<PAGE>

                                                                    Exhibit 10.6

                          REGISTRATION RIGHTS AGREEMENT

      REGISTRATION RIGHTS AGREEMENT (this "Agreement"), dated as of March 12,
2003, by and among Hemispherx Biopharma, Inc., a Delaware corporation, with
headquarters located at One Penn Center, 1617 JFK Boulevard, Suite 660,
Philadelphia, PA 19103 (the "Company"), and the undersigned buyers (each, a
"Buyer" and collectively, the "Buyers").

      WHEREAS:

      A. In connection with the Securities Purchase Agreement by and among the
parties hereto of even date herewith (the "Securities Purchase Agreement"), the
Company has agreed, upon the terms and subject to the conditions of the
Securities Purchase Agreement, to issue and sell to the Buyers (i) an aggregate
of $5,426,000 of the Company's 6% Senior Secured Convertible Debentures due
January 31, 2005 (such Senior Secured Convertible Debentures, as the same may be
amended, modified or supplemented from time to time in accordance with the terms
thereof, the "Convertible Debentures"), which will be convertible into shares of
the Company's common stock, par value $0.001 per share (the "Common Stock") (as
converted, the "Conversion Shares"), and (ii) warrants (the "Warrants") to
purchase shares of Common Stock (as exercised collectively, the "Warrant
Shares");

      B. To induce the Buyers to execute and deliver the Securities Purchase
Agreement, the Company has agreed to provide certain registration rights to the
Buyers under the Securities Act of 1933, as amended, and the rules and
regulations thereunder, or any similar successor statute (collectively, the
"1933 Act"), and applicable state securities laws; and

      C. The location of defined terms in this Agreement is set forth on the
Index of Terms attached hereto.

      NOW, THEREFORE, in consideration of the premises and the mutual covenants
contained herein and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the Company and each of the Buyers
hereby agree as follows:

      1. Definitions. As used in this Agreement, the following terms shall have
the following meanings:

            a. "Business Day" means any day other than Saturday, Sunday or any
other day on which commercial banks in The City of New York are authorized or
required by law to remain closed.

            b. "Investor" means a Buyer, any transferee or assignee thereof to
whom a Buyer assigns its rights under this Agreement and who agrees to become
bound by the provisions of this Agreement in accordance with Section 9 and any
transferee or assignee thereof to whom a transferee or assignee assigns its
rights under this Agreement and who agrees to become bound by the provisions of
this Agreement in accordance with Section 9.


<PAGE>

            c. "Person" means an individual, a limited liability company, a
partnership, a joint venture, a corporation, a trust, an unincorporated
organization and governmental or any department or agency thereof.

            d. "register" "registered," and "registration" refer to a
registration effected by preparing and filing one or more Registration
Statements (as defined below) in compliance with the 1933 Act and pursuant to
Rule 415 under the 1933 Act or any successor rule providing for offering
securities on a continuous or delayed basis ("Rule 415"), and the declaration or
ordering of effectiveness of such Registration Statement(s) by the United States
Securities and Exchange Commission (the "SEC").

            e. "Registrable Securities" means (i) the Warrant Shares issued or
issuable upon exercise of all of the Warrants, (ii) the Conversion Shares issued
or issuable upon conversion of all of the Convertible Debentures, (iii) the
Interest Shares and the Repayment Shares (each, as defined in the Convertible
Debentures) issued or issuable on the Convertible Debentures and (iv) any shares
of capital stock issued or issuable with respect to the Warrant Shares, the
Conversion Shares, the Convertible Debentures, the Interest Shares, or the
Repayment Shares as a result of any stock split, stock dividend,
recapitalization, exchange or similar event or otherwise, without regard to any
limitations on conversions of Convertible Debentures or the exercise of the
Warrants; provided, however, that shares of Common Stock which are Registrable
Securities shall cease to be Registrable Securities (A) upon any sale pursuant
to a Registration Statement, Rule 144 under the 1933 Act or upon any sale in any
manner to a person or entity which, by virtue of Section 9 of this Agreement, is
not entitled to the rights provided in this Agreement or (B) at such time as all
such shares may be sold under Rule 144(k).

            f. "Registration Statement" means a registration statement or
registration statements of the Company filed under the 1933 Act covering the
Registrable Securities.

      Capitalized terms used herein and not otherwise defined herein shall have
the respective meanings set forth in the Securities Purchase Agreement.

      2. Registration.

            a. Mandatory Registration. The Company shall prepare and, as soon as
practicable but in no event later than the earlier of (x) the date on which the
Company files the Company's Annual Report on Form 10-K for the year ended
December 31, 2002 and (y) March 31, 2003 (such earlier date being the "Filing
Deadline"), file with the SEC the Registration Statement on Form S-3 covering
the resale of all of the Registrable Securities. In the event that Form S-3 is
unavailable for such a registration, the Company shall use such other form as is
available for such a registration, subject to the provisions of Section 2(d), in
which case the "Filing Deadline" shall be forty-five (45) days after the Closing
Date. The Registration Statement prepared pursuant hereto shall register for
resale at least 6,971,256 shares of Common Stock, subject to adjustment as
provided in Section 2(e). The Company shall use its best efforts to have the
Registration Statement declared effective by the SEC as soon as practicable, but
in no event later than the date which is seventy-five (75) days after the
Closing Date. Until such time as the Registration Statement is declared
effective by the SEC, the Company shall not file with


                                       2
<PAGE>

the SEC any other registration statement under the 1933 Act with respect to the
resale or initial issuance of any Company securities.

            b. Allocation of Registrable Securities. The initial number of
Registrable Securities included in any Registration Statement and each increase
in the number of Registrable Securities included therein shall be allocated pro
rata among the Investors based on the number of Registrable Securities held by
each Investor at the time the Registration Statement covering such initial
number of Registrable Securities or increase thereof is declared effective by
the SEC. Subject to Section 9, in the event that an Investor sells or otherwise
transfers any of such Investor's Registrable Securities, each transferee shall
be allocated the portion of the then remaining number of Registrable Securities
included in such Registration Statement allocable to the transferor. The Company
shall not include any securities other than Registrable Securities on any
Registration Statement without the prior written consent of Buyers holding at
least 60% of Registrable Securities.

            c. Legal Counsel. Subject to Section 5 hereof, the Buyers holding at
least 60% of the Registrable Securities shall have the right to select one legal
counsel to review and comment upon any registration pursuant to this Section 2
("Legal Counsel"), which shall be Schulte Roth & Zabel LLP or such other counsel
as thereafter designated in writing by the holders of at least 60% of the
Registrable Securities.

            d. Ineligibility for Form S-3. In the event that Form S-3 is not
available for the registration of the resale of Registrable Securities
hereunder, the Company shall (i) register the resale of the Registrable
Securities on another appropriate form reasonably acceptable to the holders of
at least 60% of the Registrable Securities and (ii) undertake to register the
Registrable Securities on Form S-3 as soon as such form is available, provided
that the Company shall maintain the effectiveness of the Registration Statement
then in effect until such time as a Registration Statement on Form S-3 covering
the Registrable Securities has been declared effective by the SEC.

            e. Sufficient Number of Shares Registered. In the event the number
of shares available under a Registration Statement filed pursuant to Section
2(a) is insufficient to cover all of the Registrable Securities required to be
covered by such Registration Statement or an Investor's allocated portion of the
Registrable Securities pursuant to Section 2(b), the Company shall amend the
Registration Statement, or file a new Registration Statement (on the short form
available therefor, if applicable), or both, so as to cover at least 135% of the
number of such Registrable Securities as of the trading day immediately
preceding the date of the filing of such amendment or new Registration
Statement, in each case, as soon as practicable, but in any event not later than
fifteen (15) days after the necessity therefor arises. The Company shall use its
best efforts to cause such amendment and/or new Registration Statement to become
effective as soon as practicable following the filing thereof. The calculation
of the number of shares sufficient to cover all of the Registrable Securities
shall be made without regard to any limitations on the conversion of the
Convertible Debentures or the exercise of the Warrants, and such calculation
shall assume that the Convertible Debentures and the Warrants are then
convertible into shares of Common Stock and the maximum number of Interest
Shares and Repayment Shares under the Convertible Debentures, assuming the
initial outstanding principal amount of the Convertible Debentures remains
outstanding through the scheduled maturity date


                                       3
<PAGE>

and assuming no conversions or redemptions of the Convertible Debentures prior
to the scheduled maturity date, are issuable at the then prevailing Conversion
Rate (as defined in the Convertible Debentures), Warrant Exercise Price (as
defined in the Warrants), Interest Share Conversion Rate (as defined in the
Convertible Debentures), or Equity Collateral conversion Rate (as defined in the
Convertible Debentures), as applicable.

            f. Effect of Failure to File and Obtain and Maintain Effectiveness
of Registration Statement. If (i) a Registration Statement covering all the
Registrable Securities required to be covered thereby and required to be filed
by the Company pursuant to this Agreement is (A) not filed with the SEC on or
before the Filing Deadline, or (B) not declared effective by the SEC on or
before the date that is ninety (90) days after the Closing Date (the
"Effectiveness Deadline") or (ii) on any day after such Registration Statement
has been declared effective by the SEC sales of all the Registrable Securities
required to be included on such Registration Statement cannot be made (other
than during an Allowable Grace Period (as defined in Section 3(p)) pursuant to
such Registration Statement (including, without limitation, because of a failure
to keep such Registration Statement effective, to disclose such information as
is necessary for sales to be made pursuant to such Registration Statement or to
register sufficient shares of Common Stock), then, as partial relief for the
damages to any holder by reason of any such delay in or reduction of its ability
to sell the underlying shares of Common Stock (which remedy shall not be
exclusive of any other remedies available at law or in equity), the Company
shall pay to each holder of Convertible Debentures relating to such Registration
Statement an amount in cash equal to the product of (i) the outstanding
principal amount of such Investor's Convertible Debentures to which the
Registration Statement relates multiplied by (ii) the product of (I) .00067
multiplied by (II) the sum of (x) the number of days after the applicable Filing
Deadline that the Registration Statement is not filed with the SEC, plus (y) the
number of days after the applicable Effectiveness Deadline that the Registration
Statement is not declared effective by the SEC, plus (z) the number of days, in
each instance, after the Registration Statement has been declared effective by
the SEC that such Registration Statement is not available (other than during an
Allowable Grace Period) for the sale of all the Registrable Securities required
to be included on such Registration Statement. The payments to which a holder
shall be entitled pursuant to this Section 2(f) are referred to herein as
"Registration Delay Payments." Registration Delay Payments shall be paid on the
earlier of (I) the last day of the calendar month during which such Registration
Delay Payments are incurred and (II) the third Business Day after the event or
failure giving rise to the Registration Delay Payments is cured. In the event
the Company fails to make Registration Delay Payments in a timely manner, such
Registration Delay Payments shall bear interest at the rate of 1.5% per month
(prorated for partial months) until paid in full.

      3. Related Obligations.

      At such time as the Company is obligated to file a Registration Statement
with the SEC pursuant to Section 2(a), 2(d) or 2(e), the Company will use its
best efforts to effect the registration of the Registrable Securities in
accordance with the intended method of disposition thereof and, pursuant
thereto, the Company shall have the following obligations:

            a. The Company shall promptly prepare and file with the SEC a
Registration Statement with respect to the Registrable Securities (but in no
event later than the


                                       4
<PAGE>

applicable Filing Deadline) and use its best efforts to cause such Registration
Statement relating to the Registrable Securities required to be covered thereby
to become effective as soon as practicable after such filing (but in no event
later than the date which is ninety (90) days after the Closing Date). The
Company shall, subject to the terms of this Agreement, keep each Registration
Statement effective pursuant to Rule 415 at all times until the earlier of (i)
the date as of which the Investors may sell all of the Registrable Securities
covered by such Registration Statement without restriction pursuant to Rule
144(k) (or successor thereto) promulgated under the 1933 Act or (ii) the date on
which the Investors shall have sold all the Registrable Securities covered by
such Registration Statement (the "Registration Period"), which Registration
Statement, as of its filing date, (including any amendments or supplements
thereto and prospectuses contained therein, as of their respective filing dates)
shall not contain any untrue statement of a material fact or omit to state a
material fact required to be stated therein, or necessary to make the statements
therein, in light of the circumstances in which they were made, not misleading.
The term "best efforts" shall mean, among other things, that the Company shall
submit to the SEC, within five (5) Business Days after the Company learns that
no review of a particular Registration Statement will be made by the staff of
the SEC or that the staff has no further comments on the Registration Statement,
as the case may be, and the approval of Legal Counsel pursuant to Section 3(c),
a request for acceleration of effectiveness of such Registration Statement to a
time and date not later than 48 hours after the submission of such request.

            b. Subject to Section 3(p), the Company shall prepare and file with
the SEC such amendments (including post-effective amendments) and supplements to
the Registration Statement and the prospectus used in connection with such
Registration Statement, which prospectus is to be filed pursuant to Rule 424
promulgated under the 1933 Act, as may be necessary to keep such Registration
Statement effective at all times during the Registration Period, and, during
such period, comply with the provisions of the 1933 Act applicable to the
Company with respect to the disposition of all Registrable Securities covered by
such Registration Statement until such time as all of such Registrable
Securities shall have been disposed of in accordance with the intended methods
of disposition by the seller or sellers thereof as set forth in such
Registration Statement. In the case of amendments and supplements to a
Registration Statement which are required to be filed pursuant to this Agreement
(including pursuant to this Section 3(b)) by reason of the Company filing a
report on Form 10-K, Form 10-Q or Form 8-K or any analogous report under the
Securities Exchange Act of 1934, as amended (the "1934 Act"), the Company shall
have incorporated such report by reference into such Registration Statement, if
applicable, or shall file such amendments or supplements with the SEC on the
same day on which the 1934 Act report is filed which created the requirement for
the Company to amend or supplement such Registration Statement.

            c. The Company shall permit Legal Counsel to review and comment upon
(i) a Registration Statement at least three (3) Business Days prior to its
filing with the SEC and (ii) all other Registration Statements and all
amendments and supplements thereto (except for Annual Reports on Form 10-K,
Quarterly Reports on Form 10-Q and Current Reports on Form 8-K and any similar
or successor reports or other documents incorporated by reference therein)
within a reasonable number of days prior to their filing with the SEC. The
Company shall furnish to Legal Counsel, without charge, (i) copies of any
correspondence from the SEC or the staff of the SEC to the Company or its
representatives relating to any Registration Statement, (ii) promptly after the
same is prepared and filed with the SEC, one copy of any Registration


                                       5
<PAGE>

Statement and any amendment(s) thereto, including financial statements (but
excluding all schedules, all documents incorporated therein by reference, and
all exhibits, unless requested in writing by Legal Counsel) and (iii) upon the
effectiveness of any Registration Statement, one copy of the prospectus included
in such Registration Statement and all amendments and supplements thereto. The
Company agrees that it will, and it will cause its counsel to, consider in good
faith any comments or objections from Legal Counsel as to the form or content of
any Registration Statement or amendments or supplements thereto or any request
for acceleration of the effectiveness of any Registration Statement or any
amendment or supplement thereto.

            d. The Company shall furnish to each Investor whose Registrable
Securities are included in any Registration Statement, without charge, (i)
promptly after the same is prepared and filed with the SEC, at least one copy of
such Registration Statement and any amendment(s) thereto, including financial
statements (but excluding all schedules, all documents incorporated therein by
reference and all exhibits, unless requested in writing by Legal Counsel) and
each preliminary prospectus, (ii) upon the effectiveness of any Registration
Statement, ten (10) copies of the prospectus included in such Registration
Statement and all amendments and supplements thereto (or such other number of
copies as such Investor may reasonably request) and (iii) such other documents,
including copies of any preliminary or final prospectus, as such Investor may
reasonably request from time to time in order to facilitate the disposition of
the Registrable Securities owned by such Investor.

            e. Subject to Section 3(p), the Company shall use its best efforts
to (i) register and qualify, unless an exemption from registration and
qualification applies, the resale by Investors of the Registrable Securities
covered by the Registration Statement under such other securities or "blue sky"
laws of all applicable jurisdictions in the United States as any holder of
Registrable Shares reasonably requests in writing, (ii) prepare and file in
those jurisdictions, such amendments (including post-effective amendments) and
supplements to such registrations and qualifications as may be necessary to
maintain the effectiveness thereof during the Registration Period, (iii) take
such other actions as may be necessary to maintain such registrations and
qualifications in effect at all times during the Registration Period, and (iv)
take all other actions reasonably necessary or advisable to qualify the
Registrable Securities for sale in such jurisdictions; provided, however, that
the Company shall not be required in connection therewith or as a condition
thereto to (x) qualify to do business in any jurisdiction where it would not
otherwise be required to qualify but for this Section 3(e), (y) subject itself
to general taxation in any such jurisdiction, or (z) file a general consent to
service of process in any such jurisdiction. The Company shall promptly notify
Legal Counsel and each Investor who holds Registrable Securities of the receipt
by the Company of any notification with respect to the suspension of the
registration or qualification of any of the Registrable Securities for sale
under the securities or "blue sky" laws of any jurisdiction in the United States
or its receipt of actual notice of the initiation or threatening of any
proceeding for such purpose.

            f. Notwithstanding anything to the contrary set forth herein, as
promptly as practicable after becoming aware of such event, the Company shall
notify Legal Counsel and each Investor in writing of the happening of any event,
of which the Company has knowledge, as a result of which the prospectus included
in a Registration Statement, as then in effect, includes an untrue statement of
a material fact or omission to state a material fact required to be stated
therein or necessary to make the statements therein, in light of the
circumstances under which


                                       6
<PAGE>

they were made, not misleading (provided that in no event shall such notice
contain any material, nonpublic information), and, subject to Section 3(p),
promptly prepare a supplement or amendment to such Registration Statement to
correct such untrue statement or omission, and deliver ten (10) copies of such
supplement or amendment to Legal Counsel and each Investor (or such other number
of copies as Legal Counsel or such Investor may reasonably request). The Company
shall also promptly notify Legal Counsel and each Investor in writing (i) when a
prospectus or any prospectus supplement or post-effective amendment has been
filed, and when a Registration Statement or any post-effective amendment has
become effective (notification of such effectiveness shall be delivered to Legal
Counsel and each Investor by facsimile on the same day of such effectiveness and
by overnight mail), (ii) of any request by the SEC for amendments or supplements
to a Registration Statement or related prospectus or related information, and
(iii) of the Company's reasonable determination that a post-effective amendment
to a Registration Statement would be appropriate.

            g. Subject to Section 3(p), the Company shall use its best efforts
to (i) prevent the issuance of any stop order or other suspension of
effectiveness of a Registration Statement, or the suspension of the
qualification of any of the Registrable Securities for sale in any jurisdiction
and, (ii) if such an order or suspension is issued, to obtain the withdrawal of
such order or suspension at the earliest practicable moment and to notify Legal
Counsel and each Investor who holds Registrable Securities being sold of the
issuance of such order and the resolution thereof or its receipt of actual
notice of the initiation or threat of any proceeding for such purpose.

            h. Notwithstanding the provisions of Section 4(h) of the Securities
Purchase Agreement, the Company shall make available for inspection by (i) any
Investor, (ii) Legal Counsel and (iii) one firm of accountants or other agents
retained by the Investors at the Investors' expense (collectively, the
"Inspectors"), at the offices where normally kept and during reasonable business
hours, all pertinent financial and other records, and pertinent corporate
documents and properties of the Company (collectively, the "Records"), as shall
be reasonably deemed necessary by each Inspector, and cause the Company's
officers, directors and employees to supply all information which any Inspector
may reasonably request; provided, however, that each Inspector shall agree in
writing to hold in strict confidence and shall not make any disclosure (except
to an Investor) or use of any Record or other information that the Company
determines in good faith to be confidential, and of which determination the
Inspectors are so notified, unless (a) the disclosure of such Records is
necessary to avoid or correct a misstatement or omission in any Registration
Statement or is otherwise required under the 1933 Act, (b) the release of such
Records is ordered pursuant to a final, non-appealable subpoena or order from a
court or government body of competent jurisdiction, or (c) the information in
such Records has been made generally available to the public other than by
disclosure in violation of this or any other agreement of which the Inspector
has knowledge. Each Investor agrees that it shall, upon learning that disclosure
of such Records is sought in or by a court or governmental body of competent
jurisdiction or through other means, give prompt notice to the Company and allow
the Company, at its expense, to undertake appropriate action to prevent
disclosure of, or to obtain a protective order for, the Records deemed
confidential. Nothing herein (or in any other confidentiality agreement between
the Company and any Investor) shall be deemed to limit the Investors' ability to
sell Registrable Securities in a manner that is otherwise consistent with
applicable laws and regulations and the terms and provisions of this Agreement.


                                       7
<PAGE>

            i. The Company shall hold in confidence and not make any disclosure
of information concerning an Investor provided to the Company unless (i)
disclosure of such information is necessary to comply with federal or state
securities laws, (ii) the disclosure of such information is necessary to avoid
or correct a misstatement or omission in any Registration Statement, (iii) the
release of such information is ordered pursuant to a subpoena or other final,
non-appealable order from a court or governmental body of competent
jurisdiction, or (iv) such information has been made generally available to the
public other than by disclosure in violation of this Agreement or any other
agreement. The Company agrees that it shall, upon learning that disclosure of
such information concerning an Investor is sought in or by a court or
governmental body of competent jurisdiction or through other means, give prompt
written notice to such Investor and allow such Investor, at the Investor's
expense, to undertake appropriate action to prevent disclosure of, or to obtain
a protective order for, such information.

            j. The Company shall use its best efforts either to (i) cause all
the Registrable Securities covered by a Registration Statement to be listed on
each securities exchange on which securities of the same class or series issued
by the Company are then listed, if any, if the listing of such Registrable
Securities is then permitted under the rules of such exchange, or (ii) secure
designation and quotation of all the Registrable Securities covered by a
Registration Statement on The American Stock Exchange or The Nasdaq National
Market, or (iii) if, despite the Company's best efforts to satisfy the preceding
clause (i) or (ii), the Company is unsuccessful in satisfying the preceding
clause (i) or (ii), to secure the inclusion for quotation on the The Nasdaq
SmallCap Market for such Registrable Securities and, without limiting the
generality of the foregoing, to use its best efforts to arrange for at least two
market makers to register with the National Association of Securities Dealers,
Inc. ("NASD") as such with respect to such Registrable Securities. The Company
shall pay all fees and expenses in connection with satisfying its obligation
under this Section 3(j).

            k. In connection with any sale or transfer of Registrable Securities
pursuant to a Registration Statement, the Company shall cooperate with the
Investors who hold Registrable Securities being offered and, to the extent
applicable, facilitate the timely preparation and delivery of certificates (not
bearing any restrictive legend) representing the Registrable Securities to be
offered pursuant to a Registration Statement and enable such certificates to be
in such denominations or amounts, as the case may be, as the Investors may
reasonably request and, subject to compliance with applicable securities laws,
registered in such names as the Investors may request and if such certificates
are to be issued in the name of a Person other than the registered holder of
such Registrable Securities, then the Investor shall give the Company notice at
least two Business Days prior to any sale of the Registrable Securities.

            l. If requested by an Investor, the Company shall (i) as soon as
practicable incorporate in a prospectus supplement or post-effective amendment
to the Registration Statement such information as an Investor reasonably
requests to be included therein relating to the sale and distribution of
Registrable Securities to which the Company does not reasonably object,
including, without limitation, information with respect to the number of
Registrable Securities being offered or sold, the purchase price being paid
therefor and any other terms of the offering of the Registrable Securities to be
sold in such offering; (ii) as soon as practicable make all required filings of
such prospectus supplement or post-effective amendment after being notified of
the matters to be incorporated in such prospectus supplement or post-


                                       8
<PAGE>

effective amendment; and (iii) as soon as practicable, supplement or make
amendments to any Registration Statement if reasonably requested by an Investor
holding any Registrable Securities and to which the Company does not reasonably
object.

            m. The Company shall make generally available to its security
holders as soon as practical, but not later than ninety (90) days after the
close of the period covered thereby, an earnings statement (in form complying
with, and in the manner provided by, the provisions of Rule 158 under the 1933
Act) covering a twelve-month period beginning not later than the first day of
the Company's fiscal quarter next following the effective date of a Registration
Statement.

            n. The Company shall otherwise use its best efforts to comply with
all applicable rules and regulations of the SEC in connection with any
registration hereunder.

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

            p. Notwithstanding anything to the contrary herein, at any time
after the Registration Statement has been declared effective by the SEC, the
Company may delay the disclosure of material non-public information concerning
the Company the disclosure of which at the time is not, in the good faith
judgment of the Board of Directors of the Company, in the best interest of the
Company (a "Grace Period"); provided, that the Company shall promptly (i) notify
the Investors in writing of the existence of material non-public information
giving rise to a Grace Period (provided that in each notice the Company will not
disclose the content of such material non-public information to the Investors)
and the date on which the Grace Period will begin, and (ii) notify the Investors
in writing of the date on which the Grace Period ends; and, provided further,
that no Grace Period shall exceed twenty (20) days and during any three hundred
sixty five (365) day period such Grace Periods shall not exceed an aggregate of
forty-five (45) days and the first day of any Grace Period must be at least two
(2) Business Days after the last day of any prior Grace Period (an "Allowable
Grace Period"). For purposes of determining the length of a Grace Period above,
the Grace Period shall begin on and include the date the Investors receive the
notice referred to in clause (i) and shall end on and include the later of the
date the Investors receive the notice referred to in clause (ii) and the date
referred to in such notice. The provisions of Section 3(g) hereof shall not be
applicable during the period of any Allowable Grace Period. Upon expiration of
the Grace Period, the Company shall again be bound by the first sentence of
Section 3(f).

      4. Obligations Of The Investors.

            a. At least seven (7) Business Days prior to the first anticipated
filing date of a Registration Statement, the Company shall notify each Investor
in writing of the information the Company requires from each such Investor if
such Investor elects to have any of such Investor's Registrable Securities
included in such Registration Statement. It shall be a condition precedent to
the obligations of the Company to complete the registration pursuant to


                                       9
<PAGE>

this Agreement with respect to the Registrable Securities of a particular
Investor that such Investor shall furnish to the Company such information
regarding itself, the Registrable Securities held by it and the intended method
of disposition of the Registrable Securities held by it as shall be reasonably
required to effect the effectiveness of the registration of such Registrable
Securities and shall execute such documents in connection with such registration
as the Company may reasonably request. Each Investor shall promptly notify the
Company of any material change with respect to such information previously
provided to the Company by such Investor.

            b. Each Investor, by such Investor's acceptance of the Registrable
Securities, agrees to cooperate with the Company as reasonably requested by the
Company in connection with the preparation and filing of any Registration
Statement hereunder, unless such Investor has notified the Company in writing of
such Investor's election to exclude all of such Investor's Registrable
Securities from such Registration Statement.

            c. Each Investor agrees that, upon receipt of any notice from the
Company of the happening of any event of the kind described in Section 3(g),
Section 3(p) or the first sentence of 3(f), such Investor will immediately
discontinue disposition of Registrable Securities pursuant to any Registration
Statement(s) covering such Registrable Securities until such Investor's receipt
of the copies of the supplemented or amended prospectus contemplated by Section
3(g) or the first sentence of 3(f) or receipt of notice that no supplement or
amendment is required and, if so directed by the Company, such Investor shall
deliver to the Company (at the expense of the Company) or destroy (and deliver
to the Company a certificate of destruction) all copies (other than a single
file copy, which such Investor may keep) in such Investor's possession of the
prospectus covering such Registrable Securities current at the time of receipt
of such notice. Notwithstanding anything to the contrary, subject to compliance
with applicable securities laws, the Company shall cause its transfer agent to
deliver unlegended shares of Common Stock to a transferee of an Investor in
accordance with the terms of the Securities Purchase Agreement, the Convertible
Debenture and/or the Warrant, as applicable, in connection with any sale of
Registrable Securities with respect to which an Investor has entered into a
contract for sale prior to the Investor's receipt of a notice from the Company
of the happening of any event of the kind described in Section 3(g) or the first
sentence of 3(f) and for which the Investor has not yet settled.

      5. Expenses Of Registration.

      All reasonable expenses, other than underwriting discounts and
commissions, incurred in connection with registrations, filings or
qualifications pursuant to Sections 2 and 3, including, without limitation, all
registration, listing and qualifications fees, printers and accounting fees, and
fees and disbursements of counsel for the Company shall be paid by the Company.
The Company shall also reimburse the Investors for the fees and disbursements of
Legal Counsel in connection with registration, filing or qualification pursuant
to Sections 2 and 3 of this Agreement which amount shall be limited to $10,000
for the Registration Statement.

      6. Indemnification.


                                       10
<PAGE>

      In the event any Registrable Securities are included in a Registration
Statement under this Agreement:

            a. To the fullest extent permitted by law, the Company will, and
hereby does, indemnify, hold harmless and defend each Investor, the directors,
officers, members, partners, employees, agents, representatives of, and each
Person, if any, who controls any Investor within the meaning of the 1933 Act or
the 1934 Act (each, an "Indemnified Person"), against any losses, claims,
damages, liabilities, judgments, fines, penalties, charges, costs, reasonable
attorneys' fees, amounts paid in settlement or expenses, joint or several,
(collectively, "Claims") incurred in investigating, preparing or defending any
action, claim, suit, inquiry, proceeding, investigation or appeal taken from the
foregoing by or before any court or governmental, administrative or other
regulatory agency, body or the SEC, whether pending or threatened, whether or
not an indemnified party is or may be a party thereto ("Indemnified Damages"),
to which any of them may become subject insofar as such Claims (or actions or
proceedings, whether commenced or threatened, in respect thereof) arise out of
or are based upon: (i) any untrue statement or alleged untrue statement of a
material fact in a Registration Statement or any post-effective amendment
thereto or in any filing made in connection with the qualification of the
offering under the securities or other "blue sky" laws of any jurisdiction in
which Registrable Securities are offered ("Blue Sky Filing"), or the omission or
alleged omission to state a material fact required to be stated therein or
necessary to make the statements therein not misleading, (ii) any untrue
statement or alleged untrue statement of a material fact contained in any
preliminary prospectus if used prior to the effective date of such Registration
Statement, or contained in the final prospectus (as amended or supplemented, if
the Company files any amendment thereof or supplement thereto with the SEC) or
the omission or alleged omission to state therein any material fact necessary to
make the statements made therein, in light of the circumstances under which the
statements therein were made, not misleading, (iii) any violation or alleged
violation by the Company of the 1933 Act, the 1934 Act, any other law,
including, without limitation, any state securities law, or any rule or
regulation thereunder relating to the offer or sale of the Registrable
Securities pursuant to a Registration Statement or (iv) any material violation
of this Agreement (the matters in the foregoing clauses (i) through (iv) being,
collectively, "Violations"). Subject to Section 6(c), the Company shall
reimburse the Indemnified Persons, promptly as such expenses are incurred and
are due and payable, for any reasonable legal fees or other reasonable expenses
incurred by them in connection with investigating or defending any such Claim.
Notwithstanding anything to the contrary contained herein, the indemnification
agreement contained in this Section 6(a): (i) shall not apply to a Claim by an
Indemnified Person arising out of or based upon a Violation that occurs in
reliance upon and in conformity with information furnished in writing to the
Company by or on behalf of such Indemnified Person expressly for use in
connection with the preparation of the Registration Statement or any such
amendment thereof or supplement thereto, if such prospectus was timely made
available by the Company pursuant to Section 3(d); (ii) with respect to any
preliminary prospectus, shall not inure to the benefit of any such person from
whom the person asserting any such Claim purchased the Registrable Securities
that are the subject thereof (or to the benefit of any person controlling such
person) if the untrue statement or omission of material fact contained in the
preliminary prospectus was corrected in the prospectus, as then amended or
supplemented, if such prospectus was timely made available by the Company
pursuant to Section 3(d); (iii) shall not be available to the extent such Claim
is based on a failure of the Investor to deliver or to cause to be delivered the
prospectus made available by the Company, including a corrected


                                       11
<PAGE>

prospectus, if such prospectus or corrected prospectus was timely made available
by the Company pursuant to Section 3(d) and, in connection with a corrected
prospectus, the Indemnified Person was promptly advised in writing not to use
the incorrect prospectus prior to the use giving rise to a violation and such
Indemnified Person, notwithstanding such advice, used it or failed to deliver
the correct prospectus as required by the 1933 Act and such correct prospectus
was timely made available pursuant to Section 3(d); and (iv) shall not apply to
amounts paid in settlement of any Claim if such settlement is effected without
the prior written consent of the Company, which consent shall not be
unreasonably withheld or delayed. Such indemnity shall remain in full force and
effect regardless of any investigation made by or on behalf of the Indemnified
Person and shall survive the transfer of the Registrable Securities by the
Investors pursuant to Section 9.

            b. In connection with any Registration Statement in which an
Investor is participating, each such Investor agrees severally and not jointly
to indemnify, hold harmless and defend, to the same extent and in the same
manner as is set forth in Section 6(a), the Company, the directors, officers,
members, partners, employees, agents, representatives of and each Person, if
any, who controls the Company within the meaning of the 1933 Act or the 1934 Act
(each, an "Indemnified Party"), against any Claim or Indemnified Damages to
which any of them may become subject, under the 1933 Act, the 1934 Act or
otherwise, insofar as such Claim or Indemnified Damages arise out of or are
based upon any Violation, in each case to the extent, and only to the extent,
that such Violation occurs in reliance upon and in conformity with written
information furnished to the Company by or on behalf of such Investor expressly
for use in connection with such Registration Statement; and, subject to Section
6(c), such Investor will reimburse any legal or other expenses reasonably
incurred by an Indemnified Party in connection with investigating or defending
any such Claim; provided, however, that the indemnity agreement contained in
this Section 6(b) and the agreement with respect to contribution contained in
Section 7 shall not apply to amounts paid in settlement of any Claim if such
settlement is effected without the prior written consent of such Investor, which
consent shall not be unreasonably withheld or delayed; provided, further,
however, that the Investor shall be liable under this Section 6(b) for only that
amount of a Claim or Indemnified Damages as does not exceed the net proceeds to
such Investor as a result of the sale of Registrable Securities pursuant to such
Registration Statement. Such indemnity shall remain in full force and effect
regardless of any investigation made by or on behalf of such Indemnified Party
and shall survive the transfer of the Registrable Securities by the Investors
pursuant to Section 9. Notwithstanding anything to the contrary contained
herein, the indemnification agreement contained in this Section 6(b) with
respect to any preliminary prospectus shall not inure to the benefit of any
Indemnified Party if the untrue statement or omission of material fact contained
in the preliminary prospectus was corrected on a timely basis in the prospectus,
as then amended or supplemented.

            c. Promptly after an Indemnified Person or Indemnified Party under
this Section 6 has knowledge of any Claim as to which such Indemnified Person
reasonably believes indemnity may be sought or promptly after such Indemnified
Person or Indemnified Party receives notice of the commencement of any action or
proceeding (including any governmental action or proceeding) involving a Claim,
such Indemnified Person or Indemnified Party shall, if a Claim in respect
thereof is to be made against any indemnifying party under this Section 6,
deliver to the indemnifying party a written notice of such Claim, and the
indemnifying party shall have the right to participate in, and, to the extent
the indemnifying party so desires, jointly with any other indemnifying party


                                       12
<PAGE>

similarly noticed, to assume control of the defense thereof with counsel
selected by the indemnifying party and subject to the reasonable approval of the
Indemnified Person or the Indemnified Party, as the case may be; provided,
however, that an Indemnified Person or Indemnified Party shall have the right to
retain its own counsel with the fees and expenses of not more than one counsel
for such Indemnified Person or Indemnified Party to be paid by the indemnifying
party, if, in the reasonable opinion of counsel retained by the indemnifying
party, the representation by such counsel of the Indemnified Person or
Indemnified Party and the indemnifying party would be inappropriate due to
actual or potential differing interests between such Indemnified Person or
Indemnified Party and any other party represented by such counsel in such
proceeding. In the case of an Indemnified Person, legal counsel referred to in
the immediately preceding sentence shall be selected by the Investors holding at
least 60% in interest of the Registrable Securities included in the Registration
Statement to which the Claim relates. The Indemnified Party or Indemnified
Person shall cooperate fully with the indemnifying party in connection with any
negotiation or defense of any such action or Claim by the indemnifying party and
shall furnish to the indemnifying party all information reasonably available to
the Indemnified Party or Indemnified Person that relates to such action or
Claim. The indemnifying party shall keep the Indemnified Party or Indemnified
Person fully apprised at all times as to the status of the defense or any
settlement negotiations with respect thereto. No indemnifying party shall be
liable for any settlement of any action, claim or proceeding effected without
its prior written consent, provided, however, that the indemnifying party shall
not unreasonably withhold, delay or condition its consent. No indemnifying party
shall, without the prior written consent of the Indemnified Party or Indemnified
Person, consent to entry of any judgment or enter into any settlement or other
compromise that does not include as an unconditional term thereof the giving by
the claimant or plaintiff to such Indemnified Party or Indemnified Person of a
release from all liability in respect to such Claim or litigation. Following
indemnification as provided for hereunder, the indemnifying party shall be
subrogated to all rights of the Indemnified Party or Indemnified Person with
respect to all third parties, firms or corporations relating to the matter for
which indemnification has been made. The failure to deliver written notice to
the indemnifying party as provided herein shall not relieve such indemnifying
party of any liability to the Indemnified Person or Indemnified Party under this
Section 6, except to the extent that the indemnifying party is prejudiced in its
ability to defend such action.

            d. The indemnification required by this Section 6 shall be made by
periodic payments of the amount thereof during the course of the investigation
or defense, as and when bills are received or Indemnified Damages are incurred.

            e. The indemnity agreements contained herein shall be in addition to
(i) any cause of action or similar right of the Indemnified Party or Indemnified
Person against the indemnifying party or others, and (ii) any liabilities the
indemnifying party may be subject to pursuant to the law.

      7. Contribution.

      To the extent any indemnification by an indemnifying party is prohibited
or limited by law, the indemnifying party agrees to make the maximum
contribution with respect to


                                       13
<PAGE>

any amounts for which it would otherwise be liable under Section 6 to the
fullest extent permitted by law; provided, however, that: (i) no contribution
shall be made under circumstances where the maker would not have been liable for
indemnification under the fault standards set forth in Section 6, (ii) no person
involved in the sale of Registrable Securities which person is guilty of
fraudulent misrepresentation (within the meaning of Section 11(f) of the 1933
Act) in connection with such sale shall be entitled to contribution from any
person involved in such sale of Registrable Securities who was not guilty of
fraudulent misrepresentation; and (iii) contribution by any seller of
Registrable Securities shall be limited in amount to the net amount of proceeds
received by such seller from the sale of such Registrable Securities pursuant to
such Registration Statement.

      8. Reports Under The 1934 Act.

      With a view to making available to the Investors the benefits of Rule 144
promulgated under the 1933 Act or any other similar rule or regulation of the
SEC that may at any time permit the Investors to sell securities of the Company
to the public without registration ("Rule 144"), the Company agrees to use its
best efforts to:

            a. make and keep public information available, as those terms are
understood and defined in Rule 144;

            b. file with the SEC in a timely manner all reports and other
documents required of the Company under the 1933 Act and the 1934 Act so long as
the Company remains subject to such requirements (it being understood that
nothing herein shall limit the Company's obligations under Section 4(c) of the
Securities Purchase Agreement) and the filing of such reports and other
documents is required for the applicable provisions of Rule 144; and

            c. furnish to each Investor so long as such Investor owns
Registrable Securities, promptly upon request, (i) a written statement by the
Company, if true, that it has complied with the applicable reporting
requirements of Rule 144, the 1933 Act and the 1934 Act, (ii) a copy of the most
recent annual or quarterly report of the Company and copies of such other
reports and documents so filed by the Company, and (iii) such other information
as may be reasonably requested to permit the Investors to sell such securities
pursuant to Rule 144 without registration.

      9. Assignment of Registration Rights.

      The rights under this Agreement shall be automatically assignable by the
Investors to any transferee of all or any portion of such Investor's Registrable
Securities if: (i) the Investor agrees in writing with the transferee or
assignee to assign such rights, and a copy of such agreement is furnished to the
Company within a reasonable time after such assignment; (ii) the Company is,
within a reasonable time after such transfer or assignment, furnished with
written notice of (a) the name and address of such transferee or assignee, and
(b) the securities with respect to which such registration rights are being
transferred or assigned; (iii) immediately following such transfer or assignment
the further disposition of such securities by the transferee or assignee is
restricted under the 1933 Act and applicable state securities laws; (iv) at or
before the time the Company receives the written notice contemplated by clause
(ii) of this sentence the


                                       14
<PAGE>

transferee or assignee agrees in writing with the Company to be bound by all of
the provisions contained herein; (v) such transfer shall have been made in
accordance with the applicable requirements of the Securities Purchase
Agreement, the Convertible Debentures and the Warrants; (vi) such transfer shall
have been conducted in accordance with all applicable federal and state
securities laws; and (vii) such transfer or assignment does not result in more
than ten (10) holders of Registrable Securities and, after giving effect to such
transfer or assignment, such assignee or transferee holds at least 10% of the
Registrable Securities (subject to appropriate adjustment for stock splits,
stock dividends, combinations, recapitalizations and other similar transactions)
or such lesser number if such transfer involves all of the Registrable
Securities then held by such transferor.

      10. Amendment of Registration Rights.

      Provisions of this Agreement may be amended and the observance thereof may
be waived (either generally or in a particular instance and either retroactively
or prospectively), only with the written consent of the Company and Investors
who then hold at least 60% of the Registrable Securities. Any amendment or
waiver effected in accordance with this Section 10 shall be binding upon each
Investor and the Company. No such amendment shall be effective to the extent
that it applies to less than all of the holders of the Registrable Securities.
No consideration shall be offered or paid to any Person to amend or consent to a
waiver or modification of any provision of any of this Agreement unless the same
consideration also is offered to all of the parties to this Agreement.

      11. Miscellaneous.

            a. A Person is deemed to be a holder of Registrable Securities
whenever such Person owns or is deemed to own of record such Registrable
Securities. If the Company receives conflicting instructions, notices or
elections from two or more Persons with respect to the same Registrable
Securities, the Company shall act upon the basis of instructions, notice or
election received from such record owner of such Registrable Securities.

            b. Any notices, consents, waivers or other communications required
or permitted to be given under the terms of this Agreement must be in writing
and will be deemed to have been delivered: (i) upon receipt, when delivered
personally; (ii) upon receipt, when sent by facsimile (provided confirmation of
transmission is mechanically or electronically generated and kept on file by the
sending party); or (iii) one (1) Business Day after deposit with a nationally
recognized overnight delivery service, in each case properly addressed to the
party to receive the same. The addresses and facsimile numbers for such
communications shall be:


                                       15
<PAGE>

              If to the Company:

                       Hemispherx Biopharma, Inc.
                       One Penn Center
                       1617 JFK Boulevard, Suite 660
                       Philadelphia, PA  19103
                       Telephone:
                       Facsimile:
                       Attention: Chief Executive Officer

              With a copy to:

                       Ransom W. Etheridge, Esq.
                       2610 Potters Road
                       Suite 200
                       Virginia Beach, VA  23452
                       Telephone: 757-486-0599
                       Facsimile: 757-486-0792

              If to Legal Counsel:

                       Schulte Roth & Zabel LLP
                       919 Third Avenue
                       New York, New York 10022
                       Telephone: (212) 756-2000
                       Facsimile  (212) 593-5955
                       Attention: Eleazer Klein, Esq.

If to a Buyer, to its address and facsimile  number set forth on the Schedule of
Buyers attached hereto, with copies to such Buyer's representatives as set forth
on the Schedule of Buyers,  or to such other  address  and/or  facsimile  number
and/or  to the  attention  of such  other  person  as the  recipient  party  has
specified by written notice given to each other party five (5) days prior to the
effectiveness of such change.  Written  confirmation of receipt (A) given by the
recipient  of  such  notice,  consent,   waiver  or  other  communication,   (B)
mechanically  or  electronically  generated  by the sender's  facsimile  machine
containing the time, date,  recipient facsimile number and an image of the first
page of such  transmission  or (C)  provided by a courier or  overnight  courier
service shall be rebuttable  evidence of personal service,  receipt by facsimile
or receipt from a nationally recognized overnight delivery service in accordance
with clause (i), (ii) or (iii) above, respectively.

            c. Failure of any party to exercise any right or remedy under this
Agreement or otherwise, or delay by a party in exercising such right or remedy,
shall not operate as a waiver thereof.

            d. All questions concerning the construction, validity, enforcement
and interpretation of this Agreement shall be governed by the internal laws of
the State of New York, without giving effect to any choice of law or conflict of
law provision or rule (whether of the State of New York or any other
jurisdictions) that would cause the application of the laws of any


                                       16
<PAGE>

jurisdictions other than the State of New York. Each party hereby irrevocably
submits to the non-exclusive jurisdiction of the state and federal courts
sitting in the City of New York, borough of Manhattan, for the adjudication of
any dispute hereunder or in connection herewith or with any transaction
contemplated hereby or discussed herein, and hereby irrevocably waives, and
agrees not to assert in any suit, action or proceeding, any claim that it is not
personally subject to the jurisdiction of any such court, that such suit, action
or proceeding is brought in an inconvenient forum or that the venue of such
suit, action or proceeding is improper. Each party hereby irrevocably waives
personal service of process and consents to process being served in any such
suit, action or proceeding by mailing a copy thereof to such party at the
address for such notices to it under this Agreement and agrees that such service
shall constitute good and sufficient service of process and notice thereof.
Nothing contained herein shall be deemed to limit in any way any right to serve
process in any manner permitted by law. If any provision of this Agreement shall
be invalid or unenforceable in any jurisdiction, such invalidity or
unenforceability shall not affect the validity or enforceability of the
remainder of this Agreement in that jurisdiction or the validity or
enforceability of any provision of this Agreement in any other jurisdiction.
EACH PARTY HEREBY IRREVOCABLY WAIVES ANY RIGHT IT MAY HAVE, AND AGREES NOT TO
REQUEST, A JURY TRIAL FOR THE ADJUDICATION OF ANY DISPUTE HEREUNDER OR IN
CONNECTION HEREWITH OR ARISING OUT OF THIS AGREEMENT OR ANY TRANSACTION
CONTEMPLATED HEREBY.

            e. This Agreement, the Securities Purchase Agreement, the Warrants,
the Convertible Debentures, the Security Agreement, and the documents referenced
herein and therein constitute the entire agreement among the parties hereto with
respect to the subject matter hereof and thereof. There are no restrictions,
promises, warranties or undertakings, other than those set forth or referred to
herein and therein. This Agreement, the Securities Purchase Agreement, the
Warrants, the Security Agreement, and the Convertible Debentures supersede all
prior agreements and understandings among the parties hereto with respect to the
subject matter hereof and thereof.

            f. Subject to the requirements of Section 9, this Agreement shall
inure to the benefit of and be binding upon the permitted successors and assigns
of each of the parties hereto.

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

            h. This Agreement may be executed in identical counterparts, each of
which shall be deemed an original but all of which shall constitute one and the
same agreement. This Agreement, once executed by a party, may be delivered to
the other parties hereto by facsimile transmission of a copy of this Agreement
bearing the signature of the party so delivering this Agreement.

            i. Each party shall do and perform, or cause to be done and
performed, all such further acts and things, and shall execute and deliver all
such other agreements, certificates, instruments and documents, as the other
party may reasonably request in order to


                                       17
<PAGE>

carry out the intent and accomplish the purposes of this Agreement and the
consummation of the transactions contemplated hereby.

            j. All consents and other determinations required to be made by the
Investors pursuant to this Agreement shall be made, unless otherwise specified
in this Agreement, by Investors holding at least 60% of the Registrable
Securities, determined as if all of the Convertible Debentures held by Investors
then outstanding have been converted into Registrable Securities and all
Warrants then outstanding have been exercised for Registrable Securities without
regard to any limitations on conversion of the Convertible Debentures or on
exercise of the Warrants.

            k. The language used in this Agreement will be deemed to be the
language chosen by the parties to express their mutual intent and no rules of
strict construction will be applied against any party.

            l. This Agreement is intended for the benefit of the parties hereto
and their respective permitted successors and assigns, and is not for the
benefit of, nor may any provision hereof be enforced by, any other Person.

                                   * * * * * *


                                       18
<PAGE>

            IN WITNESS WHEREOF, the parties have caused this Registration Rights
Agreement to be duly executed as of day and year first above written.

COMPANY:

HEMISPHERX BIOPHARMA, INC.

By:
    -------------------------------------
    Name:
    Its:

                    [Signatures of Buyers on Following Page]


                                       19
<PAGE>

                [SIGNATURE PAGE TO REGISTRATION RIGHTS AGREEMENT]

                                BUYERS:

                                PORTSIDE GROWTH & OPPORTUNITY FUND

                                By:
                                           -----------------------------------
                                Name:
                                           -----------------------------------
                                Title:
                                           -----------------------------------

                                LEONARDO, L.P.

                                By:
                                           -----------------------------------
                                Name:
                                           -----------------------------------
                                Title:
                                           -----------------------------------


                                       20
<PAGE>

                               SCHEDULE OF BUYERS


<TABLE>
<CAPTION>
                                                                     Investor's Representative's
                                        Investor Address                       Address
    Investor                          and Facsimile Number              and Facsimile Number
    --------                          --------------------              --------------------
<S>                        <C>                                  <C>
Portside Growth &          c/o Ramius Capital Group, LLC        Schulte Roth & Zabel LLP
Opportunity Fund           666 Third Avenue, 26th Floor         919 Third Avenue
                           New York, NY 10017                   New York, NY 10022
                           Attention:  Jeffrey Smith            Attn:  Eleazer Klein
                                      Andrew Strober            Telephone:  212-756-2000
                           Telephone: (212) 845-7955            Facsimile: (212) 593-5955
                           Facsimile: (212) 845-7999

                                                                Paul, Weiss, Rifkind, Wharton &
Leonardo, L.P.             c/o Angelo Gordon, LP                Garrison LLP
                           245 Park Avenue                      1285 Avenue of the Americas
                           New York, NY 10167                   New York, NY 10019
                           Attention:  Fred Berger              Attention:  Jaideep P. Reddy, Esq.
                                             Gary Wolf          Telephone: (212) 373-3000
                           Telephone:  (212) 867-6449           Facsimile: (212) 757-3990
                           Facsimile:  (212) 867-6449
</TABLE>


<PAGE>

                                                                       EXHIBIT A

                         FORM OF NOTICE OF EFFECTIVENESS
                            OF REGISTRATION STATEMENT

[Transfer Agent]
[Address]


      Re: Hemispherx Biopharma, Inc.

Ladies and Gentlemen:

      We are counsel to Hemispherx Biopharma, Inc.., a Delaware corporation (the
"Company"), and have represented the Company in connection with that certain
Securities Purchase Agreement (the "Purchase Agreement") entered into by and
among the Company and the buyers named therein (collectively, the "Holders")
pursuant to which the Company issued to the Holders its 6% Senior Secured
Debentures, (the "Convertible Debentures") convertible into shares of the
Company's Common Stock, par value $0.001 per share (the "Common Stock") and
warrants exercisable for shares of its Common Stock (the "Warrants"). Pursuant
to the Purchase Agreement, the Company also has entered into a Registration
Rights Agreement with the Holders (the "Registration Rights Agreement") pursuant
to which the Company agreed, among other things, to register the Registrable
Securities (as defined in the Registration Rights Agreement), including the
shares of Common Stock issuable upon conversion of the Convertible Debentures
and exercise of the Warrants under the Securities Act of 1933, as amended (the
"1933 Act"). In connection with the Company's obligations under the Registration
Rights Agreement, on ____________ ___, 2003, the Company filed a Registration
Statement on Form S-3 (File No. 333-_____________) (the "Registration
Statement") with the Securities and Exchange Commission (the "SEC") relating to
the Registrable Securities, which names each of the Holders as a selling
stockholder thereunder.

      In connection with the foregoing, we advise you that a member of the SEC's
staff has advised us by telephone that the SEC has entered an order declaring
the Registration Statement effective under the 1933 Act at [ENTER TIME OF
EFFECTIVENESS] on [ENTER DATE OF EFFECTIVENESS] and we have no knowledge, after
telephonic inquiry of a member of the SEC's staff, that any stop order
suspending its effectiveness has been issued or that any proceedings for that
purpose are pending before, or threatened by, the SEC and the Registrable
Securities are available for resale under the 1933 Act pursuant to the
Registration Statement.

                                    Very truly yours,

                                    [ISSUER'S COUNSEL]

                                    By:
                                        ---------------------------


CC: [LIST NAMES OF HOLDERS]