Document:

<PAGE>   1
                                                                    EXHIBIT 10.1

                                 ANTIGENICS INC.

                           1999 EQUITY INCENTIVE PLAN

Section 1.  PURPOSE

         The purpose of the Antigenics Inc. 1999 Equity Incentive Plan (the
"Plan") is to attract and retain directors, key employees and consultants of the
Company and its Affiliates, to provide an incentive for them to achieve
long-range performance goals, and to enable them to participate in the long-term
growth of the Company.

Section 2.  DEFINITIONS

         "Affiliate" means any business entity that directly, or indirectly
through one or more intermediaries, controls, is controlled by or is under
common control with the Company. For purposes hereof, "Control" (and with
correlative meanings, the terms "controlled by" and "under common control with")
shall mean the possession of the power to direct or cause the direction of the
management and policies of the Company, whether through the ownership of voting
stock, by contract or otherwise. In the case of a corporation "control" shall
mean, among other things, the direct or indirect ownership of more than fifty
percent (50%) of its outstanding voting stock.

         "Award" means any Option, Stock Appreciation Right or Restricted Stock
awarded under the Plan.

         "Board" means the Board of Directors of the Company.

         "Code" means the Internal Revenue Code of 1986, as amended from time to
time, and any successor to such Code.

         "Committee" means a committee of not less than two members of the Board
appointed by the Board to administer the Plan. If a Committee is authorized to
grant Options to a Reporting Person or a "covered employee" within the meaning
of Section 162(m) of the Code, each member shall be a "non-employee director" or
the equivalent within the meaning of Rule 16b-3 under the Securities Exchange
Act of 1934, as amended from time to time, or any successor law, and an "outside
director" or the equivalent within the meaning of Section 162(m) of the Code,
respectively. Until such committee is appointed, "Committee" means the Board.

         "Common Stock" or "Stock" means the Common Stock, $0.01 par value, of
the Company.

         "Company" means Antigenics Inc.

         "Designated Beneficiary" means the beneficiary designated by a
Participant, in a manner determined by the Committee, to receive amounts due or
exercise rights of the Participant in the event of the Participant's death. In
the absence of an effective designation by a Participant, "Designated
Beneficiary" shall mean the Participant's estate.

                                       1
<PAGE>   2

         "Effective Date" means November 15, 1999.

         "Fair Market Value" means, with respect to Common Stock or any other
property, the fair market value of such property as determined by the Committee
in good faith or in the manner established by the Committee from time to time.

         "Incentive Stock Option" means an option to purchase shares of Common
Stock awarded to a Participant under Section 6 that is intended to meet the
requirements of Section 422 of the Code or any successor provision.

         "Nonstatutory Stock Option" means an option to purchase shares of
Common Stock awarded to a Participant under Section 6 that is not intended to be
an Incentive Stock Option.

         "Option" means an Incentive Stock Option or a Nonstatutory Stock
Option.

         "Participant" means a person selected by the Committee to receive an
Award under the Plan.

         "Reporting Person" means a person subject to Section 16 of the
Securities Exchange Act of 1934 or any successor provision.

         "Restricted Period" means the period of time selected by the Committee
during which an Award may be forfeited to the Company pursuant to the terms and
conditions of such Award.

         "Restricted Stock" means shares of Common Stock subject to forfeiture
awarded to a Participant under Section 8.

         "Stock Appreciation Right" or "SAR" means a right to receive any excess
in value of shares of Common Stock over the exercise price awarded to a
Participant under Section 7.

Section 3.  ADMINISTRATION

         The Plan shall be administered by the Committee. The Committee shall
have authority to adopt, alter and repeal such administrative rules, guidelines
and practices governing the operation of the Plan as it shall from time to time
consider advisable, and to interpret the provisions of the Plan. The Committee's
decisions shall be final and binding. To the extent permitted by applicable law,
the Committee may delegate to one or more executive officers of the Company the
power to make Awards to Participants who are not Reporting Persons or covered
employees and all determinations under the Plan with respect thereto, provided
that the Committee shall fix the maximum amount of such Awards for all such
Participants and a maximum for any one Participant.

Section 4.  ELIGIBILITY

         All employees, directors and consultants of the Company or any
Affiliate capable of contributing significantly to the successful performance of
the Company, other than a person who has irrevocably elected not to be eligible,
are eligible to be Participants in the Plan.

                                       2
<PAGE>   3

Incentive Stock Options may be granted only to persons eligible to receive such
Options under the Code.

Section 5.  STOCK AVAILABLE FOR AWARDS

         (a) Subject to adjustment under subsection (b), Awards may be made
under the Plan for up to 4,800,000 shares of Common Stock. If any Award in
respect of shares of Common Stock expires or is terminated unexercised or is
forfeited without the Participant having had the benefits of ownership (other
than voting rights), the shares subject to such Award, to the extent of such
expiration, termination or forfeiture, shall again be available for award under
the Plan. Common Stock issued through the assumption or substitution of
outstanding grants from an acquired company shall not reduce the shares
available for Awards under the Plan. Shares issued under the Plan may consist in
whole or in part of authorized but unissued shares or treasury shares.

         (b) In the event that the Committee determines that any stock dividend,
extraordinary cash dividend, creation of a class of equity securities,
recapitalization, reorganization, merger, consolidation, split-up, spin-off,
combination, exchange of shares, warrants or rights offering to purchase Common
Stock at a price substantially below fair market value, or other similar
transaction affects the Common Stock such that an adjustment is required in
order to preserve the benefits or potential benefits intended to be made
available under the Plan, then the Committee (subject, in the case of Incentive
Stock Options, to any limitation required under the Code) shall equitably adjust
any or all of (i) the number and kind of shares in respect of which Awards may
be made under the Plan, (ii) the number and kind of shares subject to
outstanding Awards, and (iii) the award, exercise or conversion price with
respect to any of the foregoing, and if considered appropriate, the Committee
may make provision for a cash payment with respect to an outstanding Award,
provided that the number of shares subject to any Award shall always be a whole
number.

         (c) Subject to adjustment under Subsection (b): (i) the maximum number
of shares of Common Stock with respect to which Options and Stock Appreciation
Rights may be granted to any Participant in the aggregate in any calendar year
shall not exceed 1,000,000 shares, and (ii) the maximum number of shares of
Common Stock that may be granted as Restricted Stock, with respect to which
performance goals apply, to any Participant in the aggregate in any calendar
year shall not exceed 1,000,000 shares.

Section 6.  STOCK OPTIONS

(a) Subject to the provisions of the Plan, the Committee may award Incentive
Stock Options and Nonstatutory Stock Options and determine the number of shares
to be covered by each Option, the option price therefor and the conditions and
limitations applicable to the exercise of the Option. The terms and conditions
of Incentive Stock Options shall be subject to and comply with Section 422 of
the Code or any successor provision and any regulations thereunder, and no
Incentive Stock Option may be granted hereunder more than ten years after the
Effective Date.

                                       3

<PAGE>   4

         (b) The Committee shall establish the option price at the time each
Option is awarded, which price shall not be less than 100% of the Fair Market
Value of the Common Stock on the date of award with respect to Incentive Stock
Options. Nonstatutory Stock Options may be granted at such prices as the
Committee may determine.

         (c) Each Option shall be exercisable at such times and subject to such
terms and conditions as the Committee may specify in the applicable Award or
thereafter. The Committee may impose such conditions with respect to the
exercise of Options, including conditions relating to applicable federal or
state securities laws, as it considers necessary or advisable.

         (d) No shares shall be delivered pursuant to any exercise of an Option
until payment in full of the option price therefor is received by the Company.
Such payment may be made in whole or in part in cash or, to the extent permitted
by the Committee at or after the award of the Option, by delivery of a note or
shares of Common Stock owned by the optionee, including Restricted Stock, or by
retaining shares otherwise issuable pursuant to the Option, in each case valued
at their Fair Market Value on the date of delivery or retention, or such other
lawful consideration as the Committee may determine.

Section 7.  STOCK APPRECIATION RIGHTS

         (a) Subject to the provisions of the Plan, the Committee may award SARs
in tandem with an Option (at or after the award of the Option), or alone and
unrelated to an Option. SARs in tandem with an Option shall terminate to the
extent that the related Option is exercised, and the related Option shall
terminate to the extent that the tandem SARs are exercised.

         (b) The Committee shall fix the exercise price of each SAR or specify
the manner in which the price shall be determined. SARs granted in tandem with
Options shall have an exercise price not less than the exercise price of the
related Option. SARs granted alone and unrelated to an Option may be granted at
such exercise prices as the Committee may determine.

Section 8.  RESTRICTED STOCK

         (a) Subject to the provisions of the Plan, the Committee may award
shares of Restricted Stock and determine the duration of the Restricted Period
during which, and the conditions under which, the shares may be forfeited to the
Company and the other terms and conditions of such Awards. The Committee may
establish performance goals for the granting or lapse of risk of forfeiture of
Restricted Stock. Such performance goals may be based on earnings per share,
revenues, sales or expense targets of the Company or any subsidiary, division or
product line thereof, stock price or such other business criteria as the
Committee may determine. Shares of Restricted Stock may be issued for no cash
consideration or such minimum consideration as may be required by applicable
law.

         (b) Shares of Restricted Stock may not be sold, assigned, transferred,
pledged or otherwise encumbered, except as permitted by the Committee, during
the Restricted Period. Shares of Restricted Stock shall be evidenced in such
manner as the Committee may determine. Any certificates issued in respect of
shares of Restricted Stock shall be registered in the name of the Participant
and unless otherwise determined by the Committee, deposited by the Participant,
together with a stock power endorsed in blank, with the Company. At the
expiration of the

                                       4
<PAGE>   5

Restricted Period, the Company shall deliver such certificates to the
Participant or if the Participant has died, to the Participant's Designated
Beneficiary.

Section 9.  GENERAL PROVISIONS APPLICABLE TO AWARDS

         (a) Documentation. Each Award under the Plan shall be evidenced by a
writing delivered to the Participant or agreement executed by the Participant
specifying the terms and conditions thereof and containing such other terms and
conditions not inconsistent with the provisions of the Plan as the Committee
considers necessary or advisable to achieve the purposes of the Plan or to
comply with applicable tax and regulatory laws and accounting principles.

         (b) Committee Discretion. Each type of Award may be made alone, in
addition to or in relation to any other type of Award. The terms of each type of
Award need not be identical, and the Committee need not treat Participants
uniformly. Except as otherwise provided by the Plan or a particular Award, any
determination with respect to an Award may be made by the Committee at the time
of award or at any time thereafter.

         (c) Settlement. The Committee shall determine whether Awards are
settled in whole or in part in cash, Common Stock, other securities of the
Company, Awards or other property. The Committee may permit a Participant to
defer all or any portion of a payment under the Plan, including the crediting of
interest on deferred amounts denominated in cash and dividend equivalents on
amounts denominated in Common Stock.

         (d) Dividends and Cash Awards. In the discretion of the Committee, any
Award under the Plan may provide the Participant with (i) dividends or dividend
equivalents payable currently or deferred with or without interest, and (ii)
cash payments in lieu of or in addition to an Award.

         (e) Termination of Employment or Service on the Board. The Committee
shall determine the effect on an Award of the disability, death, retirement or
other termination of employment or service on the Board of a Participant and the
extent to which, and the period during which, the Participant's legal
representative, guardian or Designated Beneficiary may receive payment of an
Award or exercise rights thereunder.

         (f) Change in Control. In order to preserve a Participant's rights
under an Award in the event of a change in control of the Company (as defined by
the Committee), the Committee in its discretion may, at the time an Award is
made or at any time thereafter, take one or more of the following actions: (i)
provide for the acceleration of any time period relating to the exercise or
realization of the Award, (ii) provide for the purchase of the Award upon the
Participant's request for an amount of cash or other property that could have
been received upon the exercise or realization of the Award had the Award been
currently exercisable or payable, (iii) adjust the terms of the Award in a
manner determined by the Committee to reflect the change in control, (iv) cause
the Award to be assumed, or new rights substituted therefor, by another entity,
or (v) make such other provision as the Committee may consider equitable to
Participants and in the best interests of the Company.

         (g) Loans. The Committee may authorize the making of loans or cash
payments to Participants in connection with any Award under the Plan, which
loans may be secured by any

                                       5
<PAGE>   6

security, including Common Stock, underlying or related to such Award (provided
that such Loan shall not exceed the Fair Market Value of the security subject to
such Award), and which may be forgiven upon such terms and conditions as the
Committee may establish at the time of such loan or at any time thereafter.

         (h) Withholding Taxes. The Participant shall pay to the Company, or
make provision satisfactory to the Committee for payment of, any taxes required
by law to be withheld in respect of Awards under the Plan no later than the date
of the event creating the tax liability. In the Committee's discretion, the
minimum tax obligations required by law to be withheld in respect of Awards may
be paid in whole or in part in shares of Common Stock, including shares retained
from the Award creating the tax obligation, valued at their Fair Market Value on
the date of retention or delivery. The Company and its Affiliates may, to the
extent permitted by law, deduct any such tax obligations from any payment of any
kind otherwise due to the Participant.

         (i) Foreign Nationals. Awards may be made to Participants who are
foreign nationals or employed outside the United States on such terms and
conditions different from those specified in the Plan as the Committee considers
necessary or advisable to achieve the purposes of the Plan or to comply with
applicable laws.

         (j) Amendment of Award. The Committee may amend, modify or terminate
any outstanding Award, including substituting therefor another Award of the same
or a different type, changing the date of exercise or realization and converting
an Incentive Stock Option to a Nonstatutory Stock Option, provided that the
Participant's consent to such action shall be required unless the Committee
determines that the action, taking into account any related action, would not
materially and adversely affect the Participant.

         (k) Transferability. In the discretion of the Committee, any Award may
be made transferable upon such terms and conditions and to such extent as the
Committee determines, provided that Incentive Stock Options may be transferable
only to the extent permitted by the Code. The Committee may in its discretion
waive any restriction on transferability.

Section 10.  MISCELLANEOUS

         (a) No Right To Employment or Service on the Board. No person shall
have any claim or right to be granted an Award, and the grant of an Award shall
not be construed as giving a Participant the right to continued employment or
service on the Board. The Company expressly reserves the right at any time to
dismiss a Participant free from any liability or claim under the Plan, except as
expressly provided in the applicable Award.

         (b) No Rights As Stockholder. Subject to the provisions of the
applicable Award, no Participant or Designated Beneficiary shall have any rights
as a stockholder with respect to any shares of Common Stock to be distributed
under the Plan until he or she becomes the holder thereof. A Participant to whom
Common Stock is awarded shall be considered the holder of the Stock at the time
of the Award except as otherwise provided in the applicable Award.

         (c) Effective Date. Subject to the approval of the stockholders of the
Company, the Plan shall be effective on the Effective Date. Before such
approval, Awards may be made under the Plan expressly subject to such approval.

                                       6
<PAGE>   7

         (d) Amendment of Plan. The Board may amend, suspend or terminate the
Plan or any portion thereof at any time, subject to any stockholder approval
that the Board determines to be necessary or advisable.

         (e) Governing Law. The provisions of the Plan shall be governed by and
interpreted in accordance with the laws of Delaware.

                    -----------------------------------------

This Plan was approved by the Board of Directors on November 15, 1999.
This Plan must be approved by the stockholders prior to November 15, 2000.

                                       7<PAGE>   1

                                                                   EXHIBIT 10.15

                               AMENDMENT AGREEMENT

The Letter Agreement of June 3, 1998 between Antigenics L.L.C. and Sigma-Tau
Industrie Farmaceutiche Riunite S.p.A. is hereby amended as follows:

-ON PAGE 1, THE INITIAL SIX LINES:

"Gentlemen:

We refer to our recent discussions regarding a potential scientific and business
cooperation between Antigenics L.L.C. ("Antigenics") and Sigma-Tau Industrie
Farmaceutiche Riunite S.p.A. (the "Company") in Italy (the "Territory") with
regard to Antigenics' proprietary technology on the use of Heat Shock Proteins
("HSP") for the use of HSP to boost and modulate the immune system against
cancer (the "Technology")."

are deleted and replaced by the following:

"Gentlemen:

We refer to our recent discussions regarding a potential scientific and business
cooperation between Antigenics L.L.C. ("Antigenics") and Sigma-Tau Industrie
Farmaceutiche Riunite S.p.A. (the "Company") in Italy, Spain, Portugal and
Switzerland (the "Territory") with regard to Antigenics' proprietary technology
on the use of Heat Shock Proteins ("HSP") for the use of HSP to boost and
modulate the immune system against cancer (the "Technology")."

-ON PAGE 1, PARAGRAPH 2, 10TH LINE:

"by the Company, within 12 (twelve) months from the date of this Letter
Agreement; it being"

is deleted and replaced by the following:

"by the Company, within 30 (thirty) months from the date of this Letter
Agreement; it being"

-ON PAGE 1, PARAGRAPH 3., 3RD LINE:

"parties, (ii) six months from the completion of the second Trial, if the
parties agree to only"

is deleted and replaced by the following:

"PARTIES, (ii) TWELVE MONTHS FROM THE COMPLETION OF THE SECOND TRIAL, IF THE
PARTIES AGREE TO ONLY"

<PAGE>   2

IN WITNESS WHEREOF, the parties have executed this Amendment Agreement by their
duly authorized representatives in duplicate as of the day and year below
written.

ANTIGENICS, L.L.C.                      SIGMA-TAU INDUSTRIE
                                        FARMACEUTICHE
                                        RUINITE S.p.A.

By:  /s/ Garo Armen                     By:  /s/ Emilio Plate
     --------------------------------        --------------------------------

Name/Title:  Garo Armen                 Name/Title:  Emilio Plate - Chief
           --------------------------              --------------------------
                                                            Operating Officer

Date:  10/14/99                         Date:  20-10-99
     --------------------------------        --------------------------------

Source: [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00000-of-00352.parquet"}, [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00000-of-00352.parquet"}]]