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                                                                  EXECUTION COPY

                                                                     EXHIBIT 4.2


                               Up to $350,000,000

                           Human Genome Sciences, Inc.

            3 3/4% Convertible Subordinated Notes Due March 15, 2007

                          REGISTRATION RIGHTS AGREEMENT

                                                                  March 10, 2000

Credit Suisse First Boston Corporation
Eleven Madison Avenue
New York, New York  10010-3629

Dear Sirs:

          Human Genome Sciences, Inc., a Delaware corporation (the "Company"),
proposes to issue and sell to Credit Suisse First Boston Corporation ("CSFBC"),
(the "Initial Purchaser"), upon the terms set forth in a purchase agreement of
even date herewith (the "Purchase Agreement"), $300,000,000 aggregate principal
amount (plus up to an additional $50,000,000 principal amount at the option of
the Initial Purchaser) of 3 3/4% Convertible Subordinated Notes Due March 15,
2007 (the "Notes") of the Company. The Notes will be convertible into shares of
Common Stock, par value $0.01 per share, of the Company (the "Common Stock") at
the conversion price set forth in the Offering Circular dated March 6, 2000. The
Notes will be issued pursuant to an Indenture, dated as of March 10, 2000 (the
"Indenture"), between the Company and The Bank of New York (the "Trustee"). As
an inducement to the Initial Purchaser to enter into the Purchase Agreement and
in satisfaction of a condition to the Initial Purchaser's obligations
thereunder, the Company agrees with the Initial Purchaser, (i) for the benefit
of the Initial Purchaser and (ii) for the benefit of the holders of the Notes
and the Common Stock issuable upon conversion of the Notes (collectively, the
"Securities") from time to time until such time as such Securities have been
sold pursuant to a Shelf Registration Statement (as defined below) (each of the
foregoing a "Holder" and together the "Holders"), as follows:

          1. Shelf Registration. The Company shall take the following actions:

          (a) The Company shall, at its cost, prepare and, as promptly as
practicable (but in no event more than 60 days after the first date of original



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issuance of the Notes) file with the Securities and Exchange Commission (the
"Commission") and thereafter shall use its best efforts to cause to be declared
effective as soon as practicable, but no later than 120 days after the first
date of original issuance of the Notes, a registration statement on Form S-3
(the "Shelf Registration Statement") covering the offer and sale of the Transfer
Restricted Securities (as defined in Section 5(d) hereof) by the Holders thereof
from time to time in accordance with the methods of distribution set forth in
the Shelf Registration Statement and Rule 415 under the Securities Act of 1933
(the "Securities Act") (hereinafter, the "Shelf Registration"); provided,
however, that no Holder (other than the Initial Purchaser) shall be entitled to
have the Securities held by it covered by such Shelf Registration Statement
unless such Holder agrees in writing to be bound by all the provisions of this
Agreement applicable to such Holder.

          (b) The Company shall use its best efforts to keep the Shelf
Registration Statement continuously effective, in order to permit the prospectus
included therein to be lawfully delivered by the Holders of the relevant
Securities, for a period of two years (or for such longer period if extended
pursuant to Section 2(h) below) from the date of its effectiveness or such
shorter period that will terminate when all the Securities covered by the Shelf
Registration Statement (i) have been sold pursuant thereto or (ii) may be sold
pursuant to Rule 144(k) under the Securities Act (or any successor rule
therefore), assuming for this purpose that the Holders thereof are not
affiliates of the Company (in any such case, such period being called the "Shelf
Registration Period"). The Company shall be deemed not to have used its best
efforts to keep the Shelf Registration Statement effective during the requisite
period if it voluntarily takes any action that would result in Holders of
Securities covered thereby not being able to offer and sell such Securities
during that period, unless (i) such action is required by applicable law or (ii)
upon the occurrence of any event contemplated by paragraph 2(b)(v) below, such
action is taken by the Company in good faith and for valid business reasons and
the Company thereafter complies with the requirements of paragraph 2(h).

          (c) Notwithstanding any other provisions of this Agreement to the
contrary, the Company shall cause the Shelf Registration Statement and the
related prospectus and any amendment or supplement thereto, as of the effective
date of the Shelf Registration Statement, amendment or supplement, (i) to comply
in all material respects with the applicable requirements of the Securities Act
and the rules and regulations of the Commission and (ii) not to contain any
untrue statement of a material fact or omit to state a material fact


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required to be stated therein or necessary to make the statements therein, in
light of the circumstances under which they were made, not misleading.

          2. Registration Procedures. In connection with the Shelf Registration
the following provisions shall apply:

          (a) The Company shall (i) furnish to the Initial Purchaser, prior to
the filing thereof with the Commission, a copy of the Shelf Registration
Statement and each amendment thereof and each amendment or supplement, if any,
to the prospectus included therein and, in the event that an Initial Purchaser
(with respect to any portion of an unsold allotment from the original offering)
is participating in the Shelf Registration Statement, shall use its best efforts
to reflect in each such document, when so filed with the Commission, such
comments as the Initial Purchaser reasonably may propose; and (ii) include the
names of the Holders who propose to sell Securities pursuant to the Shelf
Registration Statement, as selling security holders.

          (b) The Company shall give written notice to the Initial Purchaser and
the Holders (which notice pursuant to clauses (ii)-(v) hereof shall be
accompanied by an instruction to suspend the use of the prospectus until the
requisite changes have been made):

          (i) when the Shelf Registration Statement or any amendment thereto has
     been filed with the Commission and when the Shelf Registration Statement or
     any post-effective amendment thereto has become effective;

          (ii) of any request by the Commission for amendments or supplements to
     the Shelf Registration Statement or the prospectus included therein or for
     additional information;

          (iii) of the issuance by the Commission of any stop order suspending
     the effectiveness of the Shelf Registration Statement or the initiation of
     any proceedings for that purpose;

          (iv) of the receipt by the Company or its legal counsel of any
     notification with respect to the suspension of the qualification of the
     Securities for sale in any jurisdiction or the initiation or threatening of
     any proceeding for such purpose; and



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          (v) of the happening of any event or circumstance that would require
     the Company to make changes in the Shelf Registration Statement or the
     prospectus in order that the Shelf Registration Statement or the prospectus
     do not contain an untrue statement of a material fact nor omit to state a
     material fact required to be stated therein or necessary to make the
     statements therein (in the case of the prospectus, in light of the
     circumstances under which they were made) not misleading, which written
     notice need not provide any detail as to the nature of such event.

          (c) The Company shall make every reasonable effort to obtain the
withdrawal, at the earliest possible time, of any order suspending the
effectiveness of the Shelf Registration Statement.

          (d) The Company shall furnish to each Holder of Securities included
within the coverage of the Shelf Registration, without charge, at least one copy
of the Shelf Registration Statement and any post-effective amendment thereto,
including financial statements and schedules, and, if the Holder so requests in
writing, all exhibits thereto (including those, if any, incorporated by
reference).

          (e) The Company shall, during the Shelf Registration Period, deliver
to each Holder of Securities included within the coverage of the Shelf
Registration Statement, without charge, as many copies of the prospectus
(including each preliminary prospectus) included in the Shelf Registration
Statement and any amendment or supplement thereto as such person may reasonably
request. The Company consents, subject to the provisions of this Agreement, to
the use of the prospectus or any amendment or supplement thereto by each of the
selling Holders in connection with the offering and sale of the Securities
covered by the prospectus, or any amendment or supplement thereto, included in
the Shelf Registration Statement.

          (f) Prior to any public offering of the Securities, pursuant to the
Shelf Registration Statement, the Company shall register or qualify or cooperate
with the Holders of the Securities included therein and their respective counsel
in connection with the registration or qualification of such Securities for
offer and sale under the securities or "blue sky" laws of such states of the
United States as any such Holder reasonably requests in writing and do any and
all other acts or


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things necessary or advisable to enable the offer and sale in such jurisdictions
of the Securities covered by the Shelf Registration Statement; provided,
however, that the Company shall not be required to (i) qualify generally to do
business in any jurisdiction where it is not then so qualified or (ii) take any
action which would subject it to general service of process or to taxation in
any jurisdiction where it is not then so subject.

          (g) The Company shall cooperate with the Holders of the Securities to
facilitate the timely preparation and delivery of certificates representing the
Securities to be sold pursuant to the Shelf Registration Statement free of any
restrictive legends and in such denominations and registered in such names as
the Holders may request a reasonable period of time prior to sales of the
Securities pursuant to the Shelf Registration Statement.

          (h)  Upon the occurrence of any event contemplated by paragraphs (ii)
through (v) of Section 2(b) above during the period for which the Company is
required to maintain an effective Shelf Registration Statement, the Company
shall, as required hereby, prepare and file a post-effective amendment to the
Shelf Registration Statement or an amendment or supplement to the related
prospectus and any other required document so that, as thereafter delivered to
Holders or purchasers of Securities, the prospectus will not contain an 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 light of the
circumstances under which they were made, not misleading. If the Company
notifies the Initial Purchaser and the Holders in accordance with paragraphs
(ii) through (v) of Section 2(b) above to suspend the use of the prospectus
until the requisite changes to the prospectus have been made, then the Initial
Purchaser and the Holders shall suspend use of such prospectus and the period of
effectiveness of the Shelf Registration Statement provided for in Section 1(b)
above shall be extended by the number of days from and including the date of
giving such notice to and including the date when the Initial Purchaser and the
Holders shall have received such amended or supplemented prospectus pursuant to
this Section 2(h). Any such amendment, supplement or document will be prepared
and filed as promptly as practicable under the circumstances, as determined in
good faith by the Company, it being understood that the Company may have bona
fide reasons to delay such preparation and filing for a period which may not in
any event exceed 60 days.

          (i) Not later than the effective date of the Shelf Registration
Statement, the Company will provide CUSIP numbers for the Notes and the



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Common Stock registered under the Shelf Registration Statement, and provide the
Trustee with printed certificates for such Notes, in form eligible for deposit
with The Depository Trust Company.

          (j) The Company will comply with all rules and regulations of the
Commission to the extent and so long as they are applicable to the Shelf
Registration and will make generally available to its security holders (or
otherwise provide in accordance with Section 11(a) of the Securities Act) an
earnings statement satisfying the provisions of Section 11(a) of the Securities
Act, no later than 45 days after the end of a 12-month period (or 90 days, if
such period is a fiscal year) beginning with the first month of the Company's
first fiscal quarter commencing after the effective date of the Shelf
Registration Statement, which statement shall cover such 12-month period.

          (k) The Company shall cause the Indenture to be qualified under the
Trust Indenture Act of 1939 (the "Trust Indenture Act"), in a timely manner and
containing such changes, if any, as shall be necessary for such qualification.
In the event that such qualification would require the appointment of a new
trustee under the Indenture, the Company shall appoint a new trustee thereunder
pursuant to the applicable provisions of the Indenture.

          (l) The Company may require each Holder of Securities to be sold
pursuant to the Shelf Registration Statement to furnish to the Company such
information regarding the Holder and the distribution of the Securities as the
Company may from time to time reasonably require for inclusion in the Shelf
Registration Statement, and the Company may exclude from such registration the
Securities of any Holder that fails to furnish such information within a
reasonable time after receiving such request.

          (m) The Company shall enter into such customary agreements (including,
if requested, an underwriting agreement in customary form) and take any other
actions any Holder shall reasonably request in order to facilitate the
disposition of the Securities pursuant to the Shelf Registration Statement,
provided, however, that the Company shall not be required to facilitate an
underwritten offering pursuant to the Shelf Registration Statement by any
Holders unless the offering relates to at least $50,000,000 principal amount of
the Notes or the equivalent number of shares of Common Stock into which such
Notes are convertible.


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          (n) The Company shall (i) make reasonably available for inspection by
the Holders of the Securities, any underwriter participating in any distribution
pursuant to the Shelf Registration Statement and any attorney, accountant or
other agent retained by the Holders of the Securities or any such underwriter
all relevant financial and other records, pertinent corporate documents and
properties of the Company and (ii) cause the Company's officers, directors,
employees, accountants and auditors to supply all relevant information
reasonably requested by the Holders of the Securities or any such underwriter,
attorney, accountant or agent in connection with the Shelf Registration
Statement, in each case, as shall be reasonably necessary to enable such
persons, to conduct a reasonable investigation within the meaning of Section 11
of the Securities Act; provided, however, that the foregoing inspection and
information gathering (i) shall be coordinated on behalf of the Initial
Purchaser by you and on behalf of the other parties, by one counsel designated
by and on behalf of the other parties as described in Section 3 hereof.

          (o) The Company, if requested by any Holder of Securities covered by
the Shelf Registration Statement, shall cause (i) its counsel (which may include
the Company's general counsel for one or more of the opinions) to deliver an
opinion and updates thereof relating to the Securities in customary form
addressed to such Holders, and the managing underwriters, if any, thereof, and
dated, in the case of the initial opinion, the effective date of such Shelf
Registration Statement (it being agreed that the matters to be covered by such
opinion shall include, without limitation, the due incorporation and good
standing of the Company; the qualification of the Company to transact business
as a foreign corporation; the due authorization, execution and delivery of the
relevant agreement of the type referred to in Section 2(m) hereof; the due
authorization, execution, authentication and issuance, and the validity and
enforceability, of the applicable Securities; the absence of material legal or
governmental proceedings involving the Company; the absence of governmental
approvals required to be obtained in connection with the Shelf Registration
Statement, the offering and sale of the applicable Securities, or any agreement
of the type referred to in Section 2(m) hereof; the compliance as to form of
such Shelf Registration Statement and any documents incorporated by reference
therein and of the Indenture with the requirements of the Securities Act and the
Trust Indenture Act, respectively; and, as of the date of the opinion and as of
the effective date of the Shelf Registration Statement or most recent
post-effective amendment thereto, as the case may be, the absence from such
Shelf Registration Statement and the prospectus included therein, as then
amended or


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supplemented, and from any documents incorporated by reference therein of an
untrue statement of a material fact or the omission to state therein a material
fact required to be stated therein or necessary to make the statements therein
not misleading (in the case of any such documents, in the light of the
circumstances existing at the time that such documents were filed with the
Commission under the Securities Exchange Act of 1934 (the "Exchange Act")), (ii)
its officers to execute and deliver all customary documents and certificates and
updates thereof requested by any underwriters of the applicable Securities or
counsel for the Holders and (iii) its independent public accountants and the
independent public accountants with respect to any other entity for which
financial information is provided in the Shelf Registration Statement to provide
to the selling Holders of the applicable Securities and any underwriter therefor
a comfort letter in customary form and covering matters of the type customarily
covered in comfort letters in connection with primary underwritten offerings,
subject to receipt of appropriate documentation as contemplated, and only if
permitted, by Statement of Auditing Standards No. 72.

          (p) The Company shall use its best efforts to take all other steps
necessary to effect the registration of the Securities covered by the Shelf
Registration Statement contemplated hereby.

          3. Registration Expenses. The Company shall bear all fees and expenses
incurred in connection with the performance of its obligations under Sections 1
through 2 hereof (including the reasonable fees and expenses, if any, of
Cravath, Swaine & Moore, counsel for the Initial Purchaser, incurred in
connection with the Shelf Registration), whether or not the Shelf Registration
Statement is filed or becomes effective, and shall bear or reimburse the Holders
of the Securities covered by the Shelf Registration for the reasonable fees and
disbursements of one firm of counsel designated by the Holders of a majority in
principal amount of the Securities covered by the Shelf Registration Statement
(provided that Holders of Common Stock issued upon the conversion of the Notes
shall be deemed to be Holders of the aggregate principal amount of Notes from
which such Common Stock was converted) to act as counsel for the Holders in
connection therewith.

          4. Indemnification. (a) The Company agrees to indemnify and hold
harmless each Holder and each person, if any, who controls such Holder


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within the meaning of the Securities Act or the Exchange Act (each Holder and
such controlling persons are referred to collectively as the "Indemnified
Parties") from and against any losses, claims, damages or liabilities, joint or
several, or any actions in respect thereof (including, but not limited to, any
losses, claims, damages, liabilities or actions relating to purchases and sales
of the Securities) to which each Indemnified Party becomes subject under the
Securities Act, the Exchange Act or otherwise, insofar as such losses, claims,
damages, liabilities or actions arise out of or are based upon any untrue
statement or alleged untrue statement of a material fact contained in the Shelf
Registration Statement or prospectus, including any document incorporated by
reference therein, or in any amendment or supplement thereto or in any
preliminary prospectus relating to the Shelf Registration, or arise out of, or
are based upon, the omission or alleged omission to state therein a material
fact required to be stated therein or necessary to make the statements therein
not misleading, and shall reimburse, as incurred, the Indemnified Parties for
any legal or other expenses reasonably incurred by them in connection with
investigating or defending any such loss, claim, damage, liability or action in
respect thereof; provided, however, that (i) the Company shall not be liable in
any such case to the extent that such loss, claim, damage or liability arises
out of or is based upon any untrue statement or alleged untrue statement or
omission or alleged omission made in the Shelf Registration Statement or
prospectus or in any amendment or supplement thereto or in any preliminary
prospectus relating to the Shelf Registration in reliance upon and in conformity
with written information pertaining to such Holder and furnished to the Company
by or on behalf of such Holder specifically for inclusion therein and (ii) with
respect to any untrue statement or omission or alleged untrue statement or
omission made in any preliminary prospectus relating to the Shelf Registration
Statement, the indemnity agreement contained in this subsection (a) shall not
inure to the benefit of any Holder from whom the person asserting any such
losses, claims, damages or liabilities purchased the Securities concerned, to
the extent that a prospectus relating to such Securities was required to be
delivered by such Holder under the Securities Act in connection with such
purchase and any such loss, claim, damage or liability of such Holder results
from the fact that there was not sent or given to such person, at or prior to
the written confirmation of the sale of such Securities to such person, a copy
of the final prospectus if the Company had previously furnished copies thereof
to such Holder; provided further, however, that this indemnity agreement will be
in addition to any liability which the Company may otherwise have to such
Indemnified Party.

          The Company shall also indemnify the underwriters, their officers

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and directors and each person who controls such underwriters within the meaning
of the Securities Act or the Exchange Act to the same extent as provided above
with respect to the indemnification of the Holders if requested by such Holders.

          (b) Each Holder, severally and not jointly, will indemnify and hold
harmless the Company, its officers and directors and each person, if any, who
controls the Company within the meaning of the Securities Act or the Exchange
Act from and against any losses, claims, damages or liabilities or any actions
in respect thereof, to which the Company or any such controlling person may
become subject under the Securities Act, the Exchange Act or otherwise, insofar
as such losses, claims, damages, liabilities or actions arise out of or are
based upon any untrue statement or alleged untrue statement of a material fact
contained in a Shelf Registration Statement or prospectus or in any amendment or
supplement thereto or in any preliminary prospectus relating to a Shelf
Registration, or arise out of or are based upon the omission or alleged omission
to state therein a material fact necessary to make the statements therein not
misleading, but in each case only to the extent that the untrue statement or
omission or alleged untrue statement or omission was made in reliance upon and
in conformity with written information pertaining to such Holder and furnished
to the Company by or on behalf of such Holder specifically for inclusion
therein; and, subject to the limitation set forth immediately preceding this
clause, shall reimburse, as incurred, the Company for any legal or other
expenses reasonably incurred by the Company or any such controlling person in
connection with investigating or defending any loss, claim, damage, liability or
action in respect thereof. This indemnity agreement will be in addition to any
liability which such Holder may otherwise have to the Company or any of its
controlling persons.

          (c) Promptly after receipt by an indemnified party under this Section
4 of notice of the commencement of any action or proceeding (including a
governmental investigation), such indemnified party will, if a claim in respect
thereof is to be made against the indemnifying party under this Section 4,
notify the indemnifying party of the commencement thereof; but the omission so
to notify the indemnifying party will not, in any event, relieve the
indemnifying party from any obligations to any indemnified party other than the
indemnification obligation provided in subsections (a) or (b) above. In case any
such action is brought against any indemnified party, and it notifies the
indemnifying party of the commencement thereof, the indemnifying party will be
entitled to participate therein and, to the extent that it may wish, jointly
with any other indemnifying party similarly notified, to assume the defense
thereof, with counsel reasonably


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satisfactory to such indemnified party (who shall not, except with the consent
of the indemnified party, be counsel to the indemnifying party), and after
notice from the indemnifying party to such indemnified party of its election so
to assume the defense thereof the indemnifying party will not be liable to such
indemnified party under this Section 4 for any legal or other expenses, other
than reasonable costs of investigation, subsequently incurred by such
indemnified party in connection with the defense thereof. No indemnifying party
shall, without the prior written consent of the indemnified party, effect any
settlement of any pending or threatened action 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 any claims
that are the subject matter of such action.

          (d) If the indemnification provided for in this Section 4 is
unavailable or insufficient to hold harmless an indemnified party under
subsections (a) or (b) above, then each indemnifying party shall contribute to
the amount paid or payable by such indemnified party as a result of the losses,
claims, damages or liabilities (or actions in respect thereof) referred to in
subsections (a) or (b) above (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 on the other from the registration of the
Securities, pursuant to the Shelf Registration, or (ii) if the allocation
provided by the foregoing clause (i) 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 the indemnified party on the other in connection
with the statements or omissions that resulted in such losses, claims, damages
or liabilities (or actions in respect thereof) as well as any other relevant
equitable considerations. The relative fault of the parties 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 Company on the one hand or
such Holder or such other indemnified party, as the case may be, on the other,
and the parties' relative intent, knowledge, access to information and
opportunity to correct or prevent such statement or omission. The amount paid by
an indemnified party as a result of the losses, claims, damages or liabilities
referred to in the first sentence


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of this subsection (d) shall be deemed to include any legal or other expenses
reasonably incurred by such indemnified party in connection with investigating
or defending any action or claim which is the subject of this subsection (d).
Notwithstanding any other provision of this Section 4(d), the Holders shall not
be required to contribute any amount in excess of the amount by which the net
proceeds received by such Holders from the sale of the Securities pursuant to
the Shelf Registration Statement exceeds the amount of damages which such
Holders have 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. For purposes of this paragraph (d), each person,
if any, who controls such indemnified party within the meaning of the Securities
Act or the Exchange Act shall have the same rights to contribution as such
indemnified party and each person, if any, who controls the Company within the
meaning of the Securities Act or the Exchange Act shall have the same rights to
contribution as the Company.

          (e) The agreements contained in this Section 4 shall survive the sale
of the Securities pursuant to the Shelf Registration Statement and shall remain
in full force and effect, regardless of any termination or cancelation of this
Agreement or any investigation made by or on behalf of any indemnified party.

          5. Additional Interest Under Certain Circumstances. (a) Additional
interest (the "Additional Interest") with respect to the Notes shall be assessed
as follows if any of the following events occur (each such event in clauses (i)
through (iii) below being herein called a "Registration Default"):

          (i) if on or prior to the 60th day after the first date of original
     issuance of the Notes, the Shelf Registration Statement has not been filed
     with the Commission;

     (ii) if on or prior to the 120th day after the first date of original
     issuance of the Notes, the Shelf Registration Statement has not been
     declared effective by the Commission; or

     (iii) if after the Shelf Registration Statement is declared effective (A)
     the


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     Shelf Registration Statement thereafter ceases to be effective; or (B) the
     Shelf Registration Statement or the related prospectus ceases to be usable
     in connection with resales of Transfer Restricted Securities (as defined
     below) during the periods specified herein because either (1) any event
     occurs as a result of which the related prospectus forming part of such
     Shelf Registration Statement would include any untrue statement of a
     material fact or omit to state any material fact necessary to make the
     statements therein in the light of the circumstances under which they were
     made not misleading, or (2) it shall be necessary to amend such Shelf
     Registration Statement or supplement the related prospectus, to comply with
     the Securities Act or the Exchange Act or the respective rules thereunder.

          Additional Interest shall accrue on the Notes over and above the
interest set forth in the title of the Notes from and including the date on
which any such Registration Default shall occur, to but excluding the date on
which all such Registration Defaults have been cured, at a rate of 0.50% per
annum.

          (b) A Registration Default referred to in Section 5(a)(iii)(B) shall
be deemed not to have occurred and be continuing in relation to the Shelf
Registration Statement or the related prospectus if (i) such Registration
Default has occurred solely as a result of (x) the filing of a post-effective
amendment to the Shelf Registration Statement to incorporate annual audited
financial information with respect to the Company where such post-effective
amendment is not yet effective and needs to be declared effective to permit
Holders to use the related prospectus or (y) other material events, with respect
to the Company that would need to be described in the Shelf Registration
Statement or the related prospectus and (ii) in the case of clause (y), the
Company is proceeding in good faith to amend or supplement the Shelf
Registration Statement and related prospectus to describe such events as
required by paragraph 2(h) hereof; provided, however, that in any case if such
Registration Default occurs for a continuous period in excess of 60 days,
Additional Interest shall be payable in accordance with the above paragraph from
the day such Registration Default occurs until such Registration Default is
cured.

          (c) Any amounts of Additional Interest due pursuant to clause (a)(i),
(a)(ii) or (a)(iii) of this Section 5 will be payable in cash on the regular
interest payment dates with respect to the Notes. The amount of Additional
Interest will be determined by multiplying the applicable Additional Interest
rate by the principal amount of the Notes, multiplied by a fraction, the
numerator of


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which is the number of days such Additional Interest rate was applicable during
such period (determined on the basis of a 360-day year comprised of twelve
30-day months), and the denominator of which is 360. The indebtedness
represented by the Additional Interest shall be subordinated in right of payment
to all existing and future Senior Indebtedness (as defined in the Indenture) as
and to the same extent as the Notes.

          (d) "Transfer Restricted Securities" means each Security until (i) the
date on which such Security has been effectively registered under the Securities
Act and disposed of in accordance with the Shelf Registration Statement or (ii)
the date on which such Security is distributed to the public pursuant to Rule
144 under the Securities Act or is saleable pursuant to Rule 144(k) under the
Securities Act.

          6. Rules 144 and 144A. The Company shall use its best efforts to file
the reports required to be filed by it under the Securities Act and the Exchange
Act in a timely manner and, if at any time the Company is not required to file
such reports, it will, upon the request of any Holder, make publicly available
other information so long as necessary to permit sales of their securities
pursuant to Rules 144 and 144A. The Company covenants that it will take such
further action as any Holder of Securities may reasonably request, all to the
extent required from time to time to enable such Holder to sell Transfer
Restricted Securities without registration under the Securities Act within the
limitations of the exemptions provided by Rules 144 and 144A (including the
requirements of Rule 144A(d)(4)). The Company will provide a copy of this
Agreement to prospective purchasers of Securities identified to the Company by
the Initial Purchaser upon request. Upon the request of any Holder, the Company
shall deliver to such Holder a written statement as to whether it has complied
with such requirements. Notwithstanding the foregoing, nothing in this Section 6
shall be deemed to require the Company to register any of its securities
pursuant to the Exchange Act.

          7. Underwritten Registrations. If any of the Transfer Restricted
Securities covered by the Shelf Registration are to be sold in an underwritten
offering, the investment banker or investment bankers and manager or managers
that will administer the offering ("Managing Underwriters") will be selected by
the Holders of a majority in aggregate principal amount of such

<PAGE>   15
                                                                              15

Transfer Restricted Securities to be included in such offering (provided that
Holders of Common Stock issued upon conversion of Notes shall not be deemed
Holders of Common Stock, but shall be deemed to be Holders of the aggregate
principal amount of Notes from which such Common Stock was converted).

          No person may participate in any underwritten registration hereunder
unless such person (i) agrees to sell such person's Transfer Restricted
Securities on the basis reasonably provided in any underwriting arrangements
approved by the persons entitled hereunder to approve such arrangements and (ii)
completes and executes all questionnaires, powers of attorney, indemnities,
underwriting agreements and other documents reasonably required under the terms
of such underwriting arrangements.

          8. Miscellaneous. (a) Amendments and Waivers. The provisions of this
Agreement may not be amended, modified or supplemented, and waivers or consents
to departures from the provisions hereof may not be given, except by the Company
and the written consent of the Holders of a majority in principal amount of the
Securities (provided that Holders of Common Stock issued upon conversion of
Notes shall not be deemed Holders of Common Stock, but shall be deemed to be
Holders of the aggregate principal amount of Notes from which such Common Stock
was converted) affected by such amendment, modification, supplement, waiver or
consents.

          (b) Notices. All notices and other communications provided for or
permitted hereunder shall be made in writing by hand delivery, first-class mail,
facsimile transmission, or air courier which guarantees overnight delivery:

          (1) if to a Holder, at the most current address given by such Holder
     to the Company;

          (2) if to the Initial Purchaser, at its address as follows:

                   Credit Suisse First Boston Corporation
                   Eleven Madison Avenue
                   New York, NY 10010-3629
                   Fax No.:  (212) 325-8278
                   Attention:  Transactions Advisory Group


          with a copy to:

<PAGE>   16
                                                                              16




                   Cravath, Swaine & Moore
                   Worldwide Plaza
                   825 Eighth Avenue
                   New York, NY 10019
                   Fax No.:  (212) 474-3700
                   Attention:  Robert Rosenman, Esq.

          (3) if to the Company, at its address as follows:

                   Human Genome Sciences, Inc.
                   9410 Key West Avenue
                   Rockville, MD 20850
                   Fax No.:  (301) 309-8439
                   Attention:  James H. Davis, Esq.

          with a copy to:

                   Piper Marbury Rudnick & Wolfe LLP
                   6225 Smith Avenue
                   Baltimore, Maryland 21209-3600
                   Fax No.:  (410) 580-3001
                   Attention:  R. W. Smith, Jr., Esq.

          All such notices and communications shall be deemed to have been duly
given: at the time delivered by hand, if personally delivered; three business
days after being deposited in the mail, postage prepaid, if mailed; when receipt
is acknowledged by recipient's facsimile machine operator, if sent by facsimile
transmission; and on the day delivered, if sent by overnight air courier
guaranteeing next day delivery.

          (c) No Inconsistent Agreements. The Company has not, as of the date
hereof, entered into, nor shall it, on or after the date hereof, without the
prior consent of CSFBC, enter into, any agreement with respect to its securities
that is inconsistent with the rights granted to the Holders herein or otherwise
conflicts with the provisions hereof.

          (d) Successors and Assigns. This Agreement shall be binding


<PAGE>   17
                                                                              17


upon the Company and its successors and assigns.

          (e) 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 an original and all of which taken
together shall constitute one and the same agreement.

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

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

          By the execution and delivery of this Agreement, the Company submits
to the nonexclusive jurisdiction of any federal or state court in the State of
New York.

          (h) Severability. If any one or more of the provisions contained
herein, or the application thereof in any circumstance, is held invalid, illegal
or unenforceable, the validity, legality and enforceability of any such
provision in every other respect and of the remaining provisions contained
herein shall not be affected or impaired thereby.

          (i) Securities Held by the Company. Whenever the consent or approval
of Holders of a specified percentage of principal amount of Securities is
required hereunder, Securities held by the Company or its affiliates (other than
subsequent Holders of Securities if such subsequent Holders are deemed to be
affiliates solely by reason of their holdings of such Securities)shall not be
counted in determining whether such consent or approval was given by the Holders
of such required percentage.



<PAGE>   18
                                                                              18


          If the foregoing is in accordance with your understanding of our
agreement, please sign and return to the Company a counterpart hereof, whereupon
this instrument, along with all counterparts, will become a binding agreement
among the Initial Purchaser and the Company in accordance with its terms.

                                   Very truly yours,

                                   HUMAN GENOME SCIENCES, INC.

                                   By:/s/ Craig A. Rosen
                                         ----------------------
                           Name:  Craig A. Rosen, Ph.D.

                                    Title: Executive Vice
                                             President, Research
                                             and Development

The foregoing Registration
Rights Agreement is hereby confirmed
and accepted as of the date first
above written.

CREDIT SUISSE FIRST BOSTON CORPORATION

By:/s/  Robert A. Hansen
   ----------------------------
   Name:    Robert A. Hansen
   Title:   Director