of a written opinion of nationally-recognized counsel experienced in such
matters, reasonably determine to be required to be included therein by
applicable law and make any required filings of such Prospectus supplement or
such post-effective amendment.

     (f)  As promptly as practicable furnish to each Notice Holder, the Special
Counsel and the Initial Purchasers, without charge, at least one (1) conformed
copy of the Registration

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Statement and any amendment thereto, including financial statements but
excluding schedules, all documents incorporated or deemed to be incorporated
therein by reference and all exhibits (unless requested in writing to the
Company by such Notice Holder, Special Counsel, counsel or Initial Purchasers).

     (g)  During the Effectiveness Period, deliver to each Notice Holder, in
connection with any sale of Registrable Securities pursuant to a Registration
Statement, without charge, as many copies of the Prospectus or Prospectuses
relating to such Registrable Securities (including each preliminary prospectus)
and any amendment or supplement thereto as such Notice Holder may reasonably
request; and the Company hereby consents (except during such periods that a
Deferral Notice is outstanding and has not been revoked) to the use of such
Prospectus or each amendment or supplement thereto by each Notice Holder, in
connection with any offering and sale of the Registrable Securities covered by
such Prospectus or any amendment or supplement thereto in the manner set forth
therein.

     (h)  Prior to any public offering of the Registrable Securities pursuant to
the Shelf Registration Statement, register or qualify or cooperate with the
Notice Holders 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 Notice Holder reasonably requests
in writing (which request may be included in the Notice and Questionnaire);
prior to any public offering of the Registrable Securities pursuant to the Shelf
Registration Statement, keep each such registration or qualification (or
exemption therefrom) effective during the Effectiveness Period in connection
with such Notice Holder's offer and sale of Registrable Securities pursuant to
such registration or qualification (or exemption therefrom) and do any and all
other acts or things reasonably necessary or advisable to enable the disposition
in such jurisdictions of such Registrable Securities in the manner set forth in
the relevant Registration Statement and the related Prospectus; provided, that
nothing contained in this Section 3(h) shall require the Company to (i) qualify
as a foreign corporation or as a dealer in securities in any jurisdiction where
it would not otherwise be required to qualify but for this Agreement or (ii)
take any action that would subject it to general service of process in suits or
to taxation in any such jurisdiction where it is not then so subject.

     (i)  Upon (A) the issuance by the SEC of a stop order suspending the
effectiveness of the Shelf Registration Statement or the initiation of
proceedings with respect to the Shelf Registration Statement under Section 8(d)
or 8(e) of the Securities Act, (B) the occurrence of any event or the existence
of any fact (a "Material Event") as a result of which any Registration Statement
shall 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 not misleading, or any Prospectus shall 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, or (C) the occurrence or existence
of any pending corporate development that, in the reasonable discretion of the
Company, makes it appropriate to suspend the availability of the Shelf
Registration Statement and the related

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Prospectus, (i) in the case of clause (B) above, subject to the next sentence,
as promptly as practicable prepare and file, if necessary pursuant to applicable
law, a post-effective amendment to such Registration Statement or a supplement
to the related Prospectus or any document incorporated therein by reference or
file any other required document that would be incorporated by reference into
such Registration Statement and Prospectus so that such Registration Statement
does 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 not misleading, and such Prospectus does 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, as thereafter
delivered to the purchasers of the Registrable Securities being sold thereunder,
and, in the case of a post-effective amendment to a Registration Statement,
subject to the next sentence, use its reasonable efforts to cause it to be
declared effective as promptly as is practicable, and (ii) give notice to the
Notice Holders and the Special Counsel that the availability of the Shelf
Registration Statement is suspended (a "Deferral Notice") and, upon receipt of
any Deferral Notice, each Notice Holder agrees not to sell any Registrable
Securities pursuant to the Registration Statement until such Notice Holder's
receipt of copies of the supplemented or amended Prospectus provided for in
clause (i) above, or until it is advised in writing by the Company that the
Prospectus may be used, and has received copies of any additional or
supplemental filings that are incorporated or deemed incorporated by reference
in such Prospectus. The Company will use all reasonable efforts to ensure that
the use of the Prospectus may be resumed (x) in the case of clause (A) above, as
promptly as is practicable, (y) in the case of clause (B) above, as soon as, in
the sole judgment of the Company, public disclosure of such Material Event would
not be prejudicial to or contrary to the interests of the Company or, if
necessary to avoid unreasonable burden or expense, as soon as practicable
thereafter and (z) in the case of clause (C) above, as soon as, in the
discretion of the Company, such suspension is no longer appropriate. The Company
shall be entitled to exercise its right under this Section 3(i) to suspend the
availability of the Shelf Registration Statement or any Prospectus, without
incurring or accruing any obligation to pay liquidated damages pursuant to
Section 2(e), no more than six (6) times in any twelve (12) month period, and
any such period during which the availability of the Registration Statement and
any Prospectus is suspended (the "Deferral Period") shall, without incurring any
obligation to pay liquidated damages pursuant to Section 2(e), not exceed 30
days; provided, that in the case of a Material Event relating to an acquisition
or a probable acquisition or financing, recapitalization, business combination,
sale of the Company or other similar transaction, the Company may, without
incurring any obligation to pay liquidated damages pursuant to Section 2(e),
deliver to Notice Holders a second notice to the effect set forth above, which
shall have the effect of extending the Deferral Period by up to an additional 30
days, or such shorter period of time as is specified in such second notice,
provided, that the aggregate duration of any Deferral Periods shall not, without
incurring any obligation to pay liquidated damages pursuant to Section 2(e),
exceed 30 days in any three month period (or 60 days in any three month period
in the event of a Material Event pursuant to which the Company has delivered a
second notice as required above) or 90 days in any twelve (12) month period.

     (j)  If requested in writing in connection with a disposition of
Registrable Securities pursuant to a Registration Statement, make reasonably
available for inspection during normal

                                      -11-
<PAGE>

business hours and cause the appropriate officers, directors and employees of
the Company and its subsidiaries to make reasonably available for inspection
during normal business hours, in each case upon at least three (3) business
days' prior written notice, by a representative for the Notice Holders of such
Registrable Securities and any managing underwriter, broker-dealers, attorneys
and accountants retained by such Notice Holders, all relevant information
reasonably requested by such representative for the Notice Holders or any such
managing underwriter, broker-dealers, attorneys or accountants in connection
with such disposition, in each case as is customary for similar "due diligence"
examinations afforded to selling shareholders in secondary offerings or in the
case of any party which the special counsel deems to be potentially entitled to
a "due diligence" defense under the securities laws, afforded to underwriters in
a registered public offering; provided, however, that such persons shall first
agree in writing with the Company that any information that is reasonably and in
good faith designated by the Company in writing as confidential at the time of
delivery of such information shall be kept confidential by such persons and
shall be used solely for the purposes of exercising rights under this Agreement,
unless (i) disclosure of such information is required by court or administrative
order or is necessary to respond to inquiries of regulatory authorities, (ii)
disclosure of such information is required by law (including any disclosure
requirements pursuant to federal securities laws in connection with the filing
of any Registration Statement or the use of any Prospectus referred to in this
Agreement), (iii) such information becomes generally available to the public
other than as a result of a disclosure or failure to safeguard by any such
person or (iv) such information becomes available to any such person from a
source other than the Company and such source is not bound by any direct or
indirect obligation to the Company not disclose such information, and provided,
that the foregoing inspection and information gathering shall, to the greatest
extent possible, be coordinated on behalf of all the Notice Holders and the
other parties entitled thereto by the counsel referred to in Section 5 and
provided further, that the Company shall not be required to disclose any
information subject to the attorney-client or attorney work product privilege if
and to the extent such disclosure would constitute a waiver of such privilege.

     (k)  Use all reasonable efforts to comply with all applicable rules and
regulations of the SEC and make generally available to its securityholders
earning statements (which need not be audited) satisfying the provisions of
Section 11(a) of the Securities Act and Rule 158 thereunder (or any similar rule
promulgated under the Securities Act) no later than 45 days after the end of any
3-month period (or 90 days after the end of any 12-month period if such period
is a fiscal year) commencing on the first day of the first fiscal quarter of the
Company commencing after the effective date of a Registration Statement, which
statements shall cover said periods.

     (l)  Cooperate with each Notice Holder to facilitate the timely
preparation and delivery of certificates representing Registrable Securities
sold or to be sold pursuant to a Registration Statement, which certificates
shall not bear any restrictive legends, and cause such Registrable Securities to
be in such denominations as are permitted by the Indenture and registered in
such names as such Notice Holder may request in writing at least two (2)
Business Days prior to any sale of such Registrable Securities.

                                      -12-
<PAGE>

       (m)  Provide a CUSIP number for all Registrable Securities covered by
each Registration Statement not later than the effective date of such
Registration Statement and provide the Trustee and the transfer agent for the
Common Stock with printed certificates for the Registrable Securities that are
in a form eligible for deposit with The Depository Trust Company.

       (n)  Cooperate and assist in any filings required to be made with the
National Association of Securities Dealers, Inc.

       (o)  Upon (i) the filing of the Initial Registration Statement and (ii)
the effectiveness of the Initial Registration Statement, announce the same, in
each case by release to Reuters Economic Services and Bloomberg Business News.

     Section 4.  Holder's Obligations.  Each Holder agrees, by acquisition of
the Registrable Securities, that no Holder of Registrable Securities shall be
entitled to sell any of such Registrable Securities pursuant to a Registration
Statement or to receive a Prospectus relating thereto, unless such Holder has
furnished the Company with a Notice and Questionnaire as required pursuant to
Section 2(d) hereof (including the information required to be included in such
Notice and Questionnaire) and the information set forth in the next sentence.
Each Notice Holder agrees promptly to furnish to the Company all information
required to be disclosed in order to make the information previously furnished
to the Company by such Notice Holder not misleading and any other information
regarding such Notice Holder and the distribution of such Registrable Securities
as the Company may from time to time reasonably request. Any sale of any
Registrable Securities by any Holder shall constitute a representation and
warranty by such Holder that the information relating to such Holder and its
plan of distribution is as set forth in the Prospectus delivered by such Holder
in connection with such disposition, that such Prospectus does not as of the
time of such sale contain any untrue statement of a material fact relating to or
provided by such Holder or its plan of distribution and that such Prospectus
does not as of the time of such sale omit to state any material fact relating to
or provided by such Holder or its plan of distribution necessary to make the
statements in such Prospectus, in the light of the circumstances under which
they were made, not misleading.

     Section 5.  Registration Expenses.  The Company shall bear all fees and
expenses incurred in connection with the performance by the Company of its
obligations under Sections 2 and 3 of this Agreement whether or not any of the
Registration Statements are declared effective. Such fees and expenses shall
include, without limitation, (i) all registration and filing fees (including,
without limitation, fees and expenses (x) with respect to filings required to be
made with the National Association of Securities Dealers, Inc. and (y) of
compliance with federal and state securities or Blue Sky laws (including,
without limitation, reasonable fees and disbursements of the Special Counsel in
connection with Blue Sky qualifications of the Registrable Securities under the
laws of such jurisdictions as the Notice Holders of a majority of the
Registrable Securities being sold pursuant to a Registration Statement may
designate), (ii) printing expenses (including, without limitation, expenses of
printing certificates for Registrable Securities in a form eligible for deposit
with The Depository Trust Company), (iii) duplication expenses relating to
copies of any Registration Statement or Prospectus delivered

                                      -13-
<PAGE>

to any Holders hereunder, (iv) reasonable fees and disbursements of counsel for
the Company and the Special Counsel in connection with the Shelf Registration
Statement (provided that the Company shall not be liable for the fees and
expenses of more than one separate firm for all parties participating in any
transaction hereunder), (v) reasonable fees and disbursements of the Trustee and
its counsel and of the registrar and transfer agent for the Common Stock and
(vi) Securities Act liability insurance obtained by the Company in its sole
discretion. In addition, the Company shall pay the internal expenses of the
Company (including, without limitation, all salaries and expenses of officers
and employees performing legal or accounting duties), the expense of any annual
audit, the fees and expenses incurred in connection with the listing by the
Company of the Registrable Securities on any securities exchange on which
similar securities of the Company are then listed and the fees and expenses of
any person, including special experts, retained by the Company. Notwithstanding
the provisions of this Section 5, each seller of Registrable Securities shall
pay selling expenses, which shall include, without limitation, all underwriting
discounts, selling commissions, transfer taxes and other similar expenses and
all registration expenses, to the extent required by applicable law.

     Section 6.  Indemnification.

       (a)  Indemnification by the Company. The Company shall indemnify and hold
harmless each Notice Holder and each person, if any, who controls any Notice
Holder (within the meaning of either Section 15 of the Securities Act or Section
20 of the Exchange Act) from and against any losses, liabilities, claims,
damages and expenses (including, without limitation, any legal or other expenses
reasonably incurred in connection with defending or investigating any such
action or claim) (collectively, "Losses"), caused by any untrue statement or
alleged untrue statement of a material fact contained in any Registration
Statement or Prospectus or in any amendment or supplement thereto or in any
preliminary prospectus, or caused by any omission or alleged omission to state
therein a material fact required to be stated therein or necessary to make the
statements therein in light of the circumstances under which they were made not
misleading, provided, however, that the Company shall not be liable in any such
case to the extent that any such Losses are caused by an untrue statement or
alleged untrue statement contained in or omission or alleged omission from any
of such documents in reliance upon and conformity with any of the information
relating to the Holders furnished to the Company in writing by a Holder
expressly for use therein; provided further, that the indemnification contained
in this paragraph shall not inure to the benefit of any Holder of Registrable
Securities (or to the benefit of any person controlling such Holder) on account
of any such Losses caused by an untrue statement or alleged untrue statement or
omission or alleged omission made in any preliminary prospectus provided in each
case the Company has performed its obligations under Section 3(a) hereof if
either (A) (i) such Holder failed to send or deliver a copy of the Prospectus
with or prior to the delivery of written confirmation of the sale by such Holder
to the person asserting the claim from which such Losses arise and (ii) the
Prospectus would have corrected such untrue statement or alleged untrue
statement or such omission or alleged omission, or (B) (x) such untrue statement
or alleged untrue statement, omission or alleged omission is corrected in an
amendment or supplement to the Prospectus and (y) having previously been
furnished by or on behalf of the Company with copies of the Prospectus as so
amended or supplemented, such Holder thereafter

                                      -14-
<PAGE>

fails to deliver such Prospectus as so amended or supplemented, with or prior to
the delivery of written confirmation of the sale of a Registrable Security to
the person asserting the claim from which such Losses arise.

       (b)  Indemnification by Holders of Registrable Securities. Each Holder
agrees severally and not jointly to indemnify and hold harmless the Company and
its respective directors and officers, and each person, if any, who controls the
Company (within the meaning of either Section 15 of the Securities Act or
Section 20 of the Exchange Act) or any other Holder, from and against all Losses
caused by any untrue statement or alleged untrue statement of a material fact
contained in any Registration Statement or Prospectus or in any amendment or
supplement thereto or in any preliminary prospectus, or arising out of or based
upon any omission or alleged omission to state therein a material fact required
to be stated therein or necessary to make the statements therein in light of the
circumstances under which they were made not misleading, to the extent, but only
to the extent, that such untrue statement or alleged untrue statement or
omission or alleged omission was made in reliance upon and in conformity with
information furnished to the Company by such Holder expressly for use in such
Registration Statement or Prospectus or amendment or supplement thereto. In no
event shall the liability of any selling Holder of Registrable Securities
hereunder be greater in amount than the dollar amount of the proceeds received
by such Holder upon the sale of the Registrable Securities pursuant to the
Registration Statement giving rise to such indemnification obligation.

       (c)  Conduct of Indemnification Proceedings. In case any proceeding
(including any governmental investigation) shall be instituted involving any
person in respect of which indemnity may be sought pursuant to Section 6(a) or
6(b), such person (the "indemnified party") shall promptly notify the person
against whom such indemnity may be sought (the "indemnifying party") in writing
and the indemnifying party, upon request of the indemnified party, shall retain
counsel reasonably satisfactory to the indemnified party to represent the
indemnified party and any others the indemnifying party may designate in such
proceeding and shall pay the reasonable fees and disbursements of such counsel
related to such proceeding. In any such proceeding, any indemnified party shall
have the right to retain its own counsel, but the fees and expenses of such
counsel shall be at the expense of such indemnified party unless (i) the
indemnifying party and the indemnified party shall have mutually agreed to the
retention of such counsel or (ii) the named parties to any such proceeding
(including any impleaded parties) include both the indemnifying party and the
indemnified party and representation of both parties by the same counsel would
be inappropriate due to actual or potential differing interests between them. It
is understood that the indemnifying party shall not, in respect of the legal
expenses of any indemnified party in connection with any proceeding or related
proceedings in the same jurisdiction, be liable for the fees and expenses of
more than one separate firm (in addition to any local counsel) for all
indemnified parties, and that all such fees and expenses shall be reimbursed as
they are incurred. Such separate firm shall be designated in writing by, in the
case of parties indemnified pursuant to Section 6(a), the Holders of a majority
(with Holders of Notes deemed to be the Holders, for purposes of determining
such majority, of the number of shares of Underlying Common Stock into which
such Notes are or would be convertible or exchangeable as of the date

                                      -15-
<PAGE>

on which such designation is made) of the Registrable Securities covered by the
Registration Statement held by Holders that are indemnified parties pursuant to
Section 6(a) and, in the case of parties indemnified pursuant to Section 6(b),
the Company. The indemnifying party shall not be liable for any settlement of
any proceeding effected without its written consent, but if settled with such
consent or if there be a final judgment for the plaintiff, the indemnifying
party agrees to indemnify the indemnified party from and against any loss or
liability by reason of such settlement or judgment. No indemnifying party shall,
without the prior written consent of the indemnified party, effect any
settlement of any pending or threatened proceeding in respect of which any
indemnified party is or could have been a party and indemnity could have been
sought hereunder by such indemnified 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.

       (d)  Contribution.  To the extent that the indemnification provided for
in Section 6(a) or 6(b) is applicable by its terms but unavailable to an
indemnified party or insufficient in respect of any Losses, then each applicable
indemnifying party, in lieu of indemnifying such indemnified party, shall
contribute to the amount paid or payable by such indemnified party as a result
of such Losses (i) in such proportion as is appropriate to reflect the relative
benefits received by the indemnifying party or parties on the one hand and the
indemnified party or parties on the other hand or (ii) if the allocation
provided by clause (i) above is not permitted by applicable law, in such
proportion as is appropriate to reflect not only the relative benefits referred
to in clause (i) above but also the relative fault of the indemnifying party or
parties on the one hand and of the indemnified party or parties on the other
hand in connection with the statements or omissions that resulted in such
Losses, as well as any other relevant equitable considerations. Benefits
received by the Company shall be deemed to be equal to the total net proceeds
from the initial placement pursuant to the Purchase Agreement (before deducting
expenses) of the Registrable Securities to which such Losses relate. Benefits
received by any Holder shall be deemed to be equal to the value of receiving
Registrable Securities that are registered under the Securities Act. The
relative fault of the Holders on the one hand and the Company on the other hand
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 Holders or by
the Company, and the parties' relative intent, knowledge, access to information
and opportunity to correct or prevent such statement or omission. The Holders'
respective obligations to contribute pursuant to this paragraph are several in
proportion to the respective number of Registrable Securities they have sold
pursuant to a Registration Statement, and not joint.

     The parties hereto agree that it would not be just and equitable if
contribution pursuant to this Section 6(d) were determined by pro rata
                                                              --- ----
allocation or by any other method or allocation that does not take into account
the equitable considerations referred to in the immediately preceding paragraph.
The amount paid or payable by an indemnified party as a result of the Losses
referred to in the immediately preceding paragraph shall be deemed to include,
subject to the limitations set forth above, any legal or other expenses
reasonably incurred by such indemnified party in connection with investigating
or defending any such action or claim. Notwithstanding this




                                      -16-
<PAGE>

Section 6(d), an indemnifying party that is a selling Holder of Registrable
Securities shall not be required to contribute any amount in excess of the
amount by which the total price at which the Registrable Securities sold by such
indemnifying party and distributed to the public were offered to the public
exceeds the amount of any damages that such indemnifying party has otherwise
been required to pay by reason of such untrue or alleged untrue statement or
omission or alleged omission. 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.

       (e)  The indemnity, contribution and expense reimbursement obligations of
the parties hereunder shall be in addition to any liability any indemnified
party may otherwise have hereunder, under the Purchase Agreement or otherwise.

       (f)  The indemnity and contribution provisions contained in this Section
6 shall remain operative and in full force and effect regardless of (i) any
termination of this Agreement, (ii) any investigation made by or on behalf of
any Holder or any person controlling any Holder, or the Company, or the
Company's officers or directors or any person controlling the Company and (iii)
the sale of any Registrable Securities by any Holder.

     Section 7.  Information Requirements.  (a) The Company covenants that, if
at any time before the end of the Effectiveness Period the Company is not
subject to the reporting requirements of the Exchange Act, it will cooperate
with any Holder of Registrable Securities and take such further reasonable
action as any Holder of Registrable Securities may reasonably request in writing
(including, without limitation, making such reasonable representations as any
such Holder may reasonably request), all to the extent required from time to
time to enable such Holder to sell Registrable Securities without registration
under the Securities Act within the limitation of the exemptions provided by
Rule 144 and Rule 144A under the Securities Act and customarily taken in
connection with sales pursuant to such exemptions. Upon the written request of
any Holder of Registrable Securities, the Company shall deliver to such Holder a
written statement as to whether it has complied with such filing requirements,
unless such a statement has been included in the Company's most recent report
filed pursuant to Section 13 or Section 15(d) of Exchange Act. Notwithstanding
the foregoing, nothing in this Section 7 shall be deemed to require the Company
to register any of its securities (other than the Common Stock) under any
section of the Exchange Act.

       (b)  The Company shall file the reports required to be filed by it under
the Exchange Act and shall comply with all other requirements set forth in the
instructions to Form S-3 in order to allow the Company to be eligible to file
registration statements on Form S-3.

     Section 8.  Miscellaneous.

       (a)  No Conflicting Agreements.  The Company is not, as of the date
hereof, a party to, nor shall it, on or after the date of this Agreement, enter
into, any agreement with respect to its securities that conflicts with the
rights granted to the Holders of Registrable Securities in this Agreement,
except such conflict as has been validly waived. The Company represents and

                                      -17-
<PAGE>

warrants that the rights granted to the Holders of Registrable Securities
hereunder do not in any way conflict with the rights granted to the holders of
the Company's securities under any other agreements, other than such rights as
have been waived. The Holders acknowledge that the filing of the PSV Shelf
Registration Statement will not be deemed a breach of this Agreement.

       (b)  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 Company has obtained the written consent of Holders of a
majority of the then outstanding Underlying Common Stock constituting
Registrable Securities (with Holders of Notes deemed to be the Holders, for
purposes of this Section, of the number of outstanding shares of Underlying
Common Stock into which such Notes are or would be convertible or exchangeable
as of the date on which such consent is requested). Notwithstanding the
foregoing, a waiver or consent to depart from the provisions hereof with respect
to a matter that relates exclusively to the rights of Holders of Registrable
Securities whose securities are being sold pursuant to a Registration Statement
and that does not directly or indirectly affect the rights of other Holders of
Registrable Securities may be given by Holders of at least a majority of the
Registrable Securities being sold by such Holders pursuant to such Registration
Statement; provided, that the provisions of this sentence may not be amended,
modified, or supplemented except in accordance with the provisions of the
immediately preceding sentence. Each Holder of Registrable Securities
outstanding at the time of any such amendment, modification, supplement, waiver
or consent or thereafter shall be bound by any such amendment, modification,
supplement, waiver or consent effected pursuant to this Section 8(b), whether or
not any notice, writing or marking indicating such amendment, modification,
supplement, waiver or consent appears on the Registrable Securities or is
delivered to such Holder.

       (c)  Notices.  All notices and other communications provided for or
permitted hereunder shall be made in writing by hand delivery, by telecopier, by
courier guaranteeing overnight delivery or by first-class mail, return receipt
requested, and shall be deemed given (i) when made, if made by hand delivery,
(ii) upon confirmation, if made by telecopier, (iii) one (1) Business Day after
being deposited with such courier, if made by overnight courier or (iv) on the
date indicated on the notice of receipt, if made by first-class mail, to the
parties as follows:

             (w)  if to a Holder of Registrable Securities, at the most current
                  address given by such Holder to the Company in a Notice and
                  Questionnaire or any amendment thereto;

             (x)  if to the Company, to:

                  ViroPharma Incorporated
                  405 Eagleview Boulevard
                  Exton, PA  19341
                  Attention:  General Counsel



                                      -18-
<PAGE>

               Telecopy No.:  (610) 458-7380

               and

               Pepper, Hamilton LLP
               1235 Westlakes Drive, Suite 400
               Berwyn, PA  19312
               Attention: Jeffrey P.- Libson, Esq.
               Telecopy No.: (610) 640-7835

          (y)  if to the Initial Purchasers, to:

               Morgan Stanley & Co. Incorporated
               1585 Broadway
               New York, New York
               Attention:  Equity Capital Markets
               Telecopy No.: (212) 761-0538

               and

               Ropes & Gray
               One International Place
               Boston, MA  02110
               Attention:  Patrick O'Brien, Esq.
               Telecopy No.  (617) 951-7050

or to such other address as such person may have furnished to the other persons
identified in this Section 8(c) in writing in accordance herewith.

  (d)  Approval of Holders.  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 (as such term is
defined in Rule 405 under the Securities Act) (other than the Initial Purchasers
or subsequent Holders of Registrable Securities if such subsequent Holders are
deemed to be such affiliates solely by reason of their 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.

  (e)  Successors and Assigns.  Any person who purchases any Registrable
Securities from the Initial Purchasers shall be deemed, for purposes of this
Agreement, to be an assignee of the Initial Purchasers. This Agreement shall
inure to the benefit of and be binding upon the successors and assigns of each
of the parties and shall inure to the benefit of and be binding upon each Holder
of any Registrable Securities.

                                      -19-
<PAGE>

       (f)  Counterparts.  This Agreement may be executed in any number of
counterparts and by the parties hereto in separate counterparts, each of which
when so executed shall be deemed to be original and all of which taken together
shall constitute one and the same agreement.

       (g)  Headings.  The headings in this Agreement are for convenience of
reference only and shall not limit or otherwise affect the meaning hereof.

       (h)  Governing Law.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK, WITHOUT REGARD TO CONFLICTS
OF LAWS PRINCIPLES THEREOF.

       (i)  Severability. If any term provision, covenant or restriction of this
Agreement is held to be invalid, illegal, void or unenforceable, 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 thereby, and the parties hereto shall use their best 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
being intended that all of the rights and privileges of the parties shall be
enforceable to the fullest extent permitted by law.

       (j)  Entire Agreement.  This Agreement is intended by the parties as a
final expression of their agreement and is intended to be a complete and
exclusive statement of the agreement and understanding of the parties hereto in
respect of the subject matter contained herein and the registration rights
granted by the Company with respect to the Registrable Securities. Except as
provided in the Purchase Agreement, there are no restrictions, promises,
warranties or undertakings, other than those set forth or referred to herein,
with respect to the registration rights granted by the Company with respect to
the Registrable Securities. This Agreement supersedes all prior agreements and
undertakings among the parties with respect to such registration rights. No
party hereto shall have any rights, duties or obligations other than those
specifically set forth in this Agreement. In no event will such methods of
distribution take the form of an underwritten offering of the Registrable
Securities without the prior agreement of the Company.

       (k)  Termination.  This Agreement and the obligations of the parties
hereunder shall terminate upon the end of the Effectiveness Period, except for
any liabilities or obligations under Section 4, 5 or 6 hereof and the
obligations to make payments of and provide for liquidated damages under Section
2(e) hereof to the extent such damages accrue prior to the end of the
Effectiveness Period, each of which shall remain in effect in accordance with
its terms.

                                      -20-
<PAGE>

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

                                       VIROPHARMA INCORPORATED



                                       By: /s/ Vincent J. Milano
                                           ---------------------
                                           Name: Vincent J. Milano
                                           Title: Vice President, Chief
                                                  Financial Officer


Confirmed and accepted as of
the date first above written:

MORGAN STANLEY & CO. INCORPORATED
U.S. BANCORP PIPER JAFFRAY INC.



By: /s/ Albert L. Lord
    ------------------
    Name: Albert L. Lord
    Title: Vice President