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                                                                   EXHIBIT 10.21

                        NORTHWEST BIOTHERAPEUTICS, INC.

                                    SERIES D
                          INVESTORS' RIGHTS AGREEMENT

                                OCTOBER   , 2000
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                               TABLE OF CONTENTS

<Table>
<Caption>
                                                                     PAGE
                                                                     ----
<S>   <C>                                                            <C>
1.    Registration Rights.........................................    1
1.1   Definitions.................................................    1
1.2   Company Registration........................................    2
1.3   Obligations of the Company..................................    2
1.4   Furnish Information.........................................    3
1.5   Expenses of Registration....................................    3
1.6   Underwriting Requirements...................................    3
1.7   Delay of Registration.......................................    3
1.8   Indemnification.............................................    4
1.9   Reports Under Securities Exchange Act of 1934...............    5
1.10  Assignment of Registration Rights...........................    6
1.11  "Market Stand-Off" Agreement................................    6
1.12  Termination of Registration Rights..........................    7
2.    Covenants of the Company....................................    7
2.1   Delivery of Financial Statements............................    7
2.2   Inspection..................................................    7
2.3   Termination of Covenants....................................    7
3.    Covenants of Investors......................................    7
4.    Miscellaneous...............................................    7
4.1   Successors and Assigns......................................    7
4.2   Amendments and Waivers......................................    8
4.3   Notices.....................................................    8
4.4   Severability................................................    8
4.5   Governing Law...............................................    8
4.6   Counterparts................................................    8
4.7   Titles and Subtitles........................................    8
4.8   Aggregation of Stock........................................    8
</Table>

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                        NORTHWEST BIOTHERAPEUTICS, INC.

                                    SERIES D
                          INVESTORS' RIGHTS AGREEMENT

     This Series D Investors' Rights Agreement (the "Agreement") is made as of
the      day of             , 2000, by and among Northwest Biotherapeutics,
Inc., a Delaware corporation (the "Company"), and the Series D Preferred Stock
investors listed on Exhibit A hereto, each of which is herein referred to as a
"Series D Investor."

                                    RECITALS

     A. The Company and its Series A Investors, Series B Investors and Series C
Investors entered into an Investors' Rights Agreement dated as of February 1999
(the "Amended and Restated Rights Agreement") in order to provide such investors
with certain rights to receive or inspect information pertaining to the Company.

     B. The Series D Investors have subscribed for shares of Series D Preferred
Stock pursuant to the Confidential Private Placement Memorandum dated October
2000 and the stock purchase agreement related thereto (collectively, the
"Subscription Agreements"). A condition to the Series D Investors' obligations
under the Subscription Agreements is that the Company and the Series D Investors
enter into this Agreement in order to provide the Series D Investors with (i)
certain rights to register shares of the Company's Common Stock issuable upon
conversion of the Series D Preferred Stock held by the Series D Investors and
(ii) certain rights to receive or inspect information pertaining to the Company.

                                   AGREEMENT

     The parties hereby agree as follows:

     1. Registration Rights. The Company and the Series D Investors covenant and
agree as follows:

          1.1  Definitions. For purposes of this Section 1:

             (a) The terms "register," "registered," and "registration" refer to
        a registration effected by preparing and filing a registration statement
        or similar document in compliance with the Securities Act of 1933, as
        amended (the "Securities Act"), and the declaration or ordering of
        effectiveness of such registration statement or document;

             (b) The term "Registrable Securities" means (i) the shares of
        Common Stock issuable or issued upon conversion of the Series D
        Preferred Stock of the Company and (ii) any other shares of Common Stock
        of the Company issued as (or issuable upon the conversion or exercise of
        any warrant right or other security which is issued as) a dividend or
        other distribution with respect to, or in exchange for or in replacement
        of, the shares listed in (i); provided, however, that the foregoing
        definition shall exclude in all cases any Registrable Securities sold by
        a person in a transaction in which his or her rights under this
        Agreement are not assigned. Notwithstanding the foregoing, Common Stock
        or other securities shall only be treated as Registrable Securities if
        and so long as they have not been (A) sold to or through a broker or
        dealer or underwriter in a public distribution or a public securities
        transaction, or (B) sold in a transaction exempt from the registration
        and prospectus delivery requirements of the Securities Act under Section
        4(1) thereof so that all transfer restrictions, and restrictive legends
        with respect thereto, if any, are removed upon the consummation of such
        sale;

             (c) The number of shares of "Registrable Securities then
        outstanding" shall be determined by the number of shares of Common Stock
        outstanding which are, and the number of shares of Common Stock issuable
        pursuant to then exercisable or convertible securities which are,
        Registrable Securities;

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             (d) The term "Holder" means any person owning or having the right
        to acquire Registrable Securities or any assignee thereof in accordance
        with Section 1.10 of this Agreement;

             (e) The term "SEC" means the Securities and Exchange Commission;
        and

             (f) The term "IPO" means a firm commitment underwritten public
        offering by the Company of shares of its Common Stock pursuant to a
        registration statement on Form S-1 under the Securities Act.

          1.2  Company Registration. If (but without any obligation to do so)
     the Company proposes to register (including for this purpose a registration
     effected by the Company for stockholders other than the Holders) any of its
     stock under the Securities Act in connection with the public offering of
     such securities solely for cash (other than a registration relating solely
     to the sale of securities to participants in a Company stock plan or a
     transaction covered by Rule 145 under the Securities Act, a registration in
     which the only stock being registered is Common Stock issuable upon
     conversion of debt securities which are also being registered, or any
     registration on any form which does not include substantially the same
     information as would be required to be included in a registration statement
     covering the sale of the Registrable Securities), the Company shall, at
     such time, promptly give each Holder written notice of such registration.
     Upon the written request of each Holder given within twenty (20) days after
     mailing of such notice by the Company in accordance with Section 4.3, the
     Company shall, subject to the provisions of Section 1.6, cause to be
     registered under the Securities Act all of the Registrable Securities that
     each such Holder has requested to be registered.

          1.3  Obligations of the Company. Whenever required under this Section
     1 to effect the registration of any Registrable Securities, the Company
     shall, as expeditiously as reasonably possible:

             (a) Prepare and file with the SEC a registration statement with
        respect to such Registrable Securities and use its best efforts to cause
        such registration statement to become effective, and, upon the request
        of the Holders of a majority of the Registrable Securities registered
        thereunder, keep such registration statement effective for up to one
        hundred twenty (120) days. The Company shall not be required to file,
        cause to become effective or maintain the effectiveness of any
        registration statement that contemplates a distribution of securities on
        a delayed or continuous basis pursuant to Rule 415 under the Securities
        Act.

             (b) Prepare and file with the SEC such amendments and supplements
        to such registration statement and the prospectus used in connection
        with such registration statement as may be necessary to comply with the
        provisions of the Securities Act with respect to the disposition of all
        securities covered by such registration statement for up to one hundred
        twenty (120) days.

             (c) Furnish to the Holders such numbers of copies of a prospectus,
        including a preliminary prospectus, in conformity with the requirements
        of the Securities Act, and such other documents as they may reasonably
        request in order to facilitate the disposition of Registrable Securities
        owned by them.

             (d) Use its best efforts to register and qualify the securities
        covered by such registration statement under such other securities or
        Blue Sky laws of such jurisdictions as shall be reasonably requested by
        the Holders, provided that the Company shall not be required in
        connection therewith or as a condition thereto to qualify to do business
        or to file a general consent to service of process in any such states or
        jurisdictions.

             (e) In the event of any underwritten public offering, enter into
        and perform its obligations under an underwriting agreement, in usual
        and customary form, with the managing underwriter of such offering, each
        Holder participating in such underwriting shall also enter into and
        perform its obligations under such an agreement.

             (f) Notify each Holder of Registrable Securities covered by such
        registration statement at any time when a prospectus relating thereto is
        required to be delivered under the Securities Act of the happening of
        any event as a result of which the prospectus included in such
        registration statement, as
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        then in effect, includes an untrue statement of a material fact or omits
        to state a material fact required to be stated therein or necessary to
        make the statements therein not misleading in the light of the
        circumstances then existing, such obligation to continue for one hundred
        twenty (120) days.

             (g) Cause all such Registrable Securities registered pursuant
        hereunder to be listed on each securities exchange on which similar
        securities issued by the Company are then listed.

             (h) Provide a transfer agent and registrar for all Registrable
        Securities registered pursuant hereunder and a CUSIP number for all such
        Registrable Securities, in each case not later than the effective date
        of such registration.

             (i) Use its best efforts to furnish, at the request of any Holder
        requesting registration of Registrable Securities pursuant to this
        Section 1, on the date that such Registrable Securities are delivered to
        the underwriters for sale in connection with a registration pursuant to
        this Section 1, if such securities are being sold through underwriters,
        or, if such securities are not being sold through underwritten the date
        that the registration statement with respect to such securities becomes
        effective, (i) an opinion, dated such date, of the counsel representing
        the Company for the purposes of such registration, in form and substance
        as is customarily given to underwriters in an underwritten public
        offering, addressed to the underwriters, if any, and to the Holders
        requesting registration of Registrable Securities and (ii) a letter
        dated such date, from the independent certified public accountant, of
        the Company, in form and substance as is customarily given by
        independent certified public accountants to underwriters in an
        underwritten public offering, addressed to the underwriters, if any, and
        to the Holders requesting registration of Registrable Securities.

          1.4  Furnish Information. It shall be a condition precedent to the
     obligations of the Company to take any action pursuant to this Section 1
     with respect to the Registrable Securities of any selling Holder that such
     Holder shall furnish to the Company such information regarding itself, the
     Registrable Securities held by it, and the intended method of disposition
     of such securities as shall be required to effect the registration of such
     Holder's Registrable Securities.

          1.5  Expenses of Registration. All expenses for each Holder (which
     right may be assigned as provided in Section 1.10), other than underwriting
     discounts and commissions incurred in connection with the first four (in
     the aggregate) registrations, filings or qualifications of Registrable
     Securities pursuant to Section 1.2, including (without limitation) all
     registration, filing, and qualification fees, printers' and accounting
     fees, fees and disbursements of counsel for the Company and the reasonable
     fees and disbursements of one counsel for the selling Holder or Holders
     selected by them with the approval of the Company, which approval shall not
     be unreasonably withheld, shall be borne by the Company.

          1.6  Underwriting Requirements. In connection with any offering
     involving an underwriting of shares of the Company's capital stock, the
     Company shall not be required under Section 1.2 to include any of the
     Holders' securities in such underwriting unless they accept the terms of
     the underwriting as agreed upon between the Company and the underwriters
     selected by it (or by other persons entitled to select the underwriters),
     and then only in such quantity as the underwriters determine in their sole
     discretion will not jeopardize the success of the offering by the Company.
     If the total amount of securities, including Registrable Securities,
     requested by stockholders to be included in such offering exceeds the
     amount of securities sold other than by the Company that the underwriters
     determine in their sole discretion is compatible with the success of the
     offering, then the Company shall be required to include in the offering
     only that number of such securities, including Registrable Securities,
     which the underwriters determine in their sole discretion will not
     jeopardize the success of the offering (the securities so included to be
     apportioned pro rata among the selling stockholders according to the total
     amount of securities entitled to be included therein owned by each selling
     stockholder or in such other proportions as shall mutually be agreed to by
     such selling stockholders) but in no event shall the amount of securities
     of the selling Holders included in the offering be reduced below twenty
     percent (20%) of the total amount of securities included in such offering,
     unless such offering is the initial public offering of the Company's
     securities, in which case the selling stockholders may be excluded if the
     underwriters make the determination described above and no other
     stockholder's securities are included. For purposes of the
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     preceding parenthetical concerning apportionment, for any selling
     stockholder which is a holder of Registrable Securities and which is a
     partnership or corporation, the partners, retired partners and stockholders
     of such holder, or the estates and family members of any such partners and
     retired partners and any trusts for the benefit of any of the foregoing
     persons shall be deemed to be a single "selling stockholder," and any pro
     rata reduction with respect to such "selling stockholder" shall be based
     upon the aggregate amount of shares carrying registration rights owned by
     all entities and individuals included in such "selling stockholder," as
     defined in this sentence.

          1.7  Delay of Registration. No Holder shall have any right to obtain
     or seek an injunction restraining or otherwise delaying any such
     registration as the result of any controversy that might arise with respect
     to the interpretation or implementation of this Section 1.

          1.8  Indemnification. In the event any Registrable Securities an
     included in a registration statement under this Section 1:

             (a) To the extent permitted by law, the Company will indemnify and
        hold harmless each Holder, any underwriter (as defined in the Securities
        Act) for such Holder and each person, if any, who controls such Holder
        or underwriter within the meaning of the Securities Act or the
        Securities Exchange Act of 1934, as amended (the "Exchange Act"),
        against any losses, claims, damages, or liabilities (joint or several)
        to which they may become subject under the Securities Act, the Exchange
        Act or other federal or state law, insofar as such losses, claims,
        damages, or liabilities (or actions in respect thereof) arise out of or
        are based upon any of the following statements, omissions or violations
        (collectively a "Violation"): (i) any untrue statement or alleged untrue
        statement of a material fact contained in such registration statement,
        including any preliminary prospectus or final prospectus contained
        therein or any amendments or supplements thereto, (ii) 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, or
        (iii) any violation or alleged violation by the Company of the
        Securities Act, the Exchange Act, any state securities law or any rule
        or regulation promulgated under the Securities Act, the Exchange Act or
        any state securities law; and the Company will pay to each such Holder,
        underwriter or controlling person, as incurred, any legal or other
        expenses reasonably incurred by them in connection with investigating or
        defending any such loss, claim, damage, liability, or action; provided,
        however, that the indemnity agreement contained in this subsection
        1.8(a) shall not apply to amounts paid in settlement of any such loss,
        claim, damage, liability, or action if such settlement is effected
        without the consent of the Company (which consent shall not be
        unreasonably withheld), nor shall the Company be liable to any Holder,
        underwriter or controlling person for any such loss, claim, damage,
        liability, or action to the extent that it arises out of or is based
        upon a Violation which occurs in reliance upon and in conformity with
        written information furnished expressly for use in connection with such
        registration by any such Holder, underwriter or controlling person.

             (b) To the extent permitted by law, each selling Holder will
        indemnify and hold harmless the Company, each of its directors, each of
        its officers who has signed the registration statement, each person, if
        any, who controls the Company within the meaning of the Securities Act,
        any underwriter, any other Holder selling securities in such
        registration statement and any controlling person of any such
        underwriter or other Holder, against any losses, claims, damages, or
        liabilities (joint or several) to which any of the foregoing persons may
        become subject, under the Securities Act, the Exchange Act or other
        federal or state law, insofar as such losses, claims, damages, or
        liabilities (or actions in respect thereto) 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 by such Holder expressly for use in
        connection with such registration; and each such Holder will pay, as
        incurred, any legal or other expenses reasonably incurred by any person
        intended to be indemnified pursuant to this subsection 1.8(b), in
        connection with investigating or defending any such loss, claim, damage,
        liability, or action; provided, however, that the indemnity agreement
        contained in this subsection 1.8(b) shall not apply to amounts paid in
        settlement of any such loss, claim, damage, liability or action if such
        settlement is effected without the consent of the Holder,
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        which consent shall not be unreasonably withheld; provided, that in no
        event shall any indemnity under this subsection 1.8(b) exceed the net
        proceeds from the offering received by such Holder, except in the case
        of willful fraud by such Holder.

             (c) Promptly after receipt by an indemnified party under this
        Section 1.8 of notice of the commencement of any action (including any
        governmental action), such indemnified party will, if a claim in respect
        thereof is to be made against any indemnifying party under this Section
        1.8, deliver to the indemnifying party a written notice of the
        commencement thereof 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 similarly noticed to assume
        the defense thereof with counsel mutually satisfactory to the parties;
        provided, however, that an indemnified party (together with all other
        indemnified parties which may be represented without conflict by one
        counsel) shall have the right to retain one separate counsel, with the
        reasonable fees and expenses to be paid by the indemnifying party, if
        representation of such indemnified party by the counsel retained by the
        indemnifying party would be inappropriate due to actual or potential
        differing interests between such indemnified party and any other party
        represented by such counsel in such proceeding. The failure to deliver
        written notice to the indemnifying party within a reasonable time of the
        commencement of any such action, if prejudicial to its ability to defend
        such action, shall relieve such indemnifying party of any liability to
        the indemnified party under this Section 1.8, but the omission so to
        deliver written notice to the indemnifying party will not relieve it of
        any liability that it may have to any indemnified party otherwise than
        under this Section 1.8.

             (d) If the indemnification provided for in this Section 1.8 is held
        by a court of competent jurisdiction to be unavailable to an indemnified
        party with respect to any loss, liability, claim, damage or expense
        referred to therein, then the indemnifying party, in lieu of
        indemnifying such indemnified party hereunder, shall contribute to the
        amount paid or payable by such indemnified party as a result of such
        loss, liability, claim, damage, or expense in such proportion as is
        appropriate to reflect the relative fault of the indemnifying party on
        the one hand and of the indemnified party on the other in connection
        with the statements or omissions that resulted in such loss, liability,
        claim, damage or expense as well as any other relevant equitable
        considerations; provided, that in no event shall any contribution by a
        Holder under this subsection 1.8(d) exceed the net proceeds from the
        offering received by such Holder, except in the case of willful fraud by
        such Holder. The relative fault of the indemnifying party and of the
        indemnified party shall be determined by reference to, among other
        things, whether the untrue or alleged untrue statement of a material
        fact or the omission to state a material fact relates to information
        supplied by the indemnifying party or by the indemnified party and the
        parties, relative intent knowledge, access to information, and
        opportunity to correct or prevent such statement or omission.

             (e) Notwithstanding the foregoing, to the extent that the
        provisions on indemnification and contribution contained in the
        underwriting agreement entered into in connection with the underwritten
        public offering are in conflict with the foregoing provisions, the
        provisions in the underwriting agreement shall control.

             (f) The obligations of the Company and Holders under this Section
        1.8 shall survive the completion of any offering of Registrable
        Securities in a registration statement under this Section 1, and
        otherwise.

          1.9  Reports Under Securities Exchange Act of 1934. With a view to
     making available to the Holders the benefits of Rule 144 promulgated under
     the Securities Act and any other rule or regulation of the SEC that may at
     any time permit a Holder to sell securities of the Company to the public
     without registration, the Company agrees to:

             (a) make and keep public information available, as those terms are
        understood and defined in SEC Rule 144, at all times after ninety (90)
        days after the effective date of the first registration statement filed
        by the Company for the offering of its securities to the general public
        so long as the

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        Company remains subject to the periodic reporting requirements under
        Sections 13 or 15(d) of the Exchange Act;

             (b) file with the SEC in a timely manner all reports and other
        documents required of the Company under the Securities Act and the
        Exchange Act; and

             (c) furnish to any Holder, so long as the Holder owns any
        Registrable Securities, forthwith upon request (i) a written statement
        by the Company that it has complied with the reporting requirements of
        SEC Rule 144 (at any time after ninety (90) days after the effective
        date of the first registration statement filed by the Company), the
        Securities Act and the Exchange Act (at any time after it has become
        subject to such reporting requirements), (ii) a copy of the most recent
        annual or quarterly report of the Company and such other reports and
        documents so filed by the Company, and (iii) such other information as
        may be reasonably requested in availing any Holder of any rule or
        regulation of the SEC which permits the selling of any such securities
        without registration or pursuant to such form.

          1.10  Assignment of Registration Rights. The rights to cause the
     Company to register Registrable Securities pursuant to this Section 1 may
     be assigned (but only with all related obligations) by a Holder to a
     transferee or assignee of at least 275,350 shares of such securities unless
     such transferee or assignee is a partner or affiliate of the Holder in
     which case no minimum shareholdings are required, or all of such
     transferring Holder's securities, if less than 275,350 shares, provided,
     the Company is promptly furnished with written notice of the name and
     address of such transferee or assignee and the securities with respect to
     which such registration rights are being assigned; and provided, further,
     that such assignment shall be effective only if immediately following such
     transfer the further disposition of such securities by the transferee or
     assignee is restricted under the Securities Act. For the purposes of
     determining the number of shares of Registrable Securities held by a
     transferee or assignee, the holdings of transferees and assignees of a
     partnership who arc partners or retired partners of such partnership
     (including spouses and ancestors, lineal descendants and siblings of such
     partners or spouses who acquire Registrable Securities by gift, will or
     intestate succession) shall be aggregated together and with the
     partnership; provided that all assignees and transferees who would not
     qualify individually for assignment of registration rights shall have a
     single attorney-in-fact for the purpose of exercising any rights, receiving
     notices or taking any action under Section 1.

          1.11  "Market Stand-Off" Agreement. Each Holder hereby agrees that,
     during the period of duration (up to 180 days in the case of an initial
     public offering and up to 90 days in the case of all other public
     offerings) specified by the Company and an underwriter of Common Stock or
     other securities of the Company, following the effective date of a
     registration statement of the Company filed under the Securities Act, it
     shall not, to the extent requested by the Company and such underwriter,
     directly or indirectly sell, offer to sell, contract to sell (including,
     without limitation, any short sale), grant any option to purchase or
     otherwise transfer or dispose of (other than to donees who agree to be
     similarly bound) any securities of the Company held by it at any time
     during such period except Common Stock included in such registration;
     provided, however, that all officers and directors of the Company enter
     into similar agreements.

          In order to enforce the foregoing covenant, the Company may impose
     stop-transfer instructions with respect to the Registrable Securities of
     each Holder (and the shares or securities of every other person subject to
     the foregoing restriction) until the end of such period, and each Holder
     agrees that, if so requested, such Holder will execute an agreement in the
     form provided by the underwriter containing terms which are essentially
     consistent with the provisions of this Section 1.11.

          Notwithstanding the foregoing, the obligations described in this
     Section 1.11 shall not apply to a registration relating solely to employee
     benefit plans on Form S-1 or Form S-8 or similar forms which may be
     promulgated in the future, or a registration relating solely to an SEC Rule
     145 transaction on Form S-4 or similar forms which may be promulgated in
     the future.

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          1.12  Termination of Registration Rights. No Holder shall be entitled
     to exercise any right provided for in this Section 1 after the earlier of
     (i) five (5) years following the consummation of an IPO, or (ii) such time
     as Rule 144 or another similar exemption under the Securities Act is
     available for the sale of all of such Holder's shares during a three
     (3)-month period without registration.

     2. Covenants of the Company.

          2.1  Delivery of Financial Statements. The Company shall deliver to
     each Holder of at least 165,000 shares of Registrable Securities (other
     than a Holder reasonably deemed by the Company to be a competitor of the
     Company):

             (a) as soon as practicable, but in any event within ninety (90)
        days after the end of each fiscal year of the Company, an income
        statement for such fiscal year, a balance sheet of the Company and
        statement or stockholder's equity as of the end of such year, and a
        statement of cash flows for such year, such year-end financial reports
        to be in reasonable detail, prepared in accordance with generally
        accepted accounting principles ("GAAP"), and audited and certified by an
        independent public accounting firm of nationally recognized standing
        selected by the Company;

             (b) as soon as practicable, but in any event within thirty (30)
        days after the end of each of the first three (3) quarters of each
        fiscal year of the Company, an unaudited profit or loss statement, a
        statement of cash flows for such fiscal quarter and an unaudited balance
        sheet as of the end of such fiscal quarter, and

             (c) as soon as practicable, but in any event thirty (30) days prior
        to the end of each fiscal year, a budget for the next fiscal year,
        prepared on a monthly basis.

          2.2  Inspection. The Company shall permit each Holder of at least
     165,000 shares of Registrable Securities (except for a Holder reasonably
     deemed by the Company to be a competitor of the Company), at such Holder's
     expense, to visit and inspect the Company's properties, to examine its
     books of account and records and to discuss the Company's affairs, finances
     and accounts with its officers, all at such reasonable times as may be
     requested by the Series D Investor; provided, however, that the Company
     shall not be obligated pursuant to this Section 2.2 to provide access to
     any information which it reasonably considers to be a trade secret or
     similar confidential information,

          2.3  Termination of Covenants.

             (a) The covenants set forth in Sections 2.1 and 2.2 shall terminate
        as to each Holder and be of no further force or effect (i) immediately
        prior to the consummation of an IPO, or (ii) when the Company shall
        sell, convey, or otherwise dispose of or encumber all or substantially
        all of its property or business or merge into or consolidate with any
        other corporation (other than a wholly-owned subsidiary corporation) or
        effect any other transaction or series of related transactions in which
        more than fifty percent (50%) of the voting power of the Company is
        disposed of, provided that this subsection (ii) shall not apply to a
        merger effected exclusively for the purpose of changing the domicile of
        the Corporation.

             (b) The covenants set forth in Sections 2.1 and 2.2 shall terminate
        as to each Holder and be of no further force or effect when the Company
        first becomes subject to the periodic reporting requirements of Sections
        13 or 15(d) of the Exchange Act, if this occurs earlier than the events
        described in Section 2.3(a) above.

     3. Covenants of Investors. The Series D Investor may be permitted, subject
to compliance with applicable securities laws, to sell, transfer, distribute or
grant participations to another person or party with respect to any or all the
Securities in order to satisfy the requirements of the U.S. Internal Revenue
Service.

     4. Miscellaneous.

          4.1  Successors and Assigns. Except as otherwise provided in this
     Agreement, the terms and conditions of this Agreement shall inure to the
     benefit of and be binding upon the respective permitted successors and
     assigns of the parties (including transferees of the Preferred Stock or any
     class or series or
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<PAGE>   10

     any Common Stock issued upon conversion thereof). Nothing in this
     Agreement, express or implied, is intended to confer upon any party other
     than the parties hereto or their respective successors and assigns any
     rights, remedies, obligations, or liabilities under or by reason of this
     Agreement, except as expressly provided in this Agreement.

          4.2  Amendments and Waivers. Any term of this Agreement may be amended
     or waived only with the written consent of the Company and the holders of a
     majority of the Registrable Securities then outstanding. Any amendment or
     waiver effected in accordance with this paragraph shall be binding upon
     each bolder of any Registrable Securities then outstanding, each future
     holder of all such Registrable Securities, and the Company.

          4.3  Notices. Unless otherwise provided, any notice required or
     permitted by this Agreement shall be in writing and shall be deemed
     sufficient upon delivery, when delivered personally or by overnight courier
     or sent by telegram or fax, or forty-eight (48) hours after being deposited
     in the U.S. mail, as certified or registered mail, with postage prepaid,
     and addressed to the party to be notified at such party's address or fax
     number as set forth on the signature page or on Exhibit A hereto or as
     subsequently modified by written notice.

          4.4  Severability. If one or more provisions of this Agreement are
     held to be unenforceable under applicable law, the parties agree to
     renegotiate such provision in good faith. In the event that the patties
     cannot reach a mutually agreeable and enforceable replacement for such
     provision, then (a) such provision shall be excluded from this Agreement,
     (b) the balance of the Agreement shall be interpreted as if such provision
     were so excluded and (c) the balance of the Agreement shall be enforceable
     in accordance with its terms.

          4.5  Governing Law. This Agreement and all acts and transactions
     pursuant hereto shall be governed, construed and interpreted in accordance
     with the laws of the State of Washington, without giving effect to
     principles of conflicts of laws.

          4.6  Counterparts. This Agreement may be executed in two or more
     counterparts, each of which shall be deemed an original, but all of which
     together shall constitute one and the same instrument.

          4.7  Titles and Subtitles. The titles and subtitles used in this
     Agreement are used for convenience only and are not to be considered in
     construing or interpreting this Agreement.

          4.8  Aggregation of Stock. All shares of the Preferred Stock held or
     acquired by affiliated entities or persons shall be aggregated together for
     the purpose of determining the availability of any rights tinder this
     Agreement.

                            [Signature page follows]

                                        8
<PAGE>   11

     The parties have executed this Investors' Rights Agreement as of the date
first above written.

<Table>
<S>                                                    <C>

COMPANY:                                               SERIES D INVESTOR:
NORTHWEST BIOTHERAPEUTICS, INC.                        By:
By:                                                    -----------------------------------------------------
                                                       Name:----------------------------------------------
-----------------------------------------------------         (print)
    Daniel O. Wilds, President                         Title:-----------------------------------------------
Address:  21720 23rd Drive S.E., Suite 100
           Bothell, Washington 98021
Fax:      (206) 368-3026
</Table>

                                        9
<PAGE>   12
                                   EXHIBIT A
                     LIST OF INVESTORS - SERIES D PREFERRED


<TABLE>
<S>                                     <C>
Rosa Ayer                                  6,000

Ronald D. Crockett                        60,000

Howard L. Freedman                         5,000

Chetron Enterprise LLP                     2,000

Lochmar Enterprise LLP                     2,000

Thomas Greene                              2,400
Gerald D. Hermanson                        2,000

Mark and Kelly Kelly Trust DTD
12/18/87                                   5,000

Henry L. Kotkins, TTEE, The Henry
L. Kotkins Trust UA DTD 10/29/93          10,000

Tenwall Investment Co.                    15,000

Miller Schultz LLC                         5,000

Robert B. Olsen and Sharon M. Olsen       18,000

Robert F. Schenkkan, Jr.                   2,000

John A. Schwager, Jr. and Maria H.
Schwager                                   4,000

Gary R. Schwartz                          10,000

Patricia Sheldon                          40,000

Paul Soloway and Pamela J. Pruitt          5,000

John W. Sundsten                           6,000

Joan C. Tupper                            10,000

Nancy Weintraub                           30,000

Paul F. Seligmann                         10,000

E3 Investment Associates                  10,000

Roger W. Beckner                           2,000

Daniel L. and Julie E. Brock                 500

Edward J. Buchwald                         5,000

Martin C. Burkland and Christine S.
Burkland                                   2,000
</TABLE>


                                      A-1


<PAGE>   13
<TABLE>
<S>                                     <C>
Stephen CuUnjieng                         10,000

Jack T. Danforth and Suzan D. Danforth
JT TEN                                     5,000

Christopher C. Davis and M. Christine
Davis                                     10,000

Prudential Securities C/F Juan Carlos
del Rosario                               20,000

The Dernbach Family Trust dtd 7/16/98      2,000

Sarah P. Doud                             10,000

Russell A. Ekeblad and Sheila R.
Ekeblad, JT TEN                            7,000

A. H. Feige, Jr.                           2,000

Carol Long Feige                           2,000

Prudential Securities C/F Helen G.
Gerry                                      2,000

Michael J. Goldfarb, his separate
property                                  22,460

Jarlath J. Hume                            2,000

Kaplan & Kaplan                            2,000

Ralph Katz                                 5,000

Marc and Kari Lanphear                     6,000

Anne D. McCormick                          3,000

Jonathan Scott Miller                     10,000

Terence V. O'Keefe and Judy A.
O'Keefe                                   15,000

Jennifer T. Potter                         5,000

Prudential Securities C/F Bruce H.
Schwager                                   2,000

Prudential Securities C/F Christine E.
Schwager                                   2,000

Prudential Securities C/F Steven G.
Schwager                                   3,100

David E. Skinner                           5,000

Thomas D. Waldron                          2,000

Prudential Securities C/F Stephen
Beatty IRA Rollover                        5,000

Marnix L. and Elisabeth M. Bosch          10,000

Stephen H. Catlett                        12,000
</TABLE>


                                      A-2


<PAGE>   14
<TABLE>
<S>                                     <C>
Lynne Rogers Feldman                       2,000

Padraic Freeman and Susan Golub            2,000

Michael J. Goldfarb, his separate
property                                  25,000

Thomas Greene                              3,000

Keith D. Grinstein                         5,000

Thomas F. Grohman                          1,500

Robert D. Hamman, CLU, Inc. Profit
Sharing Plan                               5,000

Robert D. and Petra Hamman                 5,000

Kenneth M. Jafe                            2,000

Marinesa S. Letizia                        2,000

James W. Mahaffey                          6,000

Alan Bruce Montgomery                      5,000

Jonathan G. Morgan                         5,000

William W. Putts                          10,000

Prize Indemnity Ltd.                      10,000

Peter C. Rosvall                         200,000

Roger Salloch                              4,000

SCA Promotions, Inc.                       5,000

John A. Schwager, Sr.                     25,000

Maria-Helena Schwager                      2,000

James P. and Paula W. Sheehan              2,000

Michael J. Taylor and Debra A. Nicolet     1,400

Camille M. Uhlir                           5,000

Marnix L. and Elisabeth M. Bosch           1,000

Joseph W. Charlton, Jr.                    6,000

David Neiman                               4,000

Douglas C. Rosen                          10,000
</TABLE>


                                      A-3


<PAGE>   15
<TABLE>
<S>                                     <C>
Robert W. Rosen                            4,000

Michael E. Seamon and Julie Seamon,
JT TEN                                     2,000

Ronald M. Sukoneck and Judi L. Cody        2,000

Robert B. Olsen MD Pension & Profit
Sharing Trust DTD 6/19/81                 15,000

Medarex, Inc.                            800,000

R. Matluck & T. Unterberg TTEES
C.E. Unterberg, Towbin 401K Profit
Sharing Plan dtd 10/26/90 FBO Andrew
Arno                                      10,000

Andrew Arno ACF Matthew Arno
U/NY/UGMA                                  5,000

Andrew Arno ACF Jesse Benjamin
Arno U/NY/UGMA                             5,000

Anthony Charos                             2,000

Kevin Charos                               2,000

Ronald and Lisa Frumkes                    2,000

R. Matluck & T. Unterberg TTEES
C.E. Unterberg, Towbin 401 K Profit
Sharing Plan dtd 10/26/90 FBO Theresa
Garvey                                     2,000

R. Matluck & T. Unterberg TTEES
C.E. Unterberg, Towbin 401K Profit
Sharing Plan dtd 10/26/90 FBO Robert
L. Thompson                                2,000

A. Robert Towbin TTEE FBO Barry
towbin Trust dtd 5/8/96                   10,000

A. Robert Towbin TTEE FBO Lisa
Olim Trust dtd 5/8/96                     10,000

Thomas I. Unterberg TTEE Ellen Celli
Family Trust                              10,000

Emily U. Satloff                          20,000

Thomas I. Unterberg TTEE Emily U.
Satloff Family Trust                      10,000

Thomas I. Unterberg                       40,000

Marjorie and Clarence E. Unterberg
Foundation, Inc.                          40,000

Bella and Israel Unterberg Foundation
#2                                        10,000

C.E. Unterberg, Towbin LLC                50,000

C.E. Unterberg, Towbin, A California
Limited Partnership                       50,000

C.E. Unterberg, Towbin Capital
Partners I, L.P.                         100,000
</TABLE>


                                      A-4


<PAGE>   16
<TABLE>
<S>                                     <C>
William O. Batley                         10,000

Alan R. Baumann Revocable Trust
U/A/D 12/21/83                             5,000

James E. and Ann B. Borner                10,000

Julian Marie Breslow                       4,000

Neill and Linda S. Brownstein             40,000

CastleRock Partners, L.P.                110,000

CastleRock Partners II, L.P.              10,000

CastleRock Fund, Ltd.                    140,000

CRCK, LLC                                140,000

Reuben David                               2,000

Christoph Harder                          50,000

E. Paul Herbert                            2,000

Francine Hoffman                           2,500

David L. House                            20,000

Paul Huang                                 2,000

Hull Capital Corp. P.S. FBO J. Mitchell
Hull                                       5,000

George Kafkarkou                           5,000

Gloria D. Kelley                           2,000

James P. Kelley                            2,000

Jeffrey Kingsley                           2,000

Richard M. Kirshner                        2,000

Gordon S. and Pamela A. Mitchard          20,000

Theo W. Muller                             4,000

Michael J. Picker and Deborah W.
Picker                                     2,000

Roy E. Reichbach                           2,000

Daniel H. Rosenfeld                        2,500

Steven J. Sampson Trust #2                10,000
</TABLE>


                                      A-5


<PAGE>   17
<TABLE>
<S>                                     <C>
Norman A. Scott                            5,000

David and Lore Squier                      2,000

Walter Sturm, M.D. and Sandra Sturm        5,000

David and Lon Talescknick                  2,000

Wafik and Gail Abdou                       2,000

Gordon A. Bowker                           2,000

Kreg W. Boynton and Shen Boynton
DDS                                        5,000

Melinda N. Boynton                         2,300

Sidney Brownstein and Jill Meyers          5,000

Chetron Enterprise LLP                     2,000

Ira S. Cohen and Ann Cohen TTEES Ira
and Ann Cohen Farmly Trust DTD
11/5/97                                    6,000

Prudential Securities OF Juan Carlos
del Rosario                                8,500

David Goldstein MD PA Pension Trust        5,000

Thomas Greene                              4,768

Paul Jay Lewis                            10,000

Lochmar Enterprise LLP                     2,000

Terry L. Miller and Debra Shank Miller     5,000

Trust of G. Robert Osborn and Laurie
M. Osborn DTD 12/20/96                     6,000

Leslie A. Parque                           2,000

Oyvind Ragde                               2,000

Michael J. Rice                           20,000

Ronald L. Rubenstein                       4,000

Ronald L. Rubenstein MDPC TTEE
Employees Profit Sharing Trust P/S/P
Dtd. 8/31/81                              10,000

Cathy A. Sarkowsky                        15,000

Prudential Securities C/F Anne W.
Schwager IRA                               5,000

Hardwick Simmons                          20,000
</TABLE>


                                      A-6


<PAGE>   18
<TABLE>
<S>                                     <C>
Sonz Partners LP                          20,000

Gordon Wilcox, TTEE fbo The Gordon
Wilcox 401(k) Plan dtd 1/1/93             10,000

PITT & CO. FBO: Albertson's Inc.
Profit Sharing Plan                       34,000

Paine Webber as IRA Custodian FBO:
Andrew Bluestone, SEP IRA                  2,000

Steven Gersten                             2,000

Michael Glita and Joan D. Glita            2,000

Pauline de Henri Court de Grunne           2,000

David Hoffman and Heidi A. Hoffman         2,000

Paine Webber as IRA Custodian FBO:
Herbert S. Hoffman IRA                     3,000

Paine Webber as IRA Custodian FBO:
Thomas H. Law IRA                          2,000

Bruce A. MacNaughton                       5,000

Michael E. Marrus                          3,000

Sharon McFarlin                            2,000

Omnivest (Bermuda) Ltd.                   10,000

Raymond James & Associates, Inc. C/F
Charles R. Schaller SP IRA #73540455       2,000

Judy W. Soley                              2,000

Steven S. Spinell and Susan H. Spinell     2,000

Paine Webber as IRA Custodian FBO:
A. Robert Towbin IRA                       5,000

UMBTRU & CO. FBO: Wasatch Ultra
Growth Fund                               29,000

UUBTRU & CO. FBO: Wasatch
Global Technology Fund                     7,000

Paine Webber as IRA Custodian FBO:
Barry Goldsmith IRA                        5,000

John C. Kehoe and Caroline M. Kehoe        5,000

Westmont Venture Partners, LLC            10,000

Paine Webher as IRA Custodian FBO:
Morris Dangott IRA                         5,000

Scott C. Stromatt                          2,000

Daniel T. Reiner                          80,000
</TABLE>


                                      A-7