TANOX INC
S-1/A, 2000-03-27
MEDICAL LABORATORIES
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     AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON MARCH 27, 2000
                                                      REGISTRATION NO. 333-96025

================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549
                      ------------------------------------

                                 AMENDMENT NO. 3
                                       TO

                                    FORM S-1

                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933
                      ------------------------------------

                                  TANOX, INC.
             (Exact name of registrant as specified in its charter)

<TABLE>
<S>                                    <C>                                     <C>
              DELAWARE                                  2836                                 76-0196733
   (State or Other Jurisdiction of          (Primary Standard Industrial                  (I.R.S. Employer
   Incorporation or Organization)           Classification Code Number)                 Identification No.)
</TABLE>

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

                          10301 STELLA LINK, SUITE 110
                           HOUSTON, TEXAS 77025-5497
                                 (713) 664-2288
              (Address, including zip code, and telephone number,
       including area code, of registrant's principal executive offices)

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

                             NANCY T. CHANG, PH.D.
                     PRESIDENT AND CHIEF EXECUTIVE OFFICER
                                  TANOX, INC.
                          10301 STELLA LINK, SUITE 110
                           HOUSTON, TEXAS 77025-5497
                                 (713) 664-2288
           (Name, address, including zip code, and telephone number,
            including area code, of registrant's agent for service)

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

                                   COPIES TO:

<TABLE>
<S>                                    <C>
   WILBURN O. MCDONALD, JR., ESQ.                        RODD M. SCHREIBER, ESQ.
       CHAMBERLAIN, HRDLICKA,                         SKADDEN, ARPS, SLATE, MEAGHER
      WHITE, WILLIAMS & MARTIN                              & FLOM (ILLINOIS)
    1200 SMITH STREET, SUITE 1400                         333 WEST WACKER DRIVE
      HOUSTON, TEXAS 77002-4310                          CHICAGO, ILLINOIS 60606
</TABLE>

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

     APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC:  As soon
as practicable after this Registration Statement becomes effective.

     If any of the securities being registered on this Form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act,
check the following box. [ ]

     If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, check the following box and
list the Securities Act registration statement number of the earlier effective
registration statement for the same offering. [ ]

     If this Form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. [ ]

     If this Form is a post-effective amendment filed pursuant to Rule 462(d)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. [ ]

     If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box. [ ]

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

                        CALCULATION OF REGISTRATION FEE
<TABLE>
<CAPTION>
- ------------------------------------------------------------------------------------------------------------------------
                                                                         PROPOSED
                   TITLE OF EACH CLASS OF                                MAXIMUM                        AMOUNT OF
                      SECURITIES TO BE                                  AGGREGATE                      REGISTRATION
                         REGISTERED                                 OFFERING PRICE(1)                     FEE(2)
- ------------------------------------------------------------------------------------------------------------------------
<S>                                                           <C>                             <C>
Common Stock, $.01 par value per share......................           $241,500,000                      $63,756
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
(1)  Estimated solely for the purpose of calculating the amount of the
     Registration Fee in accordance with Rule 457(o) of the Securities Act of
     1933, as amended.

(2)  The company has previously paid $33,264 of the registration fee.

     THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL
FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(A) OF
THE SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(A),
MAY DETERMINE.

================================================================================

<PAGE>

                   SUBJECT TO COMPLETION, DATED MARCH 27, 2000

THE INFORMATION CONTAINED IN THIS PROSPECTUS IS NOT COMPLETE AND MAY BE CHANGED.
THESE SECURITIES MAY NOT BE SOLD UNTIL THE REGISTRATION STATEMENT FILED WITH THE
SECURITIES AND EXCHANGE COMMISSION IS EFFECTIVE. THIS PROSPECTUS IS NOT AN OFFER
TO SELL THESE SECURITIES AND IT IS NOT SOLICITING AN OFFER TO BUY THESE
SECURITIES IN ANY STATE WHERE THE OFFER OR SALE IS NOT PERMITTED.

                                7,000,000 SHARES

                                   TANOX, INC.

                                  COMMON STOCK
                               $       PER SHARE

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

This is an initial public offering of common stock of Tanox, Inc.

Tanox expects that the price to the public in the offering will be between
$27.00 and $30.00 per share. The market price of the shares after the offering
may be higher or lower than the offering price.

We have applied to include the common stock on the Nasdaq National Market under
the symbol "TNOX."

INVESTING IN THE COMMON STOCK INVOLVES RISKS. SEE "RISK FACTORS" BEGINNING ON
PAGE 8.

                                        PER SHARE       TOTAL
                                        ----------    ----------
Price to the public..................   $             $
Underwriting discount................
Proceeds to Tanox....................


Tanox has granted an over-allotment option to the underwriters. Under this
option, the underwriters may elect to purchase a maximum of 1,050,000 additional
shares from Tanox within 30 days following the date of this prospectus to cover
over-allotments.

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

NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES
COMMISSION HAS APPROVED OR DISAPPROVED OF THESE SECURITIES OR DETERMINED IF THIS
PROSPECTUS IS TRUTHFUL OR COMPLETE. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.

CIBC WORLD MARKETS
                ROBERTSON STEPHENS
                                     WARBURG DILLON READ LLC
                                                    ADAMS, HARKNESS & HILL, INC.
                                                                  KBC SECURITIES

               The date of this prospectus is            , 2000.
<PAGE>
                 (This page has been intentionally left blank)

<PAGE>
                                TABLE OF CONTENTS

                                                                           PAGE
                                                                           ----
Prospectus Summary ......................................................    4
Risk Factors ............................................................    8
Forward-Looking Statements ..............................................   19
Use of Proceeds .........................................................   20
Dividend Policy .........................................................   20
Capitalization ..........................................................   21
Dilution ................................................................   22
Selected Consolidated Financial Data ....................................   23
Management's Discussion and Analysis of Financial Condition and
 Results of Operations ..................................................   24
Business ................................................................   29
Management ..............................................................   47
Principal Stockholders ..................................................   56
Certain Transactions ....................................................   58
Description of Capital Stock ............................................   59
Shares Eligible for Future Sale .........................................   62
Underwriting ............................................................   64
Legal Matters ...........................................................   66
Experts .................................................................   66
Where You Can Find More Information .....................................   66
Index to Consolidated Financial Statements ..............................   F-1

                                       3
<PAGE>
                               PROSPECTUS SUMMARY

THIS SUMMARY HIGHLIGHTS INFORMATION CONTAINED IN OTHER PARTS OF THIS PROSPECTUS.
BECAUSE IT IS A SUMMARY, IT DOES NOT CONTAIN ALL OF THE INFORMATION THAT YOU
SHOULD CONSIDER BEFORE INVESTING IN THE SHARES. YOU SHOULD READ THE ENTIRE
PROSPECTUS CAREFULLY.

                                  THE COMPANY

                                    OVERVIEW

Tanox identifies and develops therapeutic monoclonal antibodies to address
significant unmet medical needs in the areas of immunology, infectious diseases
and cancer. Monoclonal antibodies are genetically engineered antibodies that
target a specific foreign substance, or antigen. E25, our most advanced product
in development, is an anti-immunoglobulin E, or anti-IgE, antibody. We are
developing E25 in collaboration with Novartis Pharma AG and Genentech, Inc. E25
has successfully completed Phase III clinical trials in both allergic asthma and
seasonal allergic rhinitis (hay fever). Based on the results of these trials,
our collaboration partners intend to file for marketing approval in the United
States and Europe in mid-2000. In addition, we are developing a number of
monoclonal antibodies to treat other allergic diseases or conditions, such as
severe allergic reactions to peanuts, autoimmune diseases, HIV and to restore
the suppressed immune systems of chemotherapy patients.

                        MONOCLONAL ANTIBODY THERAPEUTICS

Monoclonal antibodies represent an exciting area of novel therapeutic
development. Because of advances in antibody technologies, scientists are now
able to develop antibody products that can be administered to patients on a
chronic basis with reduced concern for adverse responses by the human immune
system. Companies can also manufacture these antibody products more cost-
effectively. As a result, a large number of monoclonal antibodies are now in
clinical and preclinical development. According to an industry survey, 74 out of
350, or 21% of all, biotechnology medicines in clinical trials in 1998 were
antibodies. The FDA has approved eight therapeutic antibodies, six of them in
the last three years. In 1999, total sales of these products exceeded $1.3
billion.

                          OUR PRODUCTS IN DEVELOPMENT

E25.  In 1987, we discovered a novel approach for treating allergies and asthma
by using monoclonal antibodies to inhibit IgE. E25, a product based on this
discovery, is a humanized (human-like) anti-IgE monoclonal antibody in
development for allergic asthma and allergic rhinitis. We estimate that, in the
United States, allergic asthma afflicts approximately 11 million people, and
allergic rhinitis afflicts approximately 40 million people, of whom
approximately 32 million are seasonal sufferers. A pivotal Phase III clinical
trial and a pivotal Phase IIb clinical trial have demonstrated E25's ability to
prevent or reduce symptoms of seasonal allergic rhinitis. Two Phase III clinical
trials in allergic asthma have demonstrated E25's ability to reduce symptoms
related to asthma. Clinicians who participated in the studies presented the
results of those trials at the American Academy of Allergy Asthma and Immunology
in March 2000. As mentioned above, Novartis and Genentech intend to file for
marketing approval for both indications in the United States and Europe in
mid-2000.

OTHER PRODUCT CANDIDATES.  Using our comprehensive understanding of the human
immune system, we are building a diverse pipeline of monoclonal antibody product
candidates. In addition to E25, we have two other products in clinical
development and we are evaluating several product candidates in preclinical and
research studies.

  o    HU-901 is a humanized anti-IgE monoclonal antibody similar to E25 that is
       in a Phase I/II trial to test its effectiveness in reducing severe
       allergic reactions to peanuts. According to a recently published survey,
       peanut or tree nut (e.g., walnut, almond and cashew) allergy affects
       about 3 million people in the United States. If our clinical trial
       indicates that Hu-901 reduces sensitivity

                                       4
<PAGE>
       to peanuts, we may also investigate its benefit to patients with other
       food allergies. Novartis and Genentech are currently disputing our right
       to independently develop this product.

  o    5D12 is an anti-CD40 monoclonal antibody that we are developing to treat
       autoimmune diseases. We are currently conducting a Phase I/II trial in
       patients with Crohn's disease. We expect the results of this trial to
       play an important role in determining clinical indications that we intend
       to pursue with this product. We have exclusive rights to 5D12 in Europe
       and Japan under a license from Chiron Corporation. We believe potential
       autoimmune disease indications, such as Crohn's disease, rheumatoid
       arthritis, multiple sclerosis and psoriasis, represent significant market
       opportunities in Europe and Japan.

  o    5A8 is an anti-CD4 antibody that is in preclinical development for
       treating HIV.

  o    166-32 is a complement factor D inhibiting antibody in research for
       treating acute inflammation.

  o    163-93 is an anti-G-CSF receptor activating antibody in research for
       treating neutropenia, or suppression of the immune system caused by
       depletion of white blood cells during chemotherapy.

                                  OUR STRATEGY

Our objective is to leverage our expertise in monoclonal antibodies and our
understanding of the human immune system to advance our product pipeline and
become a profitable biopharmaceutical company. We intend to accomplish this
through the following strategic initiatives:

  o    continuing to identify and develop novel monoclonal antibodies using our
       demonstrated expertise in immunology and monoclonal antibody technology;

  o    maximizing the market opportunity for anti-IgE antibodies by exploring
       indications beyond allergic asthma and seasonal allergic rhinitis;

  o    expanding our product pipeline through attractive acquisition and
       in-licensing opportunities;

  o    forming strategic collaborations to complement our research and
       development resources and enhance the value of our product development
       programs; and

  o    capturing additional value from our pipeline by retaining marketing
       rights for products that we can effectively sell using a small, targeted
       sales force.

                               OTHER INFORMATION

We were incorporated in Texas in March 1986 and we reincorporated in Delaware in
January 2000. Our corporate headquarters, manufacturing facility and principal
research laboratories are located at 10301 Stella Link, Houston, Texas
77025-5497 and our telephone number is 713-664-2288. Tanox and our logo are our
registered service marks.

                                       5

<PAGE>

                                  THE OFFERING

<TABLE>
<S>                                    <C>
Common stock offered.................  7,000,000 shares

Common stock to be outstanding after   40,788,731 shares
  the offering.......................

Use of proceeds......................  For research and development activities, for capital
                                       expenditures, to finance possible acquisitions and
                                       investments in technology, products or businesses and
                                       for working capital and other general corporate
                                       purposes.

Proposed Nasdaq National Market        TNOX
  symbol.............................
</TABLE>

The share amounts in the table above are based on the number of shares
outstanding at March 7, 2000 and excludes:

  o    2,747,733 shares of common stock issuable on exercise of outstanding
       options at a weighted average exercise price of $5.06 per share; and

  o    8,200,480 shares of common stock reserved for issuance pursuant to future
       grants under our stock option plans.

Unless otherwise stated, all information contained in this prospectus assumes no
exercise of the underwriters' over-allotment option. On February 1, 2000, we
effected a 1.6 for 1 stock split by paying a stock dividend. All common share
numbers in this prospectus reflect the stock split.

                                       6
<PAGE>
                   SUMMARY CONSOLIDATED FINANCIAL INFORMATION
                     (in thousands, except per share data)


                                            YEAR ENDED DECEMBER 31,
                                       ---------------------------------
                                         1997        1998        1999
                                       ---------  ----------  ----------

STATEMENT OF OPERATIONS DATA:

Revenues.............................  $   8,939  $    2,422  $    1,405

Research and development.............      6,926      11,933      17,163

General and administrative...........      2,230       3,431       8,582
                                       ---------  ----------  ----------

Total operating costs and expenses...      9,156      15,364      25,745
                                       ---------  ----------  ----------

Income (loss) from operations........       (217)    (12,942)    (24,340)

Other income, net....................      1,045       1,240       1,028
                                       ---------  ----------  ----------

Income (loss) before income taxes....        828     (11,702)    (23,312)

(Provision) benefit of income
  taxes..............................       (198)      1,533         (34)
                                       ---------  ----------  ----------

Net income (loss)....................  $     630  $  (10,169) $  (23,346)
                                       =========  ==========  ==========

Earnings (loss) per share:

     Basic...........................  $    0.02  $    (0.35) $    (0.75)
                                       =========  ==========  ==========

     Diluted.........................  $    0.02  $    (0.35) $    (0.75)
                                       =========  ==========  ==========

Shares used in computing earnings
  (loss) per share:

     Basic...........................     27,909      29,105      31,113

     Diluted.........................     31,190      29,105      31,113



                                           DECEMBER 31, 1999
                                       --------------------------
                                        ACTUAL      AS ADJUSTED
                                       ---------   --------------

BALANCE SHEET DATA:

Cash, cash equivalents and short-term
  investments........................  $  47,254      $231,704
Working capital......................     42,718       227,168
Total assets.........................     55,328       239,778
Long term debt.......................     10,000        10,000

Retained earnings (deficit)..........    (30,461)      (30,461)

Total stockholders' equity...........     40,007       224,457


The as adjusted balance sheet data in the table above reflects the sale of
7,000,000 shares of common stock in this offering at an assumed public offering
price of $28.50 per share, after deducting estimated underwriters' discounts and
commissions and offering expenses.

                                       7
<PAGE>
                                  RISK FACTORS

YOU SHOULD CAREFULLY CONSIDER THE FOLLOWING FACTORS AND OTHER INFORMATION IN
THIS PROSPECTUS BEFORE DECIDING TO INVEST IN OUR COMMON STOCK.

WE HAVE A HISTORY OF NET LOSSES; WE EXPECT TO CONTINUE TO INCUR NET LOSSES AND
WE MAY NEVER ACHIEVE OR MAINTAIN PROFITABILITY.
We have incurred net losses since our inception. As of December 31, 1999, we had
an accumulated deficit of approximately $30.5 million, including a net loss of
approximately $23.3 million for the year ended December 31, 1999. Our losses
have primarily been the result of costs incurred in our research and development
programs and from our general and administrative costs.

We have not earned any revenues from commercial sales of any of our therapeutic
products, and we do not expect sales to commence until at least 2001, if at all.
We have funded our operations principally from licensing fees and milestone
payments under our current or former collaborations and private placements of
our common stock. We expect to continue to incur substantial operating losses
for the foreseeable future, particularly as we increase our research and
development, manufacturing, clinical trial and administrative activities. We
expect that losses will continue until such time, if ever, that we are able to
generate sufficient revenue from milestone payments and royalties on our lead
product candidate, E25, to cover our expenses.

Our ability to achieve and maintain long term profitability depends to a
significant extent on obtaining regulatory approval for and successfully
commercializing E25, and also on successfully completing preclinical and
clinical trials, obtaining required regulatory approvals and successfully
developing, manufacturing and marketing our other current and future product
candidates. We cannot assure you that we will be able to achieve any of the
foregoing or that we will be profitable even if we successfully commercialize
our products.

FAILURE TO COMMERCIALIZE OUR LEAD PRODUCT CANDIDATE, E25, WOULD HAVE A
SIGNIFICANT AND ADVERSE EFFECT ON US AND OUR STOCK PRICE.

E25 is our lead product candidate. Our success will depend, to a great degree,
on the success of E25. In order to successfully commercialize E25, our
collaborators, Novartis and Genentech, must be able to, among other things,
obtain regulatory approvals for and achieve market acceptance of E25.

Neither the U.S. Food and Drug Administration, or FDA, nor any European
regulatory agency, has approved E25. Novartis and Genentech intend to file a
biologics license application, or BLA, for E25 in the United States and apply
for registration of E25 in Europe by mid-2000. We cannot assure you that the FDA
or other regulatory authorities will accept these filings, that E25 will be
approved in a timely manner or that it will be approved at all.

Even if the FDA approves E25, we cannot be certain that physicians, patients,
insurers or other third-party payors will accept E25 as a treatment for its
approved indications in the United States or in any foreign markets. A number of
factors may affect the rate and level of E25's market acceptance including:

  o    regulatory developments related to manufacturing or using E25;

  o    E25's price relative to other products or competing treatments;

  o    the effectiveness of Novartis' and Genentech's sales and marketing
       efforts;

  o    the perception by physicians and other members of the healthcare
       community of E25's safety, efficacy and benefits compared to those of
       competing products or therapies;

  o    the willingness of physicians to adopt a new allergy treatment regimen;

  o    the availability of third-party reimbursement; and

  o    unfavorable publicity concerning E25 or comparable products or therapies.

If our collaborators fail to successfully obtain regulatory approval for and
commercialize E25, our business, financial condition and results of operations
will be materially harmed. Moreover, since E25 is our most advanced product
candidate, a setback of this nature would cause a sharp drop in our stock price.
Failure of our E25 program could also reflect adversely on our Hu-901

                                       8
<PAGE>
program, which is also based on anti-IgE technology.

WE DEPEND ON NOVARTIS AND GENENTECH TO DEVELOP, OBTAIN REGULATORY APPROVAL FOR,
MANUFACTURE, MARKET AND DISTRIBUTE E25 AND OTHER ANTI-IGE PRODUCTS. WE WILL ALSO
DEPEND ON FUTURE COLLABORATION PARTNERS TO DEVELOP, MANUFACTURE, MARKET AND
DISTRIBUTE OUR OTHER PRODUCTS. FAILURE BY NOVARTIS OR GENENTECH, OR ANY OTHER
COLLABORATION PARTNER, TO PERFORM UNDER OUR CURRENT OR FUTURE COLLABORATION
AGREEMENTS MAY DELAY OR SIGNIFICANTLY IMPAIR OUR ABILITY TO GENERATE REVENUES OR
OTHERWISE ADVERSELY AFFECT OUR PROFITABILITY.

Under the terms of our collaboration agreements, Novartis and Genentech are
responsible for developing, obtaining regulatory approval for, manufacturing,
marketing and distributing E25 and other anti-IgE products that the
collaboration may select for development. We expect for the extended future that
Novartis and Genentech will manufacture, market and distribute E25 and other
selected anti-IgE products, if the FDA approves any of these products. We also
rely on Novartis and Genentech for significant financial and technical
contributions to develop products covered by our collaboration agreements. Our
ability to profit from these products depends on Novartis' and Genentech's
performance under their agreements with us. We cannot control the amount and
timing of resources Novartis and Genentech will devote to any of our products.
If Novartis or Genentech experiences manufacturing or distribution difficulties,
does not actively market E25 or other selected anti-IgE products or does not
otherwise perform under our collaboration agreements, our potential for revenue
from those products will be dramatically reduced. Novartis and Genentech may
terminate our collaboration agreements on short notice. If Novartis or Genentech
terminates our collaboration, we would experience increased capital requirements
to undertake development and marketing at our expense, and we cannot assure you
that we would be able to develop E25 or our other anti-IgE products on our own.

We will also rely on other collaboration partners to develop, manufacture,
commercialize, market and distribute our other products. Many of our competitors
are similarly seeking to develop or expand their collaboration and license
arrangements with pharmaceutical companies. The success of these efforts by our
competitors could have an adverse impact on our ability to form future
collaboration arrangements. We cannot assure you that we will be able to
negotiate acceptable collaboration agreements in the future or that efforts
under any current or future collaboration agreements will succeed. To the extent
that we choose not to or are unable to enter into future collaboration
agreements or maintain existing collaborations, we would experience increased
capital requirements to undertake research, development and marketing at our own
expense. In addition, we may encounter significant delays in introducing our
product candidates or find that the absence of these collaboration agreements
adversely affects our ability to develop, manufacture or sell our product
candidates.

Our reliance on collaboration partners poses the following additional risks:

  o    disputes with our partners may arise, delaying or terminating our product
       candidates' research, development or commercialization or resulting in
       significant litigation or arbitration;

  o    contracts with our partners may fail to provide significant protection or
       may become unenforceable if one of these partners fails to perform;

  o    our partners could develop, independently or with third parties, products
       or treatments that compete with our products;

  o    our partners may not commit enough capital or other resources to
       successfully develop our products;

  o    our partners may not continue to develop and commercialize products
       resulting from our collaborations;

  o    our partners with marketing and distribution rights to one or more of our
       products may not commit enough resources to marketing and distributing
       our products;

  o    our contracts with collaboration partners may expire or terminate and we
       may not be able to replace them; and

  o    the terms of our contracts with our partners may not be favorable to us
       in the future.

                                       9
<PAGE>

If any of these risks occur, our product development and productivity may
suffer, and our business, financial condition and results of operations would be
materially harmed.

WE ARE INVOLVED IN LITIGATION AND ARBITRATION PROCEEDINGS WITH NOVARTIS AND
GENENTECH THAT MAY BE VERY COSTLY TO US AND COULD CAUSE US TO LOSE OUR RIGHTS TO
INDEPENDENTLY DEVELOP PRODUCTS.

We are a party to arbitrations and a related federal district court lawsuit with
our collaboration partners Novartis and Genentech, relating to our rights to
develop Hu-901 and other anti-IgE antibodies independently of our collaboration
with Novartis and Genentech. Novartis and Genentech are disputing our right to
pursue development of Hu-901 independently and are claiming that we are using
their unspecified confidential and proprietary information that we have no right
to use.

If we do not succeed in these proceedings, we could incur substantial damages
that would harm our financial position and we could lose our rights to
independently develop products covered by the collaboration, including Hu-901.
Even if we succeed, we may not be able to secure any recovery or develop
anti-IgE products independently of the collaboration. In either case, we expect
these proceedings to consume substantial amounts of our financial and managerial
resources. Further, because of the substantial amount of discovery required in
connection with this type of litigation, there is a risk that disclosure might
compromise some of our confidential information.

NONE OF OUR PRODUCTS HAVE RECEIVED REGULATORY APPROVAL. IF WE DO NOT RECEIVE AND
MAINTAIN REGULATORY APPROVALS, WE WILL NOT BE ABLE TO MARKET OUR PRODUCTS.

Neither the FDA, nor any regulatory authority, has approved any of our products,
including E25. We cannot assure you that the data collected from our completed
or future clinical trials will support any regulatory approvals. We must receive
FDA approval to manufacture and market our products in the United States. Other
countries have similar requirements.

The process that pharmaceutical products must undergo to receive this approval
is extensive, and includes preclinical testing and clinical trials to
demonstrate safety and efficacy and a review of the manufacturing process to
ensure compliance with good manufacturing practices. This process can last many
years, be very costly and still be unsuccessful. The FDA can delay, limit or not
grant approval for many reasons, including:

  o    a product candidate may not be safe or effective;

  o    FDA officials may interpret data from preclinical testing and clinical
       trials in different ways than we interpret it;

  o    the FDA might not approve our manufacturing processes or facilities or
       the processes or facilities of our collaboration partners;

  o    the FDA may change its approval policies or adopt new regulations; and

  o    the FDA may approve a product candidate for fewer than all the
       indications requested.

The process of obtaining approvals in foreign countries is subject to delay and
failure for the same reasons. Any delay in or failure to receive approval for
E25 or any of our current or future product candidates could materially harm our
business, financial condition and results of operations.

Our products other than E25 require significant additional laboratory
development and/or clinical testing and investment prior to commercialization.
We cannot assure you that our product candidates will prove to be safe and
effective in clinical trials. Further, any of our products may prove to have
undesirable and unintended side effects. We may not successfully develop
products currently in research and development, successfully market our products
or produce our products in commercial quantities at an acceptable cost or in
compliance with applicable regulatory guidelines. Our products may not meet
applicable regulatory standards or obtain required regulatory approvals.

Approval of a product candidate could also depend on post-marketing studies. In
addition, any marketed product and its manufacturer continue to be subject to
strict regulation after approval. Any unforeseen problems with an approved
product or any violation of regulations could result in restrictions on the
product, including its withdrawal from the market. Delays in receiving or
failing to

                                       10
<PAGE>

receive regulatory approvals, or losing previously received approvals, would
delay or prevent product commercialization, which would adversely affect our
business, financial condition and results of operations.

WE ARE INVOLVED IN LEGAL PROCEEDINGS WITH OUR FORMER ATTORNEYS THAT MAY BE VERY
COSTLY TO US.

We have been involved in an arbitration regarding a fee dispute with our former
attorneys who represented us in our 1993 lawsuit against Genentech and F.
Hoffman-La Roche, Ltd. and its affiliates, and a 1994 lawsuit filed against us
by Genentech. The arbitration panel issued an award which entitled those
attorneys to receive approximately $3.5 million, including interest, payments
ranging from 33 1/3% to 40% of any future milestone payments received by us from
Genentech following product approval and 10% of the royalties that we receive on
sales of anti-IgE products. We sought a court order vacating the arbitration
award. However, a judgment was entered confirming the award. We intend to pursue
all available remedies, including appealing the decision.

We may not be successful in this proceeding. If this proceeding continues to
result in decisions unfavorable to us, we could lose substantial value from our
collaboration with Novartis and Genentech which could negatively affect our
stock price and harm our business, financial condition and results of
operations. Whether or not we are successful in this proceeding, we expect it to
consume substantial amounts of our financial and managerial resources.

OUR PRECLINICAL AND CLINICAL TESTING RESULTS ARE UNCERTAIN. IF TRIAL RESULTS ARE
NEGATIVE, WE MAY BE FORCED TO STOP DEVELOPING PRODUCTS IMPORTANT TO OUR FUTURE.

We must demonstrate through preclinical studies and clinical trials that our
products are safe and effective for use in each target indication before we can
obtain regulatory approvals to sell our products commercially. These studies and
trials may be very costly and time consuming. The results of preclinical studies
and initial clinical trials of our products do not necessarily predict the
results from later-stage clinical trials. Drugs in later stages of clinical
trials may fail to show the desired safety and efficacy traits despite having
progressed through initial clinical testing. We cannot assure you that the data
collected from clinical trials of our products will be sufficient to support FDA
or other regulatory approval.

The speed with which we are able to enroll patients in clinical trials is an
important factor in determining how quickly we may complete clinical trials.
Many factors affect patient enrollment, including the size of the patient
population, the proximity of patients to clinical sites and the eligibility
criteria for the study. We may target our clinical trial protocols at
indications that have small patient populations, which may make it difficult for
us to enroll enough patients to complete the trials. Delays in patient
enrollment in the trials may result in increased costs, program delays, or both,
which could slow down our product development and approval process, and could
materially harm our business.

Administering any product we develop to humans may produce undesirable side
effects. These side effects could interrupt or delay clinical trials of products
and could result in the FDA or other regulatory authorities denying approval of
our products for any or all targeted indications. The FDA, other regulatory
authorities or we may suspend or terminate clinical trials at any time. Even if
we receive FDA and other regulatory approvals, our products may later exhibit
adverse effects that limit or prevent their widespread use or that force us to
withdraw those products from the market. We cannot assure you that any of our
products will be safe for human use.

WE HAVE LIMITED EXPERIENCE AND CAPABILITY IN MANUFACTURING AND MAY ENCOUNTER
MANUFACTURING PROBLEMS OR DELAYS THAT COULD RESULT IN LOST REVENUE.

To commercialize our products successfully, we and our collaboration partners
must manufacture our products in commercial quantities in compliance with
regulatory requirements and at an acceptable cost. If the manufacturing
facilities used to produce our products cannot pass a pre-approval or periodic
plant inspection, the FDA may not approve our products or it may delay or bar
their sale. Although we expect Novartis and Genentech to manufacture E25 and
other anti-IgE products that our collaboration develops, if the FDA and

                                       11
<PAGE>

other regulatory authorities approve these products, we have reserved the right
to manufacture up to 50% of the worldwide requirements for these products. We
currently have a process development and manufacturing facility for biological
products located in Houston, Texas. However, we have no experience in
manufacturing commercial quantities of antibodies and currently have limited
manufacturing capacity. In order to obtain regulatory approvals and to create
capacity to produce our products in sufficient quantities for commercial sale at
an acceptable cost, we will have to develop or acquire additional technology for
large scale manufacturing and build or otherwise obtain access to adequate
facilities, which will require substantial additional funds. We will also be
required to demonstrate to the FDA and corresponding foreign authorities our
ability to manufacture our products using controlled, reproducible processes. We
cannot assure you that we can develop the necessary manufacturing technology or
that we will be able to fund or build an adequate commercial manufacturing
facility necessary to obtain regulatory approvals and to produce adequate
commercial supplies of our potential products on a timely basis. We cannot
assure you that we, operating alone or with the assistance of others, will be
able to successfully make the transition to commercial production.

WE LACK SALES AND MARKETING EXPERIENCE, WHICH MAKES US DEPEND ON THIRD PARTIES
FOR THEIR EXPERTISE IN THIS AREA.

If we receive the required regulatory approvals, we expect to market and sell
our products principally through distribution, co-marketing, co-promotion or
licensing arrangements with third parties. Under our current collaboration
agreement, Novartis and Genentech have exclusive marketing rights to E25 and
other selected anti-IgE products. However, commercialization rights may revert
back to us if either we or our collaborators terminate our relationship. We
currently have no sales, marketing or distribution capabilities. Any revenues we
receive from our E25 collaboration will depend primarily on the efforts of our
collaboration partners. We intend to retain marketing rights in the United
States and selected Asian countries for products that we can develop and sell
effectively with a small, targeted sales force. If we elect to market products
directly, we would require significant additional expenditures and management
resources to develop an internal sales force. We cannot assure you that we would
be able to establish a successful sales force should we choose to do so.

WE FACE INTENSE COMPETITION AND RAPID TECHNOLOGICAL CHANGE THAT COULD RESULT IN
PRODUCTS THAT ARE SUPERIOR TO THE PRODUCTS WE ARE DEVELOPING.

The biotechnology and pharmaceutical industries are subject to rapid and
significant technological change. We have numerous competitors in the United
States and abroad, including, among others, major pharmaceutical and chemical
companies, specialized biotechnology firms, universities and other research
institutions. These competitors may develop technologies and products that are
more effective or less costly than any of our current or future products or that
could render our technologies and products obsolete or noncompetitive. Many of
these competitors have substantially more resources and product development,
production and marketing capabilities than we do. In addition, many of our
competitors have significantly greater experience than we do in undertaking
preclinical testing and clinical trials of new or improved pharmaceutical
products and obtaining FDA and other regulatory approvals of products for use in
health care. If we succeed in achieving significant commercial sales of our
products, we also will be competing in manufacturing efficiency and marketing
capability, areas in which we have limited or no experience. Furthermore, our
competitors may obtain FDA approval for products sooner and be more successful
in manufacturing and marketing their products than are we or our collaborators.

Products currently exist in the market that will compete directly with the
products that we seek to develop. Any product candidate that we develop and that
obtains regulatory approval must then compete for market acceptance and market
share. Our product candidates may not gain market acceptance among physicians,
patients, healthcare payors and the medical community. Significant factors in
determining whether we will be able to compete successfully include:

  o    efficacy and safety of our products;

                                       12
<PAGE>
  o    timing and scope of regulatory approval;

  o    product availability;

  o    potential advantages over alternative treatment methods;

  o    development, marketing, distribution and manufacturing capabilities and
       support of our collaborators;

  o    reimbursement coverage from insurance companies and others;

  o    price and cost-effectiveness of our products; and

  o    patent protection.

If our products are not competitive based on these or other factors, our
business, financial condition and results of operations will be materially
harmed.

WE DEPEND ON OUR PATENTS AND PROPRIETARY RIGHTS. THE VALIDITY, ENFORCEABILITY
AND COMMERCIAL VALUE OF THESE RIGHTS ARE HIGHLY UNCERTAIN.

Our success depends in part on obtaining, maintaining and enforcing patents,
licensing the rights to patents and patent applications owned by others,
maintaining trade secrets and operating without infringing on the proprietary
rights of third parties. While we file and prosecute patent applications to
protect our inventions, our pending patent applications may not result in the
issuance of valid patents and our issued patents may not provide competitive
advantages. Also, our patent protection may not prevent others from developing
competitive products using related technology. We cannot assure you that pending
patent applications licensed to us will result in patents being issued or that,
if issued, the patents will give us an advantage over competitors with similar
technology.

We own and have licenses to certain issued patents. The patents we own that are
most material to our business are five U.S. patents and six foreign patents
relating to anti-IgE antibodies. However, the patent position of biotechnology
and pharmaceutical firms is highly uncertain and involves many complex legal and
technical issues. There is no clear policy involving the breadth of claims
allowed or the degree of protection afforded under such patents. Issued patents
can be challenged in litigation in the courts and in proceedings in the patent
and trademark office in the United States and in courts and patent offices in
foreign countries. Issuance of a patent is not conclusive as to its validity,
enforceability or the scope of its claim. We cannot assure you that our patents
will not be successfully challenged as to enforceability, invalidated or limited
in the scope of their coverage. Moreover, litigation to uphold the validity of
patents and to prevent infringement can be very costly and can result in
diverting technical and management personnel's time and attention, which may
materially harm our business, financial condition and results of operations. If
the outcome of litigation is adverse to us, third parties may be able to use our
patented technology without paying us. Moreover, we cannot assure you that our
patents will not be infringed or successfully avoided through design innovation.
Any of these events may materially and adversely effect our business.

There may be patent rights belonging to others that require us to alter our
products, pay licensing fees or cease certain activities. If our products
conflict with patent rights of others, the owners of those patent rights could
bring legal actions against us claiming damages and seeking to stop us from
manufacturing and marketing the affected products. If these legal actions are
successful, in addition to any potential liability for damages, we could be
required to obtain a license in order to continue to manufacture or market the
affected products. We cannot assure you that we would prevail in any such action
or that any license required under any such patent would be made available on
acceptable terms or at all. Any of these events may materially harm our
business, financial condition and results of operations.

Researching, developing and commercializing a biopharmaceutical product often
involves alternative development and optimization routes that are presented at
various stages in the development process. We cannot predict the preferred
routes at the outset of a research and development program, because they will
depend on subsequent discoveries and test results. There are numerous
third-party patents in our field, and it is possible that, to pursue the
preferred development route of one or more of our products, we will need to
obtain a license to a patent, which would decrease the ultimate profitability of
the applicable

                                       13
<PAGE>
product. If we cannot negotiate a license, we might have to pursue a less
desirable development route or terminate the program altogether.

We are aware that other groups have claimed discoveries similar to those covered
by our patent applications. In addition, other companies, some of which may be
our competitors, have filed applications for or have been issued patents and may
obtain additional patents and proprietary rights relating to products or
processes used in, necessary to, competitive with or otherwise related to our
patents and products. These products and processes include, among other items,
patents covering technology relating to humanized monoclonal antibodies that we
anticipate developing. Protein Design Labs, Inc. owns certain patents and patent
applications relating to such humanized antibodies. We have recently taken a
non-exclusive license to these patents and patent applications for one of our
products. We do not know if we can obtain licenses from Protein Design Labs for
our other antibody products.

We must make substantial cash payments and achieve certain milestones and
satisfy certain conditions, including filing investigational new drug
applications, obtaining product approvals and introducing products, to maintain
our rights under certain of our licenses, including our licenses from Chiron and
Biogen, Inc. We cannot assure you that we will be able to maintain our rights
under these licenses. If any of these licenses terminate, we may be unable to
commercialize any related product.

In addition to the intellectual property rights described above, we also rely on
unpatented technology, trade secrets and confidential information. We cannot
assure you that others will not independently develop substantially equivalent
information and techniques or otherwise gain access to our technology or
disclose such technology, or that we can effectively protect our rights in
unpatented technology, trade secrets and confidential information. We require
each of our employees, consultants and advisors to execute a confidentiality
agreement at the commencement of an employment or consulting relationship with
us. We cannot assure you, however, that these agreements will provide effective
protection if an unauthorized use or disclosure of this confidential information
occurs.

WE MAY EXPERIENCE DIFFICULTIES IN MANAGING GROWTH, WHICH COULD MATERIALLY HARM
OUR BUSINESS, FINANCIAL CONDITION AND RESULTS OF OPERATIONS.

If our product development efforts and the product development efforts of our
collaborators succeed, our growth could strain our operations, product
development and other managerial and operating resources. Future growth will
impose significant added responsibilities on members of management, including
the need to identify, recruit, maintain and integrate additional employees,
including management. In the future, our financial performance and our ability
to compete effectively will depend, in part, on our ability to manage any future
growth effectively. To that end, we must be able to:

  o    manage our research and development efforts effectively;

  o    expand the capacity, scalability and performance of our product
       development infrastructure;

  o    develop our administrative, accounting and management information systems
       and controls;

  o    improve coordination among our research, accounting, finance, marketing
       and operations personnel; and

  o    hire and train additional qualified personnel.

We cannot assure you that we will be able to accomplish these tasks, and our
failure to accomplish any of these tasks could materially harm our business,
financial condition and results of operations.

FAILURE TO ATTRACT AND RETAIN KEY PERSONNEL AND PRINCIPAL MEMBERS OF OUR
SCIENTIFIC AND MANAGEMENT STAFF COULD MATERIALLY HARM OUR BUSINESS, FINANCIAL
CONDITION AND RESULTS OF OPERATIONS.

Our success depends greatly on our abilities to attract and retain qualified
scientific and technical personnel, as well as to retain the services of our
existing technical management staff. To expand our research and development
programs and pursue our product development plans, we will be required to hire
additional qualified scientific and technical personnel, as well as personnel
with

                                       14
<PAGE>

expertise in clinical testing and government regulation. There is intense
competition for qualified staff, and we cannot assure you that we will be able
to attract and retain the necessary qualified staff to develop our business. The
failure to attract and retain key scientific and technical personnel and
management staff or the loss of any of our current management team could
materially harm our business and financial condition. We do not maintain, and do
not currently intend to obtain, key employee global life insurance on any of our
personnel.

WE MAY NEED ADDITIONAL FINANCING, BUT OUR ACCESS TO CAPITAL FUNDING IS
UNCERTAIN.

Our current and anticipated development projects require substantial additional
capital. We expect that the net proceeds from this offering, together with our
existing assets and revenue from operations, will sufficiently fund our
operations for the next three years. However, our future capital needs will
depend on many factors, including successfully commercializing E25, receiving
milestone payments from our collaboration partners, and making progress in our
research and development activities. Our success may also depend on the
magnitude and scope of these activities, the progress and level of unreimbursed
costs associated with preclinical studies and clinical trials, the costs
associated with acquisitions, the costs of preparing, filing, prosecuting,
maintaining and enforcing patent claims and other intellectual property rights,
competing technological and market developments, changes in or terminations of
existing collaboration and licensing arrangements, the establishment of
additional collaboration and licensing arrangements, and the cost of
manufacturing scale-up and development of marketing activities, if undertaken by
us. We do not have committed external sources of funding and we cannot assure
you that we will be able to obtain additional funds on acceptable terms, if at
all. If adequate funds are not available, we may be required to:

  o    delay, reduce the scope of or eliminate one or more of our development
       programs;

  o    obtain funds through arrangements with collaboration partners or others
       that may require us to relinquish rights to technologies, product
       candidates or products that we would otherwise seek to develop or
       commercialize ourselves; or

  o    license rights to technologies, product candidates or products on terms
       that are less favorable to us than might otherwise be available.

If we raise additional funds by issuing additional stock, further dilution to
our stockholders may result, and new investors could have rights superior to
existing stockholders. If funding is insufficient at any time in the future, we
may be unable to develop or commercialize our products, take advantage of
business opportunities or respond to competitive pressures.

WE ARE SUBJECT TO THE UNCERTAINTY RELATED TO REIMBURSEMENT POLICIES AND
HEALTHCARE REFORM MEASURES.

In recent years, there have been numerous proposals to change the healthcare
system in the United States. Some of these proposals have included measures that
would limit or eliminate payments for medical procedures and treatments or
subject pharmaceutical product pricing to government control. In addition, as a
result of the trend towards managed healthcare in the United States, as well as
legislative proposals to reduce government insurance programs, third-party
payors are increasingly attempting to contain healthcare costs by limiting both
coverage and the level of reimbursement of new drug products. Consequently,
significant uncertainty exists as to the reimbursement status of newly-approved
healthcare products. If we or any of our collaborators succeed in bringing one
or more of our products to market, we cannot assure you that third-party payors
will establish and maintain price levels sufficient for us to realize an
appropriate return on our investment in product development. Significant changes
in the healthcare system in the United States or elsewhere, including changes
resulting from adverse trends in third-party reimbursement programs, could
materially reduce our profitability. Such changes could also significantly harm
our ability to raise the capital we would need to continue our operations.
Furthermore, if these proposals affect our collaborators, the proposals may harm
our ability to commercialize the products we develop jointly with them.

                                       15
<PAGE>

WE ARE EXPOSED TO PRODUCT LIABILITY CLAIMS, AND IT IS UNCERTAIN THAT WE CAN
OBTAIN INSURANCE AGAINST THESE CLAIMS AT A REASONABLE RATE IN THE FUTURE.

Our business exposes us to potential product liability risks, which are inherent
in testing, manufacturing, marketing and selling pharmaceutical products. We may
be held liable if any product we develop, or any product that uses or
incorporates any of our technologies, causes injury or is found otherwise
unsuitable during product testing, manufacturing, marketing or sale. We cannot
assure you that we will be able to avoid product liability exposure. Product
liability insurance for the biopharmaceutical industry is generally expensive,
if available at all. We have obtained product liability insurance coverage in
the amount of $5.0 million per occurrence, subject to a $5.0 million aggregate
limitation. However, we cannot assure you that our present insurance coverage is
now or will continue to be adequate. In addition, some of our license and
collaboration agreements require us to obtain product liability insurance.
Future license and collaboration agreements may also include such a requirement.
We cannot assure you that we can obtain adequate insurance coverage at a
reasonable cost in the future. Our inability to obtain sufficient insurance
coverage at an acceptable cost or otherwise to protect against potential product
liability claims could prevent or inhibit us or our collaborators from
commercializing our products. If we are sued for any injury caused by our
products, our liability could exceed our total assets.

WE DEAL WITH HAZARDOUS MATERIALS AND MUST COMPLY WITH ENVIRONMENTAL LAWS AND
REGULATIONS, WHICH CAN BE EXPENSIVE AND RESTRICT HOW WE DO BUSINESS.

Our research and development work and manufacturing processes involve the
controlled use of hazardous materials, including chemical, radioactive and
biological materials. Our operations also produce hazardous waste products. We
are subject to federal, state and local laws and regulations governing how we
use, manufacture, store, handle and dispose of these materials. Although we
believe that we comply in all material respects with applicable environmental
laws and regulations, we cannot assure you that we will not incur significant
costs to comply with environmental laws and regulations in the future. In
addition, current or future environmental laws and regulations may impair our
research, development or production efforts.

WE COULD BE LIABLE FOR DAMAGES, PENALTIES OR OTHER FORMS OF CENSURE IF WE ARE
INVOLVED IN A HAZARDOUS WASTE SPILL OR OTHER ACCIDENT.

Despite precautionary procedures that we implement for handling and disposing of
hazardous materials, we cannot eliminate the risk of accidental contamination or
discharge or any resultant injury from these materials. If a hazardous waste
spill or other accident occurs, we could be liable for damages, penalties or
other forms of censure. In addition, we may be sued for injury or contamination
that results from our use or the use by third parties of these materials, and
our liability could exceed our total assets.

OUR EXECUTIVE OFFICERS AND DIRECTORS OWN A LARGE PERCENTAGE OF OUR VOTING STOCK
AND WILL CONTINUE TO CONTROL OUR COMPANY AND THE OUTCOME OF MATTERS PUT TO A
VOTE OF STOCKHOLDERS AFTER THIS OFFERING.

Immediately after we complete this offering, our executive officers and
directors and their affiliates will, in the aggregate, own shares representing
approximately 36% of our outstanding common stock. As a result, these
stockholders, acting together, will significantly influence our general
management and affairs, and all matters submitted to our stockholders for
approval, including electing directors and approving changes in control. Such
control could discourage others from initiating potential merger, takeover or
other change of control transactions, and may adversely affect the market price
of our common stock. For a more detailed description of our management team and
their ownership of common stock, please refer to "Management" on page 47.

                                       16
<PAGE>
OUR AMENDED AND RESTATED CERTIFICATE OF INCORPORATION AND BYLAWS AND DELAWARE
LAW CONTAIN CERTAIN PROVISIONS THAT COULD DELAY OR PREVENT A TAKEOVER AND
SUPPRESS OUR STOCK PRICE.

Provisions of our amended and restated certificate of incorporation, bylaws and
Delaware law could delay, defer or prevent a third party from acquiring us,
despite the possible benefit to our stockholders, or otherwise adversely affect
the price of our common stock.

These provisions include:

  o    the ability of our board of directors to issue shares of preferred stock
       and to determine the price and other terms, including preferences and
       voting rights, of those shares without stockholder approval;

  o    a staggered board of directors;

  o    a limitation on who may call special meetings of stockholders; and

  o    advance notice requirements for nomination for election to the board of
       directors or for proposing matters that stockholders may act on at
       stockholder meetings.

In addition to these provisions, we are subject to certain Delaware laws,
including one that prohibits us from engaging in a business combination with any
interested stockholder for a period of three years from the date the person
became an interested stockholder unless certain conditions are met. We may also
adopt a shareholder rights plan or "poison pill" after this offering. All of
this may discourage potential takeover attempts, discourage bids for our common
stock at a premium over market price or adversely affect the market price of,
and the voting and other rights of the holders of, our common stock.

YOU MAY NOT BE ABLE TO TRADE OUR COMMON STOCK IF AN ACTIVE TRADING MARKET DOES
NOT DEVELOP.

Prior to this offering, there has been no public market for our common stock. We
cannot predict the extent to which investor interest in us will lead to an
active trading market in our common stock or how liquid that market might
become. We will determine the initial public offering price for our shares by
negotiating with representatives of the underwriters. This price may not
indicate prices that will prevail in any future trading market. You may not be
able to sell shares of our common stock at or above our initial public offering
price.

MARKET VOLATILITY MAY AFFECT OUR STOCK PRICE AND THE VALUE OF YOUR INVESTMENT
MAY BE SUBJECT TO SUDDEN DECREASES.

The trading price for our common stock is likely to be volatile. Prices for our
common stock will be determined in the marketplace and may be influenced by many
factors, including variations in our financial results, changes in earnings
estimates by industry research analysts, investors' perceptions of us and our
financial prospects, results of the governmental approval process for our
products, results of clinical trials, changes in government regulations,
developments in our relationships with our collaboration partners, developments
in our litigation, announcements of new products, technologies or treatments by
us or our competitors and general economic, industry and market conditions. In
addition, the stock markets from time to time have experienced extreme price and
volume fluctuations. In particular, the market prices of the securities of
biotechnology companies have been especially volatile, and often these
fluctuations do not relate to operating performance. These broad fluctuations
may adversely affect the trading price of our common stock, regardless of our
actual operating performance.

In the past, following periods of market volatility, security holders have
instituted class action litigation. If the market value of our stock experiences
adverse fluctuations and we become involved in this type of litigation, we could
incur substantial legal costs and management's attention could be diverted,
which could materially harm our business or the market price of our common
stock.

SALES OF COMMON STOCK MAY HAVE AN ADVERSE IMPACT ON THE MARKET PRICE OF OUR
COMMON STOCK.

Sales of significant amounts of our common stock after this offering or the
perception that such sales will occur could adversely affect the market price of
our common stock or our future ability to raise capital by selling equity
securities. After this

                                       17
<PAGE>

offering is completed, 40,788,731 shares of our common stock will be issued and
outstanding. All of the shares of common stock to be sold in this offering will
be freely tradable without restriction or further registration under the federal
securities laws unless purchased by our "affiliates" within the meaning of
Rule 144 under the Securities Act. The 33,788,731 remaining shares of
outstanding common stock will be "restricted securities" under the Securities
Act, subject to restrictions on the timing, manner and volume of sales of those
shares.

Our officers and directors and stockholders who together own 32,040,783 shares
of common stock have agreed for a period of 180 days after the date of this
prospectus not to sell or otherwise dispose of any shares of our common stock,
other than shares acquired in this offering. When the lock-up period expires,
the shares owned by these persons prior to completion of this offering may be
sold into the public market without a registration statement, to the extent
permitted by Rule 144 or exemptions under the Securities Act. Moreover,
following this offering, a substantial number of shares of common stock issuable
on exercise of outstanding options will be eligible for sale in the public
market.

For a more detailed description of additional shares that may be sold in the
future, please refer to "Shares Eligible for Future Sale" on page 62 and
"Underwriting" on page 64.

AS A NEW INVESTOR, YOU WILL INCUR IMMEDIATE AND SUBSTANTIAL DILUTION IN THE BOOK
VALUE OF YOUR SHARES.

The public offering price for our shares will be substantially higher than our
net tangible book value per share, which at December 31, 1999, was $1.20 per
share. If you purchase shares of our common stock in this offering, you will
suffer immediate, substantial net tangible book value dilution of $22.93 per
share, or $22.40 per share, if the underwriters exercise their over-allotment
option in full, assuming an initial public offering price of $28.50. You will
also incur additional dilution if the holders of outstanding options to purchase
common stock at prices below our net tangible book value per share after this
offering exercise their options. For a more detailed discussion of dilution,
please refer to "Dilution" on page 22.

OUR MANAGEMENT WILL HAVE BROAD DISCRETION TO ALLOCATE THE NET PROCEEDS OF THIS
OFFERING AND MAY NOT USE THE PROCEEDS APPROPRIATELY.

Our management will have broad discretion over how we use the proceeds of this
offering. We estimate the net proceeds from this offering to be approximately
$184.5 million, after deducting underwriting discounts and commissions and
estimated offering expenses, assuming an initial public offering price of
$28.50. We plan to use these proceeds to substantially increase clinical
development of our products, expand our research and development infrastructure
and research facilities in the United States, The Netherlands and Taiwan,
acquire or in-license additional technologies and products and market our
products. We have no specific allocations for any other net proceeds of this
offering. The amount of proceeds we will actually expend on general corporate
purposes will vary depending on a number of factors, including successfully
commercializing E25. Our progress in and scope of our research and development
activities, changes in or termination of existing collaboration and licensing
arrangements, costs and magnitude of product or technology acquisitions and our
need for manufacturing capacity. Consequently, management will retain a
significant amount of discretion in spending these proceeds. Because of the
number and variability of factors that will determine the use of these proceeds,
how we spend these proceeds may vary substantially from our current intentions.

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<PAGE>
                           FORWARD-LOOKING STATEMENTS

Some of the information in this prospectus contains forward-looking statements
within the meaning of the federal securities laws. You can find these statements
under "Prospectus Summary," "Risk Factors," "Use of Proceeds,"
"Management's Discussion and Analysis of Financial Condition and Results of
Operations," "Business" and elsewhere in this prospectus.

We typically identify forward-looking statements by using terms such as "may,"
"will," "should," "could," "expect," "plan," "anticipate,"
"believe," "estimate," "predict," "potential" or "continue" or similar
words, although we express some forward-looking statements differently. You
should be aware that actual events could differ materially from those suggested
in the forward-looking statements due to a number of factors, including:

  o    the ability to develop safe and efficacious drugs;

  o    failure to achieve positive results in clinical trials;

  o    failure to successfully commercialize our products;

  o    relationships with our collaboration partners;

  o    variability of royalty, license and other revenues;

  o    ability to enter into future collaboration agreements;

  o    competition and technological change; and

  o    existing and future regulations affecting our business.

You should also consider carefully the statements under "Risk Factors" and
other sections of this prospectus, which address additional factors that could
cause our actual results to differ from those set forth in the forward-looking
statements.

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<PAGE>
                                USE OF PROCEEDS

We estimate that our net proceeds from the sale of the shares of common stock in
this offering will be approximately $184,450,000. If the underwriters fully
exercise their over-allotment option, we estimate that our net proceeds from the
offering will be $212,280,250. "Net Proceeds" are what we expect to receive
after paying the underwriters' discounts and commissions and other expenses of
the offering. For the purpose of estimating net proceeds, we are assuming an
initial public offering price of $28.50 per share.

We intend to use the net proceeds of this offering primarily for research and
development, capital expenditures and general corporate purposes, including
working capital. We may use a portion of the proceeds to acquire or invest in
technologies, products or businesses that we believe may complement our
business. We currently have no agreements or commitments in this regard. The
amount of proceeds we will actually spend on general corporate purposes will
vary depending on a number of factors, including:

  o    successfully commercializing E25;

  o    our progress in and the scope of our research and development activities;

  o    changes in or termination of existing collaboration and licensing
       arrangements;

  o    costs and magnitude of product or technology acquisitions; and

  o    our need for manufacturing capacity;

Our management will have broad discretion over how we use the net proceeds of
this offering. Pending such uses, we intend to invest the net proceeds of this
offering in short-term, interest-bearing investment grade securities.

                                DIVIDEND POLICY

We have never declared or paid any cash dividends on our capital stock. We
anticipate that we will retain earnings to support operations and to finance our
business growth and development. Therefore, we do not expect to pay cash
dividends in the foreseeable future.

                                       20
<PAGE>

                                 CAPITALIZATION

The following table shows:

  o    our actual capitalization on December 31, 1999; and

  o    our as adjusted capitalization on December 31, 1999, assuming the
       completion of the offering as adjusted for the sale of 7,000,000 shares
       of common stock by us at an assumed public offering price of $28.50 per
       share, net of estimated underwriters' discounts and commissions and
       offering expenses.


                                           DECEMBER 31, 1999
                                        ------------------------
                                         ACTUAL     AS ADJUSTED
                                        --------    ------------
                                             (IN THOUSANDS)

Long term debt.......................   $ 10,000      $ 10,000
                                        --------    ------------
Stockholders' equity:
  Preferred stock, $.01 par value;
     10,000,000 shares authorized, no
     shares issued and outstanding;
     no shares issued or outstanding,
     as adjusted.....................      --           --
  Common stock, $.01 par value;
     120,000,000 shares authorized;
     33,324,402 shares issued and
     outstanding, actual; 40,324,402
     shares issued and outstanding,
     as adjusted.....................        333           403
  Additional paid-in capital.........     71,701       256,081
  Deferred compensation..............       (651)         (651)
  Loans receivable from employees....     (1,086)       (1,086)
  Other comprehensive income,
     cumulative translation
     adjustment......................        171           171
  Retained earnings (deficit)........    (30,461)      (30,461)
                                        --------    ------------
     Total stockholders' equity......     40,007       224,457
                                        --------    ------------
       Total capitalization..........   $ 50,007      $234,457
                                        ========    ============

The shares of common stock outstanding in the actual and as adjusted columns
exclude 3,183,920 shares of common stock issuable upon exercise of options
outstanding at December 31, 1999 at a weighted average exercise price of $4.39
per share.

                                       21
<PAGE>
                                    DILUTION

Our net tangible book value on December 31, 1999 was approximately $39,965,000,
or $1.20 per share. "Net tangible book value" is total assets minus the sum of
liabilities and intangible assets. "Net tangible book value per share" is net
tangible book value divided by the total number of shares outstanding.

After giving effect to adjustments relating to the offering, our pro forma net
tangible book value on December 31, 1999, would have been $224,415,000 or $5.57
per share. The adjustments made to determine pro forma net tangible book value
per share are the following:

  o    increasing total assets to reflect the estimated net proceeds of the
       offering as described under "Use of Proceeds" assuming that the public
       offering price will be $28.50 per share; and

  o    adding the number of shares offered by this prospectus to the number of
       shares outstanding.

The following table illustrates the pro forma increase in net tangible book
value of $4.37 per share and the dilution (the difference between the offering
price per share and net tangible book value per share) to new investors:


  Assumed public offering price per
     share...........................             $   28.50
  Net tangible book value per share
     as of December 31, 1999.........  $    1.20
  Increase in net tangible book value
     per share attributable to the
     offering........................       4.37
                                       ---------
  Pro forma net tangible book value
     per share as of December 31,
     1999, after giving effect to the
     offering........................                  5.57
                                                  ---------
  Dilution per share to new investors
     in the offering.................             $   22.93
                                                  =========


The following table shows the difference between existing stockholders as of
December 31, 1999 and new investors with respect to the number of shares
purchased from Tanox, the total consideration paid and the average price paid
per share. We assume that the public offering price will be $28.50 per share.

<TABLE>
<CAPTION>
                                          SHARES PURCHASED        TOTAL CONSIDERATION
                                       ----------------------   ------------------------     AVERAGE PRICE
                                          NUMBER      PERCENT       AMOUNT       PERCENT       PER SHARE
                                       ------------   -------   --------------   -------     -------------
Existing stockholders................    33,324,402     82.6%   $   61,924,000     23.7%        $  1.86
<S>                                    <C>            <C>       <C>              <C>         <C>
New investors........................     7,000,000     17.4       199,500,000     76.3           28.50
                                       ------------   -------   --------------   -------        -------
     Total...........................    40,324,402    100.0%      261,424,000    100.0%
                                       ============   =======   ==============   =======
</TABLE>

In the discussion and tables above, we assume no exercise of outstanding options
to purchase shares of our common stock. As of December 31, 1999, there were
outstanding options to purchase a total of 3,183,920 shares of common stock at a
weighted average exercise price of $4.39 per share. To the extent that option
holders exercise their outstanding options, new investors will be further
diluted.

                                       22
<PAGE>
                      SELECTED CONSOLIDATED FINANCIAL DATA

This section presents our selected historical financial data. You should read
carefully the consolidated financial statements included in this prospectus,
including the notes to the consolidated financial statements. We do not intend
the selected data in this section to replace the consolidated financial
statements.

We derived the statement of operations data for the years ended December 31,
1997, 1998 and 1999, and balance sheet data as of December 31, 1998 and 1999
from the audited consolidated financial statements in this prospectus, which
Arthur Andersen LLP, independent public accountants, audited. We derived the
statement of operations data for the years ended December 31, 1995 and 1996 and
the balance sheet data as of December 31, 1995, 1996 and 1997 from our audited
consolidated financial statements that we have not included in the prospectus.

<TABLE>
<CAPTION>
                                                       YEAR ENDED DECEMBER 31,
                                       -------------------------------------------------------
                                         1995       1996       1997        1998        1999
                                       ---------  ---------  ---------  ----------  ----------
<S>                                    <C>        <C>        <C>        <C>         <C>
                                                (IN THOUSANDS, EXCEPT PER SHARE DATA)
STATEMENT OF OPERATIONS DATA:
Revenues.............................  $   6,957  $  15,017  $   8,939  $    2,422  $    1,405
Research and development.............      5,499      5,626      6,926      11,933      17,163
General and administrative...........      1,707      1,165      2,230       3,431       8,582
                                       ---------  ---------  ---------  ----------  ----------
Total operating costs and expenses...      7,206      6,791      9,156      15,364      25,745
                                       ---------  ---------  ---------  ----------  ----------
Income (loss) from operations........       (249)     8,226       (217)    (12,942)    (24,340)
Other income, net....................        470        433      1,045       1,240       1,028
                                       ---------  ---------  ---------  ----------  ----------
Income (loss) before income taxes....        221      8,659        828     (11,702)    (23,312)
(Provision) benefit of income
  taxes..............................        (10)    (1,922)      (198)      1,533         (34)
                                       ---------  ---------  ---------  ----------  ----------
Net income (loss)....................  $     211  $   6,737  $     630  $  (10,169) $  (23,346)
                                       =========  =========  =========  ==========  ==========
Earnings (loss) per share:
     Basic...........................  $    0.01  $    0.26  $    0.02  $    (0.35) $    (0.75)
                                       =========  =========  =========  ==========  ==========
     Diluted.........................  $    0.01  $    0.23  $    0.02  $    (0.35) $    (0.75)
                                       =========  =========  =========  ==========  ==========
Shares used in computing earnings
  (loss) per share:
     Basic...........................     26,215     26,215     27,909      29,105      31,113
     Diluted.........................     29,418     29,382     31,190      29,105      31,113
</TABLE>

<TABLE>
<CAPTION>
                                                           DECEMBER 31,
                                       -----------------------------------------------------
                                         1995       1996       1997       1998       1999
                                       ---------  ---------  ---------  ---------  ---------
<S>                                    <C>        <C>        <C>        <C>        <C>
                                                          (IN THOUSANDS)
BALANCE SHEET DATA:
Cash, cash equivalents and short-term
  investments........................  $  15,443  $  18,235  $  36,857  $  33,735  $  47,254
Working capital......................     13,847     15,575     35,871     34,323     42,718
Total assets.........................     18,147     25,871     44,831     43,422     55,328
Long term debt.......................      7,000      7,000      9,000     10,000     10,000
Retained earnings (deficit)..........     (4,313)     2,424      3,054     (7,115)   (30,461)
Total stockholders' equity...........      8,470     15,811     34,428     31,540     40,007
</TABLE>

                                       23

<PAGE>
                    MANAGEMENT'S DISCUSSION AND ANALYSIS OF
                 FINANCIAL CONDITION AND RESULTS OF OPERATIONS

YOU SHOULD READ THIS DISCUSSION TOGETHER WITH THE FINANCIAL STATEMENTS AND OTHER
FINANCIAL INFORMATION INCLUDED IN THIS PROSPECTUS.

OVERVIEW

Tanox identifies and develops therapeutic monoclonal antibodies to address
significant unmet medical needs in the areas of immunology, infectious diseases
and cancer. E25, our most advanced product in development, is an anti-IgE
antibody we are developing in collaboration with Novartis and Genentech. E25 has
successfully completed Phase III clinical trials in both allergic asthma and
seasonal allergic rhinitis (hay fever). Based on the results of these trials, in
mid-2000, our collaboration partners intend to file for marketing approval in
the United States and Europe for both indications. In addition, we are
developing a number of monoclonal antibodies to treat other allergic diseases or
conditions, such as severe allergic reactions to peanuts, autoimmune diseases,
HIV and neutropenia.

We currently have no products available for sale and are focusing on product
development, clinical trials and process development. We have incurred
substantial losses since inception and incurred an accumulated deficit through
December 31, 1999, of $30.5 million. We expect to continue to incur substantial
operating losses for the foreseeable future, particularly as we increase our
research and development, manufacturing, clinical trial and administrative
activities. We expect that losses will continue until such time, if ever, that
we generate sufficient revenue from royalties on E25 to cover our expenses.

Historically, we have earned revenues primarily from license fees, milestone
payments and sponsored research under our collaboration agreements. In the
future, we expect our principal revenues will be milestone payments, royalties
and profit-sharing payments from Novartis and Genentech. We may also receive
royalties from Hoffman-La Roche Ltd. should it participate in selling E25 in
Europe. Our revenues will depend particularly on the success of our
collaboration partners in developing, manufacturing, obtaining regulatory
approvals for and marketing E25. Because a substantial portion of our revenues
for the foreseeable future will depend on achieving development and
commercialization milestones, we anticipate that our results of operations will
vary substantially from year to year and even quarter to quarter.

RESULTS OF OPERATIONS
YEAR ENDED DECEMBER 31, 1999 COMPARED TO YEAR ENDED DECEMBER 31, 1998

REVENUES.  Revenues decreased to $1.4 million in 1999 from $2.4 million in 1998,
a decrease of $1.0 million. This decrease was primarily due to a difference of
$1.3 million in milestone and sponsored research revenues earned under our
agreements with Novartis and Genentech. These agreements accounted for 76% of
our revenues in 1999 and 98% of our revenues in 1998. The $1.3 million decrease
was partially offset by higher revenues of $0.4 million from foreign government
grants and technology licensing fees.

RESEARCH AND DEVELOPMENT EXPENSES.   Research and development expenses increased
to $17.2 million in 1999 from $II.9 million in 1998, an increase of $5.3
million. This increase was principally due to increased personnel, expansion of
preclinical and clinical development activities and a $2.7 million non-cash
charge resulting from extending the exercise periods for stock options to some
research and development employees and consultants. In addition, we incurred a
$3.4 million charge in 1999 and a $2.8 million charge in 1998 for in-process
research and development costs from the first two purchase payments made to the
former shareholders of PanGenetics B.V., a company we acquired in 1998. Please
see "Acquisition of PanGenetics B.V."

GENERAL AND ADMINISTRATIVE EXPENSES.  General and administrative expenses
increased to $8.6 million in 1999 from $3.4 million in 1998, an increase of $5.2
million. This increase was primarily attributable to recognizing an arbitration
award of $3.5 million to the attorneys who represented us in our litigation
against

                                       24
<PAGE>

Genentech, and a $1.9 million non-cash charge for stock-based compensation,
which was due to extending some employee and consultant stock options.

OTHER INCOME.  Other income decreased to $1.0 million in 1999 from $1.2 million
in 1998, a decrease of $0.2 million. This decrease was principally due to a
decline in interest income as a result of lower average cash balances and a loss
in 1999 on foreign currency transactions.

NET LOSS.  Net loss increased to $23.3 million in 1999 from $10.2 million in
1998.

YEAR ENDED DECEMBER 31, 1998 COMPARED TO YEAR ENDED DECEMBER 31, 1997

REVENUES. Revenues decreased to $2.4 million in 1998 from $8.9 million in 1997,
a decrease of $6.5 million. This decrease was due to a $4.0 million decline in
revenues from our agreements with Novartis and Genentech and a decline of $2.5
million in revenues from a former collaboration. Revenues from our collaboration
agreements with Novartis and Genentech accounted for 98% of our 1998 revenues
and 71% of our 1997 revenues.

RESEARCH AND DEVELOPMENT EXPENSES.  Research and development expenses increased
to $II.9 million in 1998 from $6.9 million in 1997, an increase of $5.0 million.
This increase was primarily due to expanding our research organization,
including $2.8 million of in-process research and development expenses
associated with purchasing PanGenetics in the first quarter of 1998.
Additionally, we incurred a $1.5 million charge in 1998 to license patents that
we may need to commercialize one of our products.

GENERAL AND ADMINISTRATIVE EXPENSES.  General and administrative expenses
increased to $3.4 million in 1998 from $2.2 million in 1997, an increase of $1.2
million. This increase was principally due to increased expenses associated with
our legal proceedings.

OTHER INCOME.  Other income increased to $1.2 million in 1998 from $1.0 million
in 1997, an increase of $0.2 million. This increase was primarily due to a $0.4
million increase in interest income due to higher average cash balances,
partially offset by a $0.2 increase in interest expense.

NET LOSS/INCOME.  Net loss totaled $10.2 million in 1998, compared to net income
of $0.6 million in 1997.

LIQUIDITY AND CAPITAL RESOURCES

We have financed our operations since inception primarily through collaboration
and grant revenues, sales of equity securities, interest income and equipment
financing agreements. From inception through 1999, we recognized approximately
$59.3 million in collaboration, grant and other revenues. Additionally, we have
raised approximately $54.9 million from sales of equity securities and $1.5
million from the exercise of stock options, and we have earned approximately
$8.8 million of interest income. As of December 31, 1999, we had approximately
$47.3 million in cash, cash equivalents and short-term investments available for
working capital.

During 1999, we used $8.2 million to finance our operating activities. The
primary use of cash for operating purposes was our net loss of $23.3 million,
although $10.0 million of this amount was attributable to non-cash items. In
1999, changes in working capital were our primary source of cash from operating
activities, including a $2.0 million income tax refund and a $3.5 million
increase in accrued liabilities for the arbitration award. Our investing
activities generated $0.9 million in 1999. Financing activities generated $23.0
million in 1999, principally from selling our common stock. The combination of
the above items resulted in a cash increase of $15.9 million during the year.

During 1998, we used $6.5 million to finance our operating activities. The
primary use of cash for operating activities was $10.2 million to finance our
net loss, although the net loss included $4.8 million of non-cash items. In
1998, we used $1.2 million of cash for working capital. The primary use of cash
for working capital was a $1.6 million increase in taxes receivable from the
carry back of the 1998 net loss to prior years. Our acquisition of PanGenetics
did not have a material impact on working capital. Investing activities used
$6.2 million of cash in 1998, primarily for the purchase of short-term
investments, purchases of fixed assets and leasehold improvements and the
initial cash purchase payment for PanGenetics.

                                       25
<PAGE>

Financing activities generated $4.2 million in 1998, primarily from the sale of
common stock to private investors and $1.0 million from the proceeds of a note
payable to Novartis. As a result of the above items, our cash balance decreased
by $8.5 million during 1998.

Our current and anticipated development projects will require substantial
additional capital to complete. We anticipate that the amount of cash we need to
fund operations will grow substantially in the future as our projects move from
research to preclinical and clinical development. We also expect that we will
need to expand our administrative, clinical development, facilities and business
development activities to support the future development of our programs and to
support the ongoing requirements of a public company.

From 1994 through 1998, Novartis advanced us $10.0 million, pursuant to a loan
agreement to finance our new clinical manufacturing facility. The loan bears
interest at the London Interbank Offered Rate, or LIBOR, plus two percent (7.3%
and 8.1% at December 31, 1998 and 1999, respectively). Through December 31,
1999, Novartis has agreed to forgive interest on the loan. For the years 1997,
1998 and 1999, the interest Novartis has forgiven has been reflected as interest
expense and a capital contribution. Although the loan is currently scheduled to
be due in full on December 31, 2005, Novartis may partially or totally forgive
the principal and future interest payments based on the future use of the
facility.

From inception through December 31, 1999, we have invested approximately $II.6
million in property and equipment, primarily to support research and product
development activities and to construct our new clinical manufacturing facility.
We pledged all of the assets of the new clinical manufacturing facility as
security for the Novartis loan.

We have agreed to loan some employees up to $1.5 million in April 2000 to pay
tax obligations resulting from their stock option exercises in 1999.

At December 31, 1999, we had a net operating loss of approximately $6.6 million
for federal income tax reporting purposes, which begins to expire in 2019. We
also have a foreign net operating loss carryforward of approximately $5.2
million. Additionally, we have an unused U.S. research and development tax
credit carryforward of approximately $1.0 million, which begins to expire in
20II. Because we have incurred cumulative losses to date and there is no
assurance of future taxable income, we have established a valuation allowance to
fully offset the deferred tax asset at December 31, 1999.

We expect to incur substantial additional capital, research and development,
manufacturing and other costs as we continue to develop our products.
Consequently, we may need to raise substantial additional funds. We expect that
the net proceeds from this offering, together with our existing assets and
revenue from operations, will fund our operations for the next three years.
However, our future capital needs will depend on many factors, including
successfully commercializing E25, receiving payments from our collaboration
partners, progress in our research and development activities, the magnitude and
scope of these activities, the progress and level of unreimbursed costs
associated with preclinical studies and clinical trials, the costs and magnitude
of product or technology acquisitions, the cost of preparing, filing,
prosecuting, maintaining and enforcing patent claims and other intellectual
property rights, competing technological and market developments, changes in or
terminations of existing collaboration and licensing arrangements, the
establishment of additional collaboration and licensing arrangements, and
manufacturing scale-up costs and marketing activities, if we undertake those
activities. We do not have committed external sources of funding and we cannot
assure that we will be able to obtain additional funds on acceptable terms, if
at all. If adequate funds are not available, we may be required to:

  o   delay, reduce the scope of or eliminate one or more of our programs;

  o   obtain funds through arrangements with collaboration partners or others
       that may require us to relinquish rights to technologies, product
       candidates or products that we would otherwise seek to develop or
       commercialize ourselves; or

  o   license rights to technologies, product candidates or products on terms
       that are less favorable to us than might otherwise be available.

                                       26
<PAGE>

We are currently engaged in litigation and arbitration relating to a fee dispute
with the law firms that represented us in connection with the Genentech
litigation. An arbitration panel issued an award entitling the attorneys to
receive approximately $3.5 million, including interest, payments ranging from
33 1/3% to 40% of the future payments we would receive from Genentech following
product approval, and 10% of the royalties that we would receive on all sales of
anti-IgE products by Genentech and Novartis. We are contesting this award.
During the appeals process, we will either post a bond or place amounts in
escrow to secure payment of the award. See "Business -- Pending Legal
Proceedings."

ACQUISITION OF PANGENETICS B.V.

In March 1998, we purchased PanGenetics B.V., now our subsidiary Tanox Pharma
B.V., for an initial payment to its shareholders of $0.5 million in cash and
226,409 shares of our common stock, valued at $II.25 per share, for a total
initial consideration of $3.1 million. In addition, we agreed to pay future
consideration, in two installments, totaling up to $0.7 million in cash and
484,147 shares of our common stock upon occurrence of certain future events.

In September 1999, we paid the second installment of $0.3 million in cash and
242,075 shares of our common stock, valued at $12.50 per share, for a total
consideration of $3.4 million. If we make the final future payment as scheduled
in March 2001, we will record an additional purchase price amount based on the
cash paid and the fair value of the common stock issued at the time of payment.
If we make the final payment in March 2001, we will allocate the final payment
to acquired in-process research and development and goodwill based on the
appraisal obtained as of the date of the acquisition.

Tanox engaged an independent firm, KPMG LLP, to perform an appraisal of the
assets acquired in the transaction. The appraisal was completed and the report
issued in 1998. We accounted for the costs to acquire PanGenetics under the
purchase method of accounting. At the time of the acquisition, we valued the
total current and future consideration for accounting purposes at $9.2 million,
based on the total of the cash and then fair value of common stock paid, or
expected to be paid, to PanGenetics shareholders. Of this amount, we allocated
approximately $7.2 million to in-process research and development and $2.0
million to goodwill and other assets. See "Note 2 of the Notes to Consolidated
Financial Statements."

QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISK

We are exposed to a variety of risks, including foreign currency exchange
fluctuations and changes in interest rates. In the normal course of business, we
have established policies and procedures to manage these risks.

FOREIGN CURRENCY EXCHANGE RATES.  During 1999, our operating results reflect
foreign exchange losses of $0.1 million and our balance sheet reflects a foreign
currency translation adjustment of $0.2 million. We are subject to foreign
currency exchange risk because:

  o    we invest in our foreign subsidiaries;

  o    we incur a significant portion of our costs and expenses and a smaller
       portion of our revenues in the local currencies of the countries where we
       do business; and

  o    we finance part of the cost of our subsidiaries' operations through
       dollar denominated inter-company loans and equity investments that are
       recorded on their books in the respective local currencies.

Fluctuations in exchange rates have not had a material impact on our revenues or
costs and expenses, but have affected the value of our equity investments and
inter-company loans. As a result of our international operations and our current
financing approach, fluctuations in exchange rates of the local currencies
versus the U.S. dollar impact our operating results. We are primarily exposed to
gains and losses with respect to Dutch guilders and Taiwan dollars because our
subsidiaries conduct business in these currencies. To date, we have not
implemented a program to hedge our foreign currency risk, but we may do so in
the future.

INTEREST RATE RISK.   Cash and short-term investments were approximately $47.3
million at December 31, 1999. These assets were primarily invested in investment
grade commercial paper which we hold to

                                       27
<PAGE>

maturity. We do not invest in derivative securities. Although our portfolio is
subject to fluctuations in interest rates and market conditions, no gain or loss
on any security would actually be recognized in earnings unless we sell the
asset. In addition, our loan from Novartis is based on a premium over LIBOR. As
such, if general interest rates increase, our interest costs will increase.

THE YEAR 2000

During 1998 and 1999, we had a Year 2000 Project (Y2K Project) in place to
address the potential exposures related to the impact on our computer systems
and scientific and manufacturing equipment containing computer-related
components for the Year 2000 and beyond. As of December 31, 1999, all scheduled
Y2K work was completed. As of the date of this prospectus, we have not
encountered any material Y2K system problems and we have not experienced any
impact on operations or expenses.

Nevertheless, we do use and rely on a wide variety of information technologies,
computer systems and scientific and manufacturing equipment containing
computer-related components (such as programmable logic controllers and other
embedded systems). As a result, time-sensitive functions of those software
programs and equipment may yet misinterpret dates after January 1, 2000, to
refer to the twentieth century rather than the twenty-first century. Although we
do not anticipate any material problems, we could suffer system or equipment
shutdowns, failures or miscalculations. Such conditions could result in
inaccuracies in computer output or disruptions of operations, including, among
other things, inaccurate processing of financial information and/or temporary
inabilities to process transactions, manufacture products, or engage in similar
normal business activities.

In addition, although all of our significant suppliers and our significant
service providers indicated that they were or expected to be Year 2000 compliant
by December 31, 1999, and although as of the date of this prospectus we are not
aware of any material Year 2000 compliance problems with these third parties'
systems, we cannot be certain that these third parties made accurate
representations or that their systems are or will continue to be Year 2000
compliant. If any of our significant suppliers or significant service providers
experience Year 2000 compliance problems and we cannot replace them with
alternate sources, our business would be harmed.

                                       28
<PAGE>
                                    BUSINESS

OVERVIEW

Tanox identifies and develops therapeutic monoclonal antibodies to address
significant unmet medical needs in the areas of immunology, infectious diseases
and cancer. In 1987, we discovered a novel approach for treating allergies and
asthma by using monoclonal antibodies capable of inhibiting IgE. E25, our most
advanced product in development is a genetically engineered monoclonal antibody
that attaches to, and inhibits, the activity of IgE. We are developing E25 in
collaboration with Novartis and Genentech for treatment of allergic asthma and
allergic rhinitis (hay fever). We estimate that in the United States, allergic
asthma afflicts approximately 11 million people, and allergic rhinitis afflicts
approximately 40 million people, of whom approximately 32 million are seasonal
sufferers. E25 has successfully completed Phase III clinical trials in allergic
asthma and seasonal allergic rhinitis patients. Based on the results of these
trials, our collaboration partners intend to file a BLA with the FDA
concurrently with European registration for both indications in mid-2000.

Using our extensive understanding of the human immune system, we are building a
diverse pipeline of monoclonal antibody product candidates. We are conducting
clinical trials with two additional monoclonal antibodies. Hu-901, an anti-IgE
antibody distinct from E25, is currently in a Phase I/II trial for treating
severe peanut allergy. 5D12, an anti-CD40 antibody, is currently in a Phase I/II
trial for treating Crohn's disease and in preclinical studies for treating other
autoimmune diseases. We are conducting preclinical and research studies with
5A8, an anti-CD4 antibody for treating HIV, 166-32, a complement factor D
inhibiting antibody for treating acute inflammation and 163-93, an anti-G-CSF
receptor activating antibody for treating neutropenia.

STRATEGY

Our objective is to leverage our expertise in monoclonal antibodies and
understanding of the human immune system to advance our product pipeline and
become a profitable biopharmaceutical company. We intend to accomplish this
through the following strategic initiatives:

  o    CONTINUING TO IDENTIFY AND DEVELOP NOVEL MONOCLONAL ANTIBODIES. We have
       focused on the research and development of monoclonal antibodies since
       our inception and have successfully identified and obtained patents for
       novel monoclonal antibodies with potential therapeutic applications. We
       believe that monoclonal antibodies will be one of the primary areas for
       pharmaceutical development for the foreseeable future, particularly as
       genomic research identifies novel disease targets. We will continue to
       apply our expertise in immunology to identify new antibodies that may
       bind to these novel targets. We will focus on diseases for which current
       therapies are substandard or unavailable and the market opportunities are
       large. With product candidates targeting autoimmune diseases, HIV,
       inflammation and cancer, we believe that we have a diverse monoclonal
       antibody product pipeline.

  o    MAXIMIZING THE MARKET OPPORTUNITY FOR ANTI-IGE ANTIBODIES. We are focused
       on identifying diseases and conditions for which anti-IgE antibodies are
       likely to provide a safe and effective therapy. In this regard, we intend
       to work with our partners, Novartis and Genentech, in establishing market
       awareness for E25 in asthma, allergic rhinitis and potential future
       indications targeted through the collaboration. Concurrently with this
       effort, we are pursuing the independent development of Hu-901 to
       demonstrate the efficacy of anti-IgE antibodies for indications our
       partners are not currently pursuing.

  o    IDENTIFYING ATTRACTIVE ACQUISITION AND IN-LICENSING CANDIDATES. In
       addition to our in-house development efforts, we will continue our
       efforts to identify opportunities to acquire or in-license products and
       technologies. We believe that we are well positioned to continue to
       attract in-licensing and acquisition candidates as a result of our
       demonstrated expertise in immunology and monoclonal antibodies. Our
       anti-CD40 and anti-CD4 programs are the result of in-licensing.

                                       29
<PAGE>
  o    FORMING STRATEGIC COLLABORATIONS TO SUPPORT DEVELOPMENT AND
       COMMERCIALIZATION OF OUR PRODUCTS. We often deem it advantageous to
       partner with large pharmaceutical and biotechnology companies to obtain
       funding and marketing support for our development activities. These
       collaborations generally:

       -   enable us to develop a greater number of products than otherwise
           would be possible;

       -   lower the substantial financial investment that is required of us to
           develop our products; and

       -   provide us with domestic and international marketing and sales
           expertise for our partnered products once approved.

    Under future collaborations, we expect to retain strategically important
    development, manufacturing or marketing rights in order to optimize the
    value of our drug development opportunities.

  o    RETAINING STRATEGIC MARKETING RIGHTS TO OUR PRODUCTS. As we pursue
       strategic collaborations, we intend to reserve strategic marketing rights
       for our products. We will focus initially on markets for which our
       products have a clear advantage over other therapies or that we can
       market using a relatively small, targeted sales force.

HUMAN IMMUNE SYSTEM

The human immune system has three general mechanisms that protect the body
against infections, toxins and cancer by responding to and clearing foreign
agents, or antigens, that have penetrated the body's protective barriers. These
mechanisms are:

  o    ANTIBODIES. The body produces proteins called antibodies that deactivate
       and help remove the antigens from the body. Each antibody matches an
       antigen much as a key matches a lock. Antibodies are made by specialized
       white blood cells called B cells, with the help of other white blood
       cells, called helper T cells. When a B cell encounters its triggering
       antigen, it manufactures millions of identical antibody molecules and
       releases them into the bloodstream.

  o    CELL-MEDIATED IMMUNE RESPONSE. The second protective immune mechanism
       relies on cells to recognize and destroy foreign antigens. This mechanism
       is known as cell-mediated immunity. T cells are important in
       cell-mediated immune responses. Some T cells, called killer T cells, seek
       out and kill cancer cells or virus-infected cells. Helper T cells also
       play a role in cell-mediated immune response by stimulating inflammatory
       cells called macrophages to actively destroy foreign antigens,
       microorganisms and cancer cells.

  o    INNATE IMMUNE RESPONSE. Innate immunity is the body's first line of
       defense against injury and infection and serves a surveillance and
       maintenance role. One part of the innate immune system involves large
       white blood cells called phagocytes (literally, "cell-eaters") that can
       engulf and digest foreign microorganisms and other antigens. Important
       phagocytes include macrophages, that rid the body of dead cells and other
       debris, and granulocytes, including neutrophils, that contain granules
       filled with potent chemicals. These chemicals, in addition to destroying
       microorganisms, play a key role in acute inflammatory reactions. Another
       part of the innate immune response is known as the complement system. The
       complement system includes a series of proteins that work to "complement"
       the activity of antibodies in destroying bacteria, either by stimulating
       the macrophages and neutrophils, or by puncturing the bacterial cell
       membrane to kill the cells. These proteins also cause the neutrophils to
       accumulate at the site of infection or tissue damage.

Most of the time, the immune system protects us from infections, cancer and some
toxic agents. However, sometimes the immune system actually causes the disease
or symptoms of the disease. For example, in the case of allergic diseases, such
as asthma and hay fever, the immune system responds to the antigen, or allergen
in this case, by producing an IgE form of antibody. IgE is instrumental in
triggering the symptoms of the disease. In other cases, T and B cells may
recognize a part of the body as foreign, triggering an immune response against
the body resulting in an autoimmune disease and associated tissue destruction.

MONOCLONAL ANTIBODIES AS THERAPEUTICS

Monoclonal antibodies represent an exciting area of novel therapeutic product
development. Genetically engineered monoclonal antibodies are man-made
antibodies that target a specific antigen. Most monoclonal antibodies are
derived from animals such as mice. Advances in antibody design technologies have
enabled

                                       30
<PAGE>

scientists to develop humanized (human-like) and fully human antibody products
that can be administered to patients on a chronic basis with reduced concern for
adverse responses by the human immune system. Advances in antibody production
technologies, such as high productivity fermentation and transgenic plants and
animals, have enabled manufacturers to produce antibody products more
cost-effectively. Because of these advances, a large number of monoclonal
antibodies are currently undergoing clinical and preclinical investigation.
According to a survey conducted by the Pharmaceutical Research and Manufacturers
of America, 74 out of 350, or 21% of all, biotechnology medicines in clinical
trials in 1998 were antibodies. The FDA has approved eight therapeutic
antibodies, six of them in the last three years, with total sales in 1999 in
excess of $1.3 billion.

Generally speaking, there are three basic methods for using monoclonal
antibodies as therapeutics. Each of the approaches described below capitalizes
on the monoclonal antibody's ability to precisely target selected receptors on
specific cells:

  o    BLOCKING CELL ACTIVITY AND IMMUNE FUNCTIONS -- monoclonal antibodies can
       be produced to bind to specific bioactive molecules or cell receptors to
       prevent undesirable cell responses, such as allergic reactions and
       autoimmune diseases.

  o    ACTIVATING CELL ACTIVITY AND IMMUNE FUNCTIONS -- monoclonal antibodies
       can be produced to bind to specific cell receptors in order to activate a
       desired cellular response.

  o    DELIVERING THERAPEUTIC AGENTS -- monoclonal antibodies bind to specific
       target receptors. Consequently, antibodies can be used to deliver active
       agents, such as radioactive isotopes and toxins, to specific cells and
       tissues targeted for destruction.

PRODUCT DEVELOPMENT PROGRAMS

We have three products in clinical development and are evaluating several
product candidates in preclinical and research studies. Our drugs target various
elements or malfunctions of the immune system, and all are monoclonal
antibodies. We have designed drugs to deactivate or reduce the activity of the
immune system for diseases caused by over-activation or inappropriate activation
of immune responses, such as autoimmune and allergic diseases and acute
inflammation. We also have designed drugs to activate the immune system for
treatment of diseases where boosting immune protection is desirable, such as
AIDS, infectious diseases and cancer. Our products have either resulted from our
internal research and development activities or were in-licensed or acquired.

The table below summarizes the development status for our principal product
candidates.

<TABLE>
<CAPTION>
                           ANTIBODY
             PRODUCT      DESCRIPTION      INDICATION         STATUS          PARTNERS
           --------------------------------------------------------------------------------
<S>        <C>         <C>              <C>              <C>              <C>                <C>

           E25         Anti-IgE         Allergic Asthma  Phase III        Novartis/Genentech
                                                         Completed

                                        Seasonal Allergic Phase III       Novartis/Genentech
                                        Rhinitis         Completed

           Hu-901      Anti-IgE         Severe Peanut    Phase I/II              --
                                        Allergy

           5D12        Anti-CD40        Crohn's Disease  Phase I/II              --

                                        Other Autoimmune Preclinical             --
                                        Diseases

           5A8         Anti-CD4         HIV/AIDS         Preclinical             --

           166-32      Anti-complement  Acute            Research                --
                       Factor D         Inflammation
           163-93      Anti-G-CSF       Chemotherapy-Induced Research            --
                       Receptor         Neutropenia

           Novartis and Genentech are disputing our rights to independently develop Hu-901.
           We discuss the dispute in this prospectus in the section entitled
           "Business--Pending Legal Proceedings." In addition, we have the right to
           develop and commercialize 5D12 only in Europe and Japan.
</TABLE>

                                       31
<PAGE>
ANTI-IGE DEVELOPMENT -- E25

E25 is a humanized anti-IgE monoclonal antibody designed to prevent symptoms of
allergic asthma and allergic rhinitis. E25 works by preventing the ability of
allergens to activate the immune system. We are developing E25 in collaboration
with Novartis and Genentech. The product has successfully completed Phase III
clinical trials in both allergic asthma and seasonal allergic rhinitis. Our
partners intend to file a BLA with the FDA concurrently with European
registration for both indications by mid-2000. We expect E25 to be administered
by subcutaneous injections once or twice per month.

In allergic diseases, the immune system responds to the antigen, or allergen in
this case, by producing IgE. IgE binds to the surface of mast cells and
basophils. These cells, which are found in tissue and also circulate in the
blood, contain chemicals such as histamine and leukotrienes, which induce
inflammation. The first time an allergy-prone person is exposed to an allergen,
he or she makes large amounts of an IgE antibody specific to that allergen.
These IgE molecules attach to the surfaces of mast cells or basophils. When an
IgE antibody sitting on a mast cell or basophil next comes in contact with its
specific allergen, the IgE antibody signals the mast cell or basophil to release
its powerful chemicals, causing tissue inflammation and asthma and allergy
symptoms, including wheezing, bronchospasm, sneezing, runny nose, watery eyes
and itching. E25 blocks IgE from binding to mast cells and basophils, thereby
inhibiting the allergic response.

The diagram below shows how allergic reactions are triggered (The Allergy
Cascade) and how E25 works to prevent or reduce allergy symptoms (How E25
Works).

<TABLE>
<CAPTION>

          The Allergy Cascade                              How E25 Works

<S>            <C>                                  <C>            <C>
[graphic]      When an allergy-prone                [graphic]      E25 binds
               person is exposed to an                             to IgE and
               allergen such as ragweed,

[graphic]      he or she makes large                [graphic]      prevents IgE from
               amounts of IgE antibody                             attaching to mast
               to ragweed.                                         cells.

[graphic]      These IgE antibodies bind
               to receptors on mast cells.

[graphic]      The next time a person               [graphic]      Without IgE on the mast
               encounters ragweed, it cross-                       cell, ragweed cannot cause
               links the IgE antibodies on                         the mast cell to release its
               mast cells and the IgE-primed                       powerful chemicals,
               mast cell releases its powerful
               chemicals, including histamine.

[graphic]      These chemicals cause the            [graphic]      and the person's asthma
               person to suffer asthma                             and allergy symptoms
               and allergy symptoms, such as                       are prevented or reduced.
               wheezing, bronchospasm,
               sneezing, runny nose, watery
               eyes and itching.

</TABLE>

                                       32
<PAGE>

MARKET OPPORTUNITY

Allergic reactions triggered by IgE include allergic rhinitis and allergic
asthma.

ALLERGIC RHINITIS.  Allergic rhinitis is a disease characterized by runny nose,
sneezing, congestion, itchy eyes and similar symptoms, and includes hay fever.
Allergic rhinitis afflicts at least 39.5 million people in the United States,
most of whom have seasonal allergies. In 1993, it was estimated that the total
cost associated with all forms of allergic rhinitis in the United States was
$3.4 billion. Doctors commonly treat the symptoms of allergic rhinitis with
antihistamines, decongestants, nasal steroids and other drugs. For many
patients, however, these medications do not completely alleviate the allergic
reactions or eliminate the symptoms. According to a 1997 study, only 26% of
treated allergic rhinitis patients reported symptoms as "well" or completely
controlled. Doctors sometimes prescribe allergy shots, called hyposensitization
therapy, for severely allergic persons to treat allergic rhinitis and systemic
allergic reactions. If the treating health care professional knows the allergen
to which the patient is reacting, hyposensitization can be effective. However,
it is difficult to identify which allergen causes the patient's allergy, and the
frequent and lengthy treatment protocols, as well as the potential for serious
adverse side effects, generally make hyposensitization undesirable.

ASTHMA.  Asthma makes breathing difficult and is potentially life threatening.
According to a 1998 report by the Centers for Disease Control, approximately 17
million people in the United States suffer from asthma. Published reports also
indicate that asthma's prevalence in the United States has increased 75% from
1980 to 1994. Approximately two-thirds of these patients have allergic asthma.
The American Lung Association estimates that over $9.8 billion is spent annually
on asthma-related costs in the United States. Corticosteroids and beta-agonists,
the mainstay of asthma therapy, are sometimes effective, yet each is associated
with specific safety drawbacks. Particular side effects of corticosteroid
treatment include growth retardation in children, osteoporosis and cataracts.
Beta-agonists offer only short-term relief and do not control the underlying
inflammation. Leukotriene modifiers, a new class of controller medications with
the potential to reduce steroid requirements, appear to be modestly effective
for some patients. However, these modifiers have been associated with drug
interactions and adverse events including liver injury. Increasing use of
beta-agonists indicates inadequate control of the underlying inflammation
causing asthma.

DEVELOPMENT STATUS AND CLINICAL DATA

SEASONAL ALLERGIC RHINITIS.  A Phase III clinical trial of E25 for seasonal
allergic rhinitis was conducted in Scandinavia during the spring 1998 birch
season. The randomized, placebo-controlled, multicenter clinical trial examined
symptoms of rhinoconjunctivitis and rescue medication usage in 251 adult and
adolescent patients with a history of birch pollen allergy. Patients were
treated with 300 mg of E25 or placebo via subcutaneous injection every three or
four weeks and treatment lasted eight or nine weeks. Patients received rescue
medications, such as antihistamines, when their symptoms were severe enough to
require additional medication. This Phase III clinical trial confirmed the
statistically significant results of an earlier pivotal Phase II trial in 536
patients conducted during the 1997 ragweed season in the United States. The
statistical significance of clinical results is determined by a widely-used
statistical method that establishes the par value, or p value, of the clinical
results. A par value of less than 0.01 (p<0.01) means that the chance of the
clinical results occurring by accident is less than 1 in 100.

Results from the Phase III seasonal allergic rhinitis trial were reported in May
1999 and showed that:

  o    E25 DECREASED THE SEVERITY OF NASAL AND OCULAR ALLERGY SYMPTOMS. Compared
       to placebo, E25 treatment improved average daily nasal (p<0.001) and
       ocular (p=0.031) symptom severity scores. Patients treated with placebo
       experienced recurring sneezing, itchy nose and similar symptoms that
       increased over the pollen season and were greatest during highest pollen
       exposure. In contrast, patients treated with E25 did not experience an
       increase in nasal symptoms over the whole E25 treatment period compared
       to symptoms reported before the pollen season began and before treatment
       began. 21% of patients treated with E25 reported complete control of
       symptoms and an additional 59% reported symptom control was improved
       compared to previous seasons. In addition, 6 of 7 patients

                                       33
<PAGE>
       who discontinued treatment because of unsatisfactory treatment effects
       were in the placebo group, even though there were twice as many patients
       receiving E25.

  o    E25 DECREASED USE OF RESCUE MEDICATION. Patients receiving E25 used an
       average 0.5 antihistamine tablets per day versus 1.3 tablets per day in
       the placebo group (p<0.001), and required rescue medication on half as
       many days (p<0.001). Increased use of rescue medication by placebo group
       patients may have blunted the difference in symptom scores reported.

  o    E25 IMPROVED PATIENTS' QUALITY OF LIFE. E25 treatment delivered
       clinically meaningful and statistically significant improvement over
       placebo in total rhinitis quality of life score (p < 0.001) and in the
       specific categories of activity limitations, nasal symptoms, non-nose and
       non-eye symptoms and practical problems. E25 treated patients also
       experienced improvements in sleep, eye symptoms and emotional state.

  o    E25 TREATMENT WAS SAFE AND WELL TOLERATED. In this and in the previous
       trials, no antibodies against E25 were detected and no serum sickness,
       immune complex disease, severe allergic reactions or other
       allergy-related side effects were reported. Less than two percent of
       patients experienced side effects of headache (1.8%) and upper
       respiratory infection (1.2%). Three subjects reported urticaria (skin
       itching and hives) following E25 injections (0.5% of all E25 injections).
       The incidence of adverse events was similar for the E25 and placebo
       groups.

The difference between placebo and E25 in all instances was statistically
significant. In addition, Novartis has announced the initiation of a Phase IV
evaluation in perennial (year-round) allergic rhinitis in the third quarter of
1999.

ALLERGIC ASTHMA. Two Phase III clinical trials in allergic asthma have been
completed in the United States in adults (12-75 years) and children (6-12
years). The two randomized, placebo-controlled, multicenter clinical trials
included 525 patients in the adult study and 334 patients in the pediatric
study. Clinicians who participated in the Phase III clinical trials presented
the results at the American Academy of Allergy Asthma & Immunology meeting in
San Diego, California in March 2000.

In both trials, patients who were experiencing asthma symptoms, despite taking
inhaled corticosteroid therapy, were given either E25 or placebo via
subcutaneous injection every two or four weeks. Patients were monitored for
asthma exacerbations, which were defined as symptoms requiring a doubling of
inhaled corticosteroids or initiation of oral corticosteroids to maintain
adequate asthma control. These trials were conducted over 28 weeks in two
phases, followed by the following treatment extensions:

  o    a 16-week stable treatment period that monitored patients taking either
       E25 or placebo in addition to ongoing treatment with inhaled
       corticosteroids and rescue beta-agonists;

  o    a 12-week steroid reduction period (immediately following the stable
       treatment period), where the dosage of inhaled corticosteroids was
       gradually reduced in both the E25 and placebo groups; and

  o    a treatment extension so that long term safety could be evaluated for a
       one year period.

In patients receiving E25 versus placebo, the reduction in steroid dosage was
greater in both adults (p<0.001) and children (p=0.001).

In the 525 patient adult trial, steroid dosage was reduced at least 75% in half
of the patients receiving E25 compared to at least 50% in half of the patients
receiving placebo. The percentage of E25 patients experiencing asthma
exacerbations was reduced in both the stable treatment period and the steroid
reduction period. Additionally, the average daily dose of rescue medication
during this period was less in both treatment phases for the E25 group than for
the placebo group. During the stable treatment period, 14.6% (39 of 268
patients) of patients receiving E25 demonstrated asthma exacerbations compared
to 23.3% (60 of 257) of patients receiving placebo (p=0.009). During the steroid
reduction period, 21.3% (57 of 268) of patients receiving E25 experienced asthma
exacerbations compared to 32.3% (83 of 257) of patients receiving placebo
(p=0.026). Additionally, 40% (107 of 268) of patients receiving E25 completely
withdrew from inhaled corticosteroids during this period compared to 19% (49 of
257) of patients receiving placebo.

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<PAGE>

In the 334 patient pediatric trial, steroid dosage was reduced by 100% in more
than half of the patients receiving E25, compared to at least 71% in half of the
patients receiving placebo. The percentage of E25 patients experiencing asthma
exacerbations also was reduced during both the stable treatment period and the
steroid reduction period and the average daily dose of rescue medication was
less in both treatment phases for the E25 group than in the placebo group.
During the stable treatment period, 16% (35 of 225 patients) of patients
receiving E25 demonstrated asthma exacerbations compared to 23% (25 of 109) of
patients receiving placebo (p=0.095). During the steroid reduction period, 18%
(41 of 225) of patients receiving E25 experienced asthma exacerbations compared
to 39% (42 of 109) of patients receiving placebo (p<0.001). 55% (124 of 225) of
patients receiving E25 completely withdrew from inhaled corticosteroids during
this phase compared to 39% (43 of 109) of patients receiving placebo.

Results from the two trials showed that despite decreasing dosages of
conventional therapies, E25 treatment reduced the number of serious asthma
exacerbations. These studies corroborated results from an earlier Phase II
clinical trial in allergic asthma recently reported in THE NEW ENGLAND JOURNAL
OF MEDICINE. Headache and upper respiratory tract infection were the most
frequently reported adverse events. The incidence of adverse events during the
Phase III trials was similar in both the E25 and placebo groups. Safety data
from the five-month follow-up phase of the trials are pending.

Novartis and Genentech intend to file a BLA for E25 in the United States and to
concurrently file for registration in Europe in mid-2000 for both allergic
asthma and seasonal allergic rhinitis.

We and our collaboration partners have begun clinical development of E26, a
humanized anti-IgE monoclonal antibody with improved binding to IgE. Although
E26 may be several years behind the development of E25, we believe the product
may require lower doses to achieve the same clinical benefits as E25.

ANTI-IGE DEVELOPMENT -- HU-901

Hu-901 is a humanized anti-IgE monoclonal antibody that we are developing to
prevent symptoms of peanut induced anaphylaxis, a severe, potentially
life-threatening allergic reaction. Hu-901 binds IgE in a way similar to E25,
and is designed to prevent allergic reactions. At the initiation of our
collaboration with Novartis and Genentech in 1996, Novartis and we were
developing Hu-901, and Genentech was developing E25. We agreed with our
collaboration partners to select E25 for joint development primarily because
commercial-scale manufacturing capability existed for E25.

In July 1999, we initiated our first independent clinical trial with Hu-901. As
discussed in this prospectus under the heading "Business -- Pending Legal
Proceedings," Novartis and Genentech are contesting our right to independently
develop Hu-901. Regardless of the outcome, we believe that our development
program will highlight the potential for additional indications for the use of
anti-IgE antibodies and will encourage our partners to expand the development of
anti-IgE more rapidly into additional indications. If we lose the litigation, we
will terminate independent development of Hu-901.

MARKET OPPORTUNITY.  We believe anti-IgE antibodies, such as Hu-901, have
potential applications beyond asthma and allergic rhinitis in treating other
allergic reactions and diseases, including peanut and other food allergies. For
example, patients with severe peanut allergy suffer gastrointestinal, skin and
respiratory symptoms, and may also suffer potentially life-threatening
anaphylaxis in response to ingesting peanuts. According to a recently published
survey, peanut or tree nut (e.g., walnut, almond and cashew) allergy affects
about 3 million people in the United States, 1.1% of the U.S. population.
Current treatment is avoiding peanuts and peanut oil, which is used in preparing
many food products. Complete avoidance requires constant vigilance and is
difficult because prepared food labeling does not always identify peanut-derived
ingredients. Accidental exposures can result in serious allergic reactions and
sometimes death. Patients with severe peanut allergy take antihistamines for
accidental exposure and epinephrine for severe anaphylactic reactions.

                                       35
<PAGE>

Approximately 2 to 4% of children and 1 to 2% of adults in the United States
suffer from food allergies. If Hu-901 effectively reduces sensitivity to
peanuts, we may also investigate its use in other food allergies. There is no
approved preventive therapeutic for food allergies.

Another indication for possible treatment with our anti-IgE product is atopic
dermatitis, or eczema, a disease resulting in itching, blisters, redness,
swelling and scaling or hardness of the skin. Eczema correlates with higher IgE
levels and is prevalent in asthma patients. Severe eczema causes much distress
to patients and greatly impairs their quality of life. Eczema is usually treated
with skin hydration and topical steroids, and sometimes treated with
antihistamines. Some individuals resist conventional therapies or develop
unacceptable side effects. Eczema is the most common skin condition in children
under the age of eleven. The percentage of children diagnosed with eczema has
increased in the United States from 3% in the 1960s to 10% in the 1990s.

DEVELOPMENT STATUS.  We recently initiated a randomized, placebo-controlled,
multicenter Phase I/II trial with Hu-901. We designed the trial to determine the
extent to which Hu-901 treatment of patients with histories of severe reactions
to peanut products decreases sensitivity to eating small amounts of a peanut
preparation.

ANTI-CD40 DEVELOPMENT -- 5D12

5D12 is an anti-CD40 monoclonal antibody that we are developing for treatment of
autoimmune diseases. 5D12 blocks the CD40 pathway. The CD40 pathway enables B
cells to produce antibodies and regulates cellular immune responses, including
activation of macrophages and killer T cells. T and B cells sometimes recognize
a part of the body as "non-self," triggering an immune response against the
body that results in an autoimmune disease and associated tissue destruction. We
believe that 5D12 is the only antibody in clinical development that binds to
CD40 and inhibits cellular activation. We have exclusive rights to 5D12 in
Europe and Japan under a license from Chiron.

Preclinical studies have shown that interfering with the CD40 pathway may be
beneficial in treating autoimmune diseases, including multiple sclerosis and
lupus, and in preventing grafted organ rejection. 5D12 has potently inhibited
activation of B cells and macrophages in tissue culture systems. 5D12 also was
biologically active in preventing or delaying the appearance of clinical signs
and symptoms in a primate model for multiple sclerosis.

MARKET OPPORTUNITY.  Based on our research and preclinical studies with 5D12, we
believe the product could reduce production of antibodies and the activation of
cellular immune responses that cause autoimmune diseases. Examples of such
diseases and estimated potential market sizes in Europe and Japan include:

  o    Crohn's disease, an inflammatory disease of the bowel (over 165,000
       people);

  o    rheumatoid arthritis (approximately 3 million people);

  o    multiple sclerosis (approximately 200,000 people);

  o    lupus (approximately 320,000 people); and

  o    psoriasis (approximately 1 million people).

DEVELOPMENT STATUS.  We have initiated a 20 patient Phase I/II clinical study
with 5D12 in Crohn's disease in Europe. The trial is a single dose,
dose-escalating study designed to provide data regarding the safety of the
product, its behavior, including half-life and clearance characteristics, and
its biological activity using histological examination of biopsy tissue. We
expect the results of this study to play an important role in determining
clinical indications that we will pursue with 5D12.

                                       36
<PAGE>
ANTI-CD4 DEVELOPMENT -- 5A8

5A8 is a humanized anti-CD4 monoclonal antibody that is in preclinical
development to treat human immunodeficiency virus, or HIV. The virus enters the
host cell by binding to the CD4 receptors on these cells. In lab studies, our
5A8 antibody binds to the CD4 receptor on the cell surface and prevents viral
entry into the cell, thereby blocking infection. We have exclusive worldwide
rights to 5A8 through a license with Biogen.

MARKET OPPORTUNITY.  According to the World Health Organization, HIV infects
approximately 1.4 million people in North America and Western Europe. A number
of drugs targeting viral replication are being used, often in combination. About
30% of the patients treated with drug combinations, however, no longer respond
since HIV has become drug resistant. In addition, many drug combinations produce
a variety of undesirable toxic side effects.

DEVELOPMENT STATUS.  5A8 is now in preclinical development to determine its
usefulness to treat HIV-infected patients. In preliminary preclinical testing,
5A8 has potently blocked infections in all twenty-five strains of primary HIV
isolates tested in cell culture. In a primate model for HIV infection, 5A8
showed robust antiviral activity. Preclinical tests with our 5A8 antibody showed
no reduction in CD4-positive cell numbers, no evidence that any function of the
immune system was suppressed and no toxic effects.

OTHER PRODUCT CANDIDATES

ANTI-FACTOR D DEVELOPMENT.  166-32 is a monoclonal antibody that binds to Factor
D, a component of the complement system, and is in research to treat acute
inflammation. The complement system is the body's first line of defense against
infection. 166-32 binds to Factor D and inhibits complement activation.

While the complement system generally functions to protect the body, complement
system activation can become excessive and uncontrolled resulting in
inflammation and tissue damage. This can occur in cases of acute tissue injury
or surgical procedures that reduce blood flow into a tissue. In addition, during
heart surgery involving cardiopulmonary bypass (CPB), the shunting of blood
outside the body for circulation through mechanical devices has been shown to
activate complement and result in tissue injury. Research studies with 166-32 in
laboratory models for heart injury showed that low levels of 166-32 prevented
heart damage, and that 166-32 inhibited complement activation in a model for
CPB.

ANTI-G-CSF RECEPTOR DEVELOPMENT.  163-93 is a monoclonal antibody that binds to
the receptor for granulocyte colony stimulating factor (G-CSF) to treat a
condition known as neutropenia, which often afflicts cancer patients who have
undergone chemotherapy. Neutropenia is a deficiency of a type of granulocyte (a
specialized white blood cell that contains granules filled with potent
chemicals) called neutrophils, which results in a compromised immune system and
susceptibility to infection. Granulocytes engulf and destroy bacteria and other
microbial pathogens. G-CSF binds specific receptors on the surface of
granulocytes and stimulates their proliferation and controls their activities.
Recombinant G-CSF is sold by Amgen as Neupogen and has been used in cancer
patients to help restore their immune protection. Amgen reported 1999 sales of
$1.3 billion for Neupogen.

We are developing 163-93 to stimulate proliferation and activity of granulocytes
just as does G-CSF. 163-93 activates the same intracellular signaling pathway as
G-CSF and supports the production of granulocytes from human bone marrow as does
G-CSF. In a preliminary study, injecting 163-93 into primates stimulated
granulocyte production without affecting any other blood cells or causing any
toxicity. We believe that 163-93 could provide an alternative to G-CSF for
therapeutic use in cancer patients during chemotherapy and may have the
advantage of less frequent dosing.

ADDITIONAL PRODUCTS IN RESEARCH AND DEVELOPMENT.  We have additional discovery
research projects directed towards developing new products and technologies to
treat immunological diseases and cancer.

                                       37
<PAGE>
COLLABORATION AND LICENSING AGREEMENTS

COLLABORATION WITH NOVARTIS AND GENENTECH.  We are developing our lead product,
E25, in collaboration with Novartis and Genentech. In 1990, we established a
collaboration with Novartis to jointly develop anti-IgE antibodies to treat
allergic diseases. In 1996, Genentech joined our collaboration with Novartis and
we agreed to combine our respective anti-IgE development programs in a
three-party collaboration. We and our collaboration partners selected E25 as the
lead product for development and commercialization.

Currently, under the terms of the collaboration agreements:

  o    DEVELOPMENT. Novartis and Genentech are responsible for completing the
       development of and obtaining the regulatory approval for E25 and the
       other anti-IgE products developed through the collaboration. Novartis and
       Genentech share all development costs relating to E25 and other anti-IgE
       products that the collaboration may select for development in the United
       States and Europe. We and Novartis equally share development costs
       relating to China, Hong Kong, Korea, Singapore and Taiwan. Novartis is
       responsible for development costs in the rest of the world.

  o    MANUFACTURING. Novartis and Genentech are responsible for manufacturing
       E25 and other selected anti-IgE products worldwide, subject to our right
       to manufacture up to 50% of the worldwide requirements of those products.

  o    MARKETING. Novartis and Genentech share U.S. marketing rights, and
       Novartis has marketing rights in Europe (with Roche retaining the option
       to participate in Europe) to products developed through the
       collaboration. Novartis is responsible for marketing these products in
       the rest of the world, including China, Hong Kong, Korea, Singapore and
       Taiwan.

  o    PAYMENTS. We may receive payments of up to $63.5 million based on
       completing development and marketing objectives for E25, $6.5 million of
       which we have already received. We may also receive payments of up to
       $14.0 million based on completing development objectives for E26, $1.0
       million of which we have already received. Our next payments totaling $12
       million are due on filing the E25 BLA.

  o    ROYALTIES AND PROFIT SHARING. We may receive royalties based on net sales
       of E25 and other selected anti-IgE products in the United States and a
       share of Novartis' profits on these sales. We also may receive royalties
       on net sales of E25 and other selected anti-IgE products in Europe and
       the rest of the world (except for China, Hong Kong, Korea, Singapore and
       Taiwan) and an equal share of Novartis' profits from sales of E25 and
       other selected anti-IgE products in China, Hong Kong, Korea, Singapore
       and Taiwan.

Our rights to the full amount of the payments, royalties and profits that we
receive could be affected by the on-going legal proceedings with our former
attorneys described under "Business -- Pending Legal Proceedings."

The collaboration agreements provide that we may independently develop, without
our collaboration partners, any anti-IgE product that the partners are not
developing through the collaboration. If we choose to independently develop a
product, Novartis has a right of first refusal on the licensing of that anti-IgE
product. As described in this prospectus under the heading "Business -- Pending
Legal Proceedings," we are currently involved in a dispute with our
collaboration partners regarding our rights to independently develop Hu-901 and
our right to use certain know-how and other information in our independent
development efforts.

Roche has an option to participate in commercializing E25 and other anti-IgE
products which the collaboration selects for development in Japan and Europe.
Roche may exercise this option if specified events relating to commercializing
the product occur. Roche has waived this option for E25 in Japan. In addition,
Roche has an option to assume Genentech's position in the collaboration if
Genentech withdraws from the collaboration, as described below.

Either Novartis or Genentech may withdraw from the collaboration on short
notice. If either Novartis or Genentech withdraws, rights to E25 and any other
products developed by the collaboration (including by

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the withdrawing partner) revert to us and the remaining collaborator and, if
Genentech is the withdrawing party, to Roche if Roche exercises the option
described above. If the collaboration is dissolved in its entirety, we would
continue to retain rights to develop anti-IgE antibodies under the terms of
separate agreements with Novartis and Genentech.

In addition to the collaboration agreements, we and Genentech entered into a
cross-license agreement under which each party has an option to license the
other party's patents relating to the development of anti-IgE antibodies for use
in developing specific products. This option may be exercised at any time if
either party chooses to independently develop a product as permitted under the
collaboration agreements, if our collaboration with Novartis and Genentech
terminates or if we and Genentech may mutually agree.

OTHER COLLABORATIONS AND LICENSE AGREEMENTS

CHIRON LICENSE.  In 1998, our European subsidiary Tanox Pharma, B.V. entered
into an agreement to license from Chiron exclusive research and development
rights (except as to Chiron) to Chiron's murine monoclonal antibodies against
CD40. Subject to our obligations to develop an anti-CD40 product and, under
certain circumstances, to pay maintenance fees, we have an option to obtain a
commercial license to Chiron's anti-CD40 antibodies, patents and technology for
Europe and Japan. Chiron retains its commercial rights in the United States and
the rest of the world. Additionally, Chiron has an option to obtain a commercial
license for the United States and the rest of the world outside Europe and Japan
to use anti-CD40 patents and technology that Tanox Pharma develops. Chiron has
two awarded U.S. patents and has patents pending in Europe, Japan and Canada.

Upon registering a product in Japan, the United Kingdom, France or Germany,
Tanox Pharma has agreed to pay Chiron a registration fee and royalties based on
its European and Japanese sales. Tanox Pharma has also agreed to pay Chiron a
milestone payment if Tanox Pharma develops and commercializes a product. Chiron
has agreed to pay us royalties based on its sales in the United States and the
rest of the world. We may make a $1.0 million product development milestone
payment, in addition to royalty payments under this agreement. The license
terminates on the later of the expiration of 10 years following the first
commercial product sale or the expiration of the last to expire of licensed
patents. The currently licensed patent expires in 2013.

BIOGEN.  In 1998, we entered into an agreement to license from Biogen its
anti-CD4 monoclonal antibody and intellectual property on an exclusive worldwide
basis with limited sublicense rights. Biogen owns issued U.S. and European
patents and has pending applications in Australia, Canada and Japan. We paid
Biogen a license fee and agreed to make additional development milestone
payments and royalty payments to Biogen based on annual net sales revenue
levels. Additionally, we agreed to make milestone payments to Biogen that
increase as product development continues and if specified corporate development
events occur. In addition to royalty payments, we may make up to an aggregate of
$10.4 million in product development milestone payments under this agreement.
The license terminates on a country-by-country basis on the later of the
expiration of 12 years following the first commercial product sale or the
expiration or invalidity of applicable patents. The licensed patents expire in
Europe in 2011 and in the United States in 2016.

BIOVATION.  We entered into an agreement in 1999 with Biovation Limited of
Aberdeen, United Kingdom, to apply Biovation's proprietary deimmunization
technology to certain of our monoclonal antibodies and protein products.
Biovation also licensed the patents needed to develop and commercialize any
deimmunized monoclonal antibodies it developed, and if the deimmunized
monoclonal antibodies meet certain criteria, we will pay royalties to Biovation.
Additionally, Tanox Pharma and Biovation have agreed to jointly develop their
respective protein engineering technologies. We paid Biovation a license fee
that will decrease if Biovation products transferred to us under the agreement
do not achieve certain specifications. In addition to royalty payments, we may
make up to an aggregate of $1.3 million in product development milestone
payments under this agreement. The license terminates on the earlier of 10 years
from the first commercial product sale, a formal determination that any licensed
patents are invalid or unenforceable or such patents have expired. Biovation has
filed a patent application for the licensed technology; if a patent issues under
the application, it will expire in 2018.

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<PAGE>
PATENTS AND PROPRIETARY RIGHTS

We pursue patent protection for our proprietary technology and products. We
typically file U.S. patent applications, then international treaty applications,
usually followed by filing foreign patent applications on our technology and
products in those regions or countries where business considerations warrant
filings. These countries include Japan, Canada, Australia, countries of the
European Union, other European countries, and certain other Asian countries.

We have five U.S. patents that cover and/or relate to the use of anti-IgE and
other allergy/asthma products. We also hold patents in Europe, Canada, Japan,
Singapore, Hong Kong and Australia covering such products. We have additional
anti-IgE patents pending in the United States and internationally. Some of our
patents are co-owned with Novartis. We also cross license Genentech's patents
covering anti-IgE products.

We have filed U.S. and international patent applications relating to anti-Factor
D antibodies and anti-G-CSF receptor antibodies. We anticipate filing
corresponding national phase applications in selected jurisdictions at the
appropriate time. We have a number of other U.S. and foreign patents covering
certain other proprietary technology and products, with over forty U.S. patents
granted to date. Our issued patents expire between 2008 and 2014. We cannot
assure you that one or more of the patents noted above would not be rescinded,
held invalid, successfully opposed or revoked or narrowed or held unenforceable.

Patenting biotechnology-related products and processes can involve uncertain and
complex legal and factual questions and, to date, policies regarding the breadth
of claims allowed in biotechnology patents are not necessarily consistent.
Patents, if issued, may be challenged, invalidated, limited in their scope of
coverage, circumvented or held unenforceable. Thus, any patents that we own or
license from third parties may not provide any protection against competitors.
Our pending patent applications, those we may file in the future, or those we
may license from third parties, may not result in patents being issued. Also,
patent rights may not provide us with proprietary protection or competitive
advantages against competitors with similar technology or different technology.
Furthermore, others may independently develop similar technologies or duplicate
any technology that we have developed. Moreover, the laws of certain foreign
countries do not protect our intellectual property rights to the same extent as
do the laws of the United States.

Litigation may be necessary to enforce any patents issued or licensed to us or
to determine the scope and validity of these patents. We could incur substantial
costs and divert technical and management personnel's time and attention if we
must participate in litigation or if we must defend ourselves against patent
suits against us. If the outcome of litigation is adverse to us, third parties
may be able to use our patented invention without paying us. Moreover, we cannot
assure you that our patents will not be infringed or successfully avoided
through design innovation. Any of these events may materially and adversely
affect our business.

In addition, other companies, some of which may be our competitors, have filed
applications for or have been issued patents, and may obtain additional patents
and proprietary rights, relating to products or processes used in, necessary to,
competitive with or otherwise related to our patents and products. These
products and processes include, among other items, patents covering technology
relating to the type of humanized monoclonal antibodies that we anticipate
developing. Protein Design Labs, Inc. owns certain patents and patent
applications relating to these humanized antibodies. We have recently taken a
non-exclusive license to these patents and patent applications for one of our
products. We do not know if licenses from Protein Design Labs will be available
for our other antibody products. The Medical Research Council also owns patents
relating to humanized antibodies, for which we hold a license. Our Medical
Research Council license includes a sublicense to the Boss patent relating to
antibody co-expression owned by Celltech, Ltd. Genentech has a pending
application (the Cabilly Application) that is involved in an interference before
the U.S. Patent & Trademark Office with the Celltech Boss patent.

We may also develop products that are chimeric antibodies. Genentech owns a
patent (the Cabilly Patent) relating to chimeric antibodies and instituted suit
against us in 1994 claiming that we infringed this patent. We settled the
lawsuit and, pursuant to the settlement, we acquired a non-exclusive license to
the Cabilly Patent, the Cabilly Application and other Genentech patents (or
patents to which Genentech has a license

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<PAGE>

and is free to grant a sublicense) relating to our anti-IgE antibody products.
We also have certain rights to acquire a non-exclusive license from Genentech
for the Cabilly Patent, the Cabilly Application and certain other Genentech
patents for products not exclusively or co-exclusively licensed by Genentech to
a third party and for certain products that do not compete with those of
Genentech or its affiliates. In addition, other parties also own patents
covering chimeric and/or deimmunized antibodies and/or processes applicable to
making these antibodies.

In addition, we hold a non-exclusive license to certain patents and patent
applications, including two U.S. patents, owned by Immunex Corporation, relating
to the G-CSF receptor. The patents and applications cover certain reagents that
may be involved in making our anti-G-CSF receptor antibodies and other products
we are developing, under which we must pay license execution and maintenance
fees, milestone payments for each developed product and royalties on net sales
of the product.

The scope, enforceability and validity of these patents, the extent to which we
must obtain licenses under these patents or under other proprietary rights and
the cost and availability of licenses are unknown, but these factors may limit
our ability to market our products. Moreover, even if a license were available,
the payments that would be required could render uneconomic our efforts to
market certain of our products. If we elect to manufacture or market these
products without either a license or a favorable result in litigation, damages
could be assessed that could be materially adverse to us. Further, failure to
obtain a license could result in an injunction prohibiting us from manufacturing
or selling the affected lines of products.

In addition to patents, we rely on trade secrets and proprietary know-how. We
seek protection, in part, through confidentiality and proprietary information
agreements. These agreements may not provide meaningful protection or adequate
remedies for our technology if unauthorized use or disclosure of this
information occurs. The parties to these agreements may breach them.
Furthermore, our trade secrets may otherwise become known to, or be
independently developed by, our competitors.

We require our employees, directors, consultants, advisors, outside scientific
collaborators and sponsored researchers and other advisors to execute
confidentiality agreements on commencing an employment, consulting or other
contractual relationships with us. These agreements provide that all
confidential information developed or made known to the individual during the
course of the relationship is to be kept confidential and not disclosed to third
parties except in specific circumstances. In the case of employees and certain
other parties, the agreements provide that all inventions conceived by the
individual shall be our exclusive property. We cannot assure you, however, that
these agreements will provide meaningful protection for our confidential
information or trade secrets against or in the event of unauthorized use or
disclosure of such information.

GOVERNMENT REGULATION

Producing and marketing our products and our research and development activities
are subject to regulations relating to product safety and efficacy by numerous
governmental authorities in the United States and other countries. In the United
States, drugs are subject to rigorous FDA regulation. The Federal Food, Drug and
Cosmetic Act and other federal and state statutes and regulations govern, among
other things, the testing, manufacture, safety, effectiveness, labeling,
storage, record keeping, approval, advertising and promotion of our products.

Before we may market a pharmaceutical product in the United States, the FDA
requires us to complete the following steps:

  o    preclinical laboratory and animal tests;

  o    submission to the FDA of an investigational new drug application, or IND,
       which must become effective before human clinical trials may commence;

  o    adequate and well controlled human clinical trials conforming with good
       laboratory and clinical practices to establish the safety and efficacy of
       the product;

  o    submission to the FDA of a New Drug Application, or NDA, with respect to
       drugs, and a BLA with respect to biological products; and

  o    FDA approval of the NDA or BLA before any commercial sale or shipment of
       the product.

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<PAGE>

In addition, the FDA requires the registration of each drug and approval of each
manufacturing establishment. For our monoclonal antibody products we are subject
to the simplified, interim procedure for well-characterized biologicals.
Domestic manufacturing establishments are subject to FDA inspection and must
comply with current good manufacturing practices, or cGMP, for pharmaceutical
products. To supply products for use in the United States, foreign manufacturing
establishments must comply with cGMP and are subject to periodic FDA or other
regulatory authority inspection under reciprocal agreements with the FDA.

Preclinical tests include laboratory evaluation and animal studies to assess the
potential safety and efficacy of the product and its formulation. To comply with
FDA regulations, laboratories must conduct these preclinical safety tests
according to Good Laboratory Practices. The results of the preclinical tests are
submitted to the FDA as part of an IND, and the FDA reviews the results before
the commencement of human clinical trials. Unless the FDA objects, the IND will
become effective 30 days following its receipt. There is no certainty that
submission of an IND will result in FDA authorization to commence clinical
trials. If we will ship a biological product produced within the United States
to a foreign country for clinical trials, the product must comply with export
regulations promulgated by the FDA before shipment.

Human clinical trials involve the administration of the investigational compound
to patients or other volunteers under the supervision of a qualified principal
investigator. Clinical trials are conducted in accordance with protocols that
detail the objectives of the study, the parameters to be used to monitor safety
and the efficacy criteria to be evaluated. Each protocol must be submitted to
the FDA as part of the IND. Further, each clinical study must be conducted under
the auspices of an independent institutional review board, or IRB, at the
institution where the study will be conducted. The IRB will consider, among
other things, ethical factors, the safety of human subjects and the possible
liability of the institution.

Clinical trials are typically conducted in four sequential phases, which may
overlap. In Phase I, the initial introduction of the product into human
subjects, the product is tested for safety (adverse effects), dosage tolerance,
metabolism, distribution, excretion and clinical pharmacology. Phase II involves
studies in a limited patient population:

  o    to determine the efficacy of the product for specific, targeted
       indications;

  o    to determine dosage tolerance and optimal dosage; and

  o    to identify possible adverse effects and safety risks.

When a product is found to be effective and to have an acceptable safety profile
in Phase II evaluations, Phase III trials are undertaken:

  o    to continue to evaluate clinical efficacy; and

  o    to test further for safety within an expanded patient population at
       geographically dispersed clinical study sites.

We cannot assure you that we will successfully complete clinical testing of our
products within any specified time period, if at all. Furthermore, the FDA or we
may suspend clinical trials at any time if it is felt that the subjects or
patients are being exposed to an unacceptable health risk. Phase IV studies are
typically done post-FDA approval to address safety issues not addressed in the
Phase I/II/III programs, for example, chronic use of the product.

In the case of agents for life-threatening diseases, the initial human testing
is generally done in patients rather than in healthy volunteers. Since these
patients already are afflicted with the target disease, it is possible that
these studies may provide results traditionally obtained in Phase II trials,
potentially expediting the approval process. These trials are frequently
referred to as "Phase I/II" trials.

The results of the pharmaceutical development, preclinical studies and clinical
studies are submitted to the FDA in the form of an NDA or BLA to approve
marketing and commercial shipment of the product. The testing and approval
process frequently requires substantial time and effort and we cannot assure you
that any approval will be granted on a timely basis, if at all. The FDA may deny
an NDA or BLA if applicable regulatory criteria are not satisfied, require
additional testing or information or require postmarketing testing and
surveillance to monitor the safety and efficacy of the product. Notwithstanding
the submission of this data, the FDA may ultimately decide that the application
does not satisfy its regulatory criteria for

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<PAGE>
approval. Finally, product approvals may be withdrawn if compliance with
regulatory standards is not maintained or if problems occur following initial
marketing.

Among the conditions for NDA or BLA approval is that the prospective
manufacturer's quality control and manufacturing procedures conform to cGMP. In
complying with standards set forth in these regulations, manufacturers must
continue to expend time, monies and effort in the area of production and quality
control to ensure full compliance.

In addition to FDA regulations, we are subject to regulation under the
Occupational Safety and Health Act, the Environmental Protection Act, the Toxic
Substances Control Act, the Resource Conservation and Recovery Act and other
present and future federal, state or local regulations.

For marketing outside the United States, we also are subject to foreign
regulatory requirements governing human clinical trials and marketing approval
for pharmaceutical products. The requirements governing the conduct of clinical
trials, product licensing, pricing and reimbursement vary widely from country to
country. Whether or not we obtain FDA approval,we must obtain approval of a
product by the comparable regulatory authorities of foreign countries before
manufacturing or marketing the product in those countries. The approval process
varies from country to country and the time required for these approvals may
differ substantially from that required for FDA approval. We cannot assure you
that clinical trials conducted in one country will be accepted by other
countries or that approval in one country will result in approval in any other
country. For clinical trials conducted outside the United States, the clinical
stages are comparable to the phases of clinical development established by the
FDA.

COMPETITION

The pharmaceutical and biotechnology industries are characterized by rapidly
evolving technology and intense competition. Many companies, including major
pharmaceutical and chemical companies, as well as specialized biotechnology
companies, perform activities similar to ours. Many of these companies have
substantially greater financial and other resources, larger research and
development staffs and more extensive marketing and manufacturing organizations
than ours. Many of these companies have significant experience in preclinical
testing, human clinical trials and other regulatory approval procedures.
Consequently, we chose to enter into the collaboration agreements with Novartis
and Genentech, in part to secure the benefit of their experience in these areas,
as well as the contribution of their greater financial resources. In addition,
colleges, universities, governmental agencies and other public and private
research organizations conduct research and may market commercial products on
their own or through joint ventures. These institutions are becoming more active
in seeking patent protection and licensing arrangements to collect royalties for
using technology that they have developed. We compete with these institutions in
recruiting and retaining highly qualified scientific personnel.

The diseases that we have targeted, including allergic diseases, autoimmune
diseases, transplantation, cancer, inflammation, and HIV infection, are
intensely competitive areas targeted by both pharmaceutical companies and other
biotechnology companies, including our collaborators, Novartis and Genentech.
All of these companies may have competitive products on the market, may be
testing their products in clinical trials or may be focusing on product
approaches that could prove to be superior to our approaches. For instance, we
are aware that some of these companies, which may be our competitors, have filed
applications for or have been issued patents and may obtain additional patent
and proprietary rights relating to products or processes used in, necessary to,
competitive with or otherwise related to, our products or processes. These
patents include, among other items, patents relating to humanized monoclonal
antibodies.

Additionally, our competition will be determined in part by the potential
indications for which our antibodies are developed and ultimately approved by
regulatory authorities. For some of our potential products, an important factor
in competition may be the timing of market introduction of our products or
competitive products. Accordingly, we expect the relative speed with which we
develop our products, complete the necessary approval processes and are able to
generate and market commercial quantities of the products to be important
competitive factors. We expect that competition among products approved for sale
will also be based, among other factors, on product efficacy and safety, timing
and scope of regulatory

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<PAGE>
approval, product availability, advantages over alternative treatment methods,
price and cost-effectiveness, development, distribution and marketing
capabilities, third-party reimbursement and patent position.

We are aware that several companies, including Novartis, have existing products
that will compete with E25, if it is approved for sale, including
corticosteriods, beta-agonists, antihistamines, leukotriene inhibitors and
allergen immunotherapy. In addition, several companies have products in
development that may compete with E25. These companies include, but are not
limited to, IDEC (Anti-CD23), Immunex (sIL-4R), CellTech/Schering-Plough
(Anti-IL5), Merck/Biogen (VLA-4 inhibitors), Magainin/Genentech (anti-IL-9) and
Protein Design Labs (Anti-IL4).

Our competitive position also depends upon our ability to:

  o    attract and retain qualified personnel;

  o    obtain patent protection or otherwise develop proprietary products or
       processes;

  o    discover new therapeutic products that successfully treat human diseases;

  o    secure sufficient capital resources to complete product development and
       regulatory processes;

  o    build or obtain manufacturing facilities; and

  o    build or obtain a sales organization.

MANUFACTURING

We have a small-scale production and purification facility in which we have
produced our products in compliance with cGMP standards for use in Phase I
and/or Phase II clinical trials. With funding from Novartis, we recently
completed construction of a pilot manufacturing facility that we may use for
larger-scale process development and cGMP production of animal cell culture
derived products. The facility includes a 1500L bioreactor and occupies
approximately 14,000 square feet of space now under lease to us in Houston,
Texas. The new manufacturing facility is not yet operational, and the Company is
pursuing required cGMP validation. The facility is available to both us and
other companies for production of monoclonal antibodies and other biologic
products for large-scale clinical trials and initial market launch.

Under our agreements with Novartis and Genentech, Novartis and Genentech must
manufacture E25 and any other anti-IgE products selected by us and our
collaboration partners for development, although we have retained the right to
manufacture and supply up to 50% of the worldwide requirements for E25 and the
other selected products. Genentech will supply the initial quantities of E25 for
product launch. Novartis has announced that it intends to supply E25 from a
facility now under construction that has a capacity of more than one ton of
active substance per year.

Our current facility will not be adequate for commercial scale manufacturing
requirements if we successfully develop our products. If we decide to establish
a full-scale manufacturing facility, we will require substantial additional
funds and must hire and train significant numbers of employees and comply with
the extensive FDA regulations applicable to that facility.

MARKETING AND SALES

Novartis and Genentech will market E25 and the other products selected for
development by the collaboration. Novartis and Genentech share U.S. marketing
rights, and Novartis has marketing rights in Europe (with Roche retaining the
option to participate in Europe). Novartis can market these products in the rest
of the world, including China, Hong Kong, Korea, Singapore and Taiwan, where we
will share costs and profits with Novartis.

To effectively serve the worldwide markets, we intend to continue to collaborate
with major pharmaceutical companies or prominent pharmaceutical sales and
distribution organizations that can successfully market our products on a
worldwide basis or within specific geographic territories. As we pursue
strategic collaborations, we intend to reserve marketing rights for our
products, to the extent commercially reasonable, including rights in the United
States and selected Asian countries. We will focus initially on markets for
which our products have a clear advantage over other therapies or which we may
target using a

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relatively small sales force. We currently do not have an internal sales and
marketing capability. If we elect to retain marketing rights, we will have to
build a sales and marketing infrastructure.

FACILITIES

We currently lease 35,624 square feet of laboratory and office space in Houston,
Texas, under a lease which expires in March 2002, subject to a five-year renewal
option. This space includes a biological product manufacturing facility
occupying approximately 14,000 square feet of space. We lease approximately
2,690 square feet of temporary laboratory and office space in the Amsterdam
Science Park in The Netherlands and intend to lease approximately 10,760 square
feet of space there when it becomes available in 2001. We also lease
approximately 6,500 square feet of space in the Hsinchu Science Based Industrial
Park in Taiwan.

We plan to obtain temporary additional space, before the end of 2000, to expand
our research, clinical development and production capabilities. We are exploring
alternatives to meet our longer term facility requirements for our U.S.
operations. We do not foresee any significant difficulties in obtaining
additional facilities.

HUMAN RESOURCES

Including the employees of our subsidiaries, we have 71 full-time employees, 58
of whom are based in the United States, 12 of whom are based in The Netherlands
and one of whom is based in Taiwan. Approximately 55 of our employees are
involved in research and product development activities. Twenty-nine of our
employees hold Ph.D., M.D. or Sc.D. degrees and ten employees hold other
advanced degrees. We consider our relations with our employees to be good. None
of our employees are covered by collective bargaining agreements. We enter into
confidentiality agreements with all of our employees, directors and consultants.
We do not maintain, and do not currently intend to obtain, key employee life
insurance on any of our personnel.

PENDING LEGAL PROCEEDINGS

TANOX BIOSYSTEMS, INC. VS. AKIN, GUMP, STRAUSS, HAUER AND FELD, L.L.P., THE
ROBINSON LAW FIRM AND WILLIAMS, BIRNBERG & ANDERSON, AMERICAN ARBITRATION
ASSOCIATION NO. 70-199-0167-96.  We are arbitrating a fee dispute with the law
firms that represented us in connection with a lawsuit involving Genentech and
Roche relating to, among other things, the intellectual property rights of the
parties surrounding the development of anti-IgE technology. We settled the
litigation contemporaneously with the formation of our collaboration with
Genentech.

We initiated the arbitration proceeding after we and our attorneys could not
reach agreement on the fee owed pursuant to the terms of our written fee
agreement. The arbitration panel issued an award entitling the attorneys to
receive approximately $3.5 million, including interest, payments ranging from
33 1/3% to 40% of the future milestone payments received by us from Genentech
under the collaboration following product approval and 10% of the royalties that
we receive on sales of anti-IgE products.

We sought a court order vacating the arbitration award on the grounds that the
arbitration decision exceeds the scope of the fee agreement in dispute, deviates
materially from the evidence presented and is unsupportable as a matter of law.
However, a judgment was entered confirming the award. We intend to pursue all
available remedies, including appealing the decision.

TANOX, INC. VS. NOVARTIS PHARMA AG, AMERICAN ARBITRATION ASSOCIATION NO. 50 T
153 00119 99; GENENTECH, INC., GENENTECH INTERNATIONAL LIMITED AND NOVARTIS
PHARMA AG VS. TANOX, INC., CASE NO. C 99-2060; AND TANOX, INC. VS. GENENTECH,
INC. AND GENENTECH INTERNATIONAL LIMITED, AMERICAN ARBITRATION ASSOCIATION NO.
74 Y 181 1113 99.  We are currently pursuing clinical development of Hu-901 to
determine its potential in treating peanut induced anaphylaxis independently of
Novartis and Genentech. Novartis and Genentech have disputed our right to pursue
development of Hu-901 independently and have claimed that we are using
unspecified confidential and proprietary information of Novartis and Genentech
that we have no right to use. We believe our agreements with Novartis and
Genentech allow us to pursue the development of Hu-901.

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In an effort to resolve this dispute, we initiated an arbitration with Novartis
in March 1999 pursuant to our agreement with Novartis. In the arbitration, we
are seeking to confirm our rights to independently develop certain anti-IgE
products, including Hu-901, and to use know-how we received from Novartis.
Novartis has claimed that the dispute does not constitute a dispute which we
must arbitrate under our agreements and that our claimed rights to independently
develop Hu-901 do not exist. Novartis has also claimed damages arising from our
action.

In response to the arbitration initiated by us against Novartis, Novartis and
Genentech jointly filed suit against us in April 1999 in the United States
District Court for the Northern District of California. In the lawsuit, Novartis
and Genentech seek declarations that we cannot develop Hu-901 independently,
that we cannot use confidential and proprietary information obtained from
Novartis or Genentech for independent product development, and that we cannot
pursue separate arbitrations on these matters against both Novartis and
Genentech. Novartis and Genentech also claim undetermined actual and punitive
damages resulting from our independent development of Hu-901, and they also seek
a permanent injunction stopping our Hu-901 development and preventing us from
continuing with our arbitrations on these matters.

At the time the lawsuit was filed, Novartis also asked the Federal court for an
emergency temporary stay of the arbitration requested by us. The United States
District Court Judge denied Novartis' request and ordered Novartis to proceed
with the arbitration.

In July 1999, we initiated an arbitration proceeding against Genentech. In this
arbitration, we are seeking to confirm that Genentech expressly acknowledged our
independent development rights in our agreement with them and that Genentech
agreed to allow us to use and disclose their confidential and proprietary
information for purposes contemplated by our separate agreement with Novartis.
We also are seeking to confirm that we have not used any of their confidential
and proprietary information and that Genentech's lawsuit claims are
unsupportable and made in bad faith, have impaired our ability to exercise our
rights under our agreements with Novartis, violate our agreed dispute resolution
procedures and violate their agreement not to interfere in our separate disputes
with Novartis. In response, Genentech asserts that their disputes with us are
not subject to arbitration, and should remain in Federal court. Additionally,
Genentech asserts that our arbitration with Novartis should be joined with
Genentech's arbitration.

In September 1999, the United States District Court Judge issued an order
staying all proceedings in the lawsuit and both arbitrations, except for the
parties' opportunity to engage in limited written discovery in the form of
requests for production of documents and written questions to each other. The
Judge's order required the parties, with their respective chief executive
officers present, to undertake to mediate all matters in dispute between them.
The mediation took place on November 2 and 3, 1999 in San Francisco, California,
and concluded unsuccessfully. To continue our arbitration proceedings, we must
ask the court to allow our respective arbitrations with Novartis and Genentech
to proceed.

We intend to continue to pursue independent development of Hu-901 during the
pendency of the above-described actions. If we are unsuccessful in these
actions, we may not independently develop Hu-901 and other anti-IgE products
covered by the collaboration with Novartis and Genentech and could be required
to pay damages that could be significant.

                                       46

<PAGE>
                                   MANAGEMENT

DIRECTORS, EXECUTIVE OFFICERS AND KEY EMPLOYEES

Our directors, executive officers and key employees, and their ages and
positions as of March 7, 2000 are:


NAME                                  AGE            POSITION
- -----------------------------------   --- ------------------------------
Nancy T. Chang, Ph.D...............   50  Chairman of the Board,
                                          President and Chief Executive
                                            Officer
John C. Morris.....................   56  Senior Vice President of
                                          Operations
David W. Thomas, Ph.D. ............   52  Senior Vice President of
                                          Research and Development
David Duncan, Jr...................   51  Vice President of Finance and
                                          Chief Financial Officer
John Blickenstaff..................   46  Vice President of
                                          Administration, Secretary and
                                            Treasurer
George Y. Wang, Sc.D...............   46  Vice President of Process
                                          Development and Production
Eric P. Mirabel, J.D., LL.M. ......   43  Vice President of Intellectual
                                          Property
Tse Wen Chang, Ph.D.(1)(2).........   52  Director
Osama I. Mikhail, Ph.D.(1)(2)......   52  Director
William J. Jenkins, M.D.(2)........   52  Director

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

(1)  Member of the compensation committee.

(2)  Member of the audit committee.

NANCY T. CHANG, PH.D. is one of our co-founders and has served as our President
and Chairman of the Board of Directors since our organization in March 1986. Dr.
Chang has served as our Chief Executive Officer since June 1990. From 1986 to
1992, Dr. Chang served as an Associate Professor at Baylor College of Medicine
in the Division of Molecular Virology. Between 1981 and 1986, Dr. Chang was
employed by Centocor, Inc., serving as the Director of Research, Molecular
Biology Group, from 1984 to 1986. From 1980 to 1981, she was employed by Roche
Institute of Molecular Biology. Dr. Chang received her Ph.D. in biological
chemistry from Harvard University.

JOHN C. MORRIS has served as our Senior Vice President of Operations since
September 1997. From September 1996 to September 1997, he served as our Vice
President of Quality Assurance. Prior to coming to us, Mr. Morris served as
President at Oread Laboratories, Inc. from 1995 to 1996. From 1992 to 1995, Mr.
Morris served as Vice President of Production Development, Regulatory Affairs
and Quality Assurance at Sanofi Animal Health Inc. Mr. Morris received an M.S.
in microbial biochemistry from the University of Missouri.

DAVID W. THOMAS, PH.D. has served as our Senior Vice President of Research and
Development since November 1997. Prior to joining us, Dr. Thomas served as Vice
President of Biological Research at Hybridon, Inc. in 1997. From 1988 to 1997,
he served as Director of Cell Biology and Immunology at Biogen, Inc. Prior to
his positions at Biogen and Hybridon, Dr. Thomas held professorships at the
University of Michigan Medical School and Washington University in St. Louis,
Missouri. He received his Ph.D. degree in microbiology and immunology from the
University of Colorado Health Sciences Center, followed by a postdoctoral
fellowship in the Laboratory of Immunology of the National Institute of Allergy
and Infectious Disease at the National Institutes of Health.

DAVID DUNCAN, JR. has served as our Vice President of Finance and Chief
Financial Officer since August 1998. Prior to joining us, from 1994 to March
1998, he served as Chief Financial Officer at Neuromedical Systems, Inc., which
filed a voluntary petition for bankruptcy under Chapter 11 of the U.S.
bankruptcy code on March 26, 1999. From 1988 to 1994, Mr. Duncan served as Chief
Financial Officer at Telios

                                       47
<PAGE>
Pharmaceuticals, Inc. From 1983 to 1988, he served as the Controller at
Hybritech Incorporated. Mr. Duncan received an MBA from Indiana University.

JOHN BLICKENSTAFF has served as our Vice President of Administration since April
1989 and has also served as Vice President of Finance from that time until March
1996. Mr. Blickenstaff also served as our Chief Financial Officer from June 1990
until March 1996. He joined us in March 1987 as Director of Finance and
Administration, becoming our Secretary and Treasurer in September 1987. Between
1984 and 1987, Mr. Blickenstaff served as Operations Manager at Montgomery
Engineering Company. Mr. Blickenstaff holds a B.S. in health sciences and an MBA
from Brigham Young University.

GEORGE Y. WANG, SC.D. has served as our Vice President of Process Development
and Production since October 1994. He joined us in 1991 as our Assistant
Director, BioProcessing Development. Prior to joining us, Dr. Wang held various
engineering and management positions at IDEC Pharmaceuticals Corp., BP Amoco
Corporation and MGI Pharmaceuticals, Inc. Dr. Wang received his Sc.D. in
biochemical engineering from Massachusetts Institute of Technology.

ERIC MIRABEL has served as our Vice President of Intellectual Property since
1994. He joined us in 1990 as our Patent Counsel. From 1986 to 1990 Mr. Mirabel
practiced intellectual property law with the Houston firm of Butler and Binion.
Mr. Mirabel received a B.S. in biochemistry from the University of British
Columbia, a J.D. from Northwestern School of Law at Lewis and Clark College, and
an LL.M. in patent and trade regulation law from the National Law Center at
George Washington University.

TSE WEN CHANG, PH.D. is one of our co-founders and has served as a member of our
board of directors since our organization in March 1986. Dr. Chang served as our
Vice President of Research and Development and Chief Scientific Officer from
March 1986 until January 1997 when he resigned that position to assume the
position of Dean of the College of Life Sciences at National Tsing Hua
University in Hsinchu, Taiwan. Dr. Chang is currently a professor there. Dr.
Chang was a professor at Baylor College of Medicine in the Division of Molecular
Virology from 1986 to 1991. From 1984 to 1986, Dr. Chang served as Vice
President of Research at Centocor, Inc. Dr. Chang obtained his Ph.D. from
Harvard University in cell and developmental biology.

OSAMA MIKHAIL, PH.D. has served as a member of our board of directors since
1994, and also has served as a consultant to us since 1993. Dr. Mikhail is
currently Senior Vice President and Chief Strategic Officer at St. Luke's
Episcopal Health System and Professor of Management and Policy Sciences at the
University of Texas, School of Public Health, both in the Texas Medical Center,
Houston, Texas. Dr. Mikhail has been associated with St. Luke's and the
University of Texas School of Public Health since 1989. Dr. Mikhail received an
MBA from the University of Pennsylvania's Wharton School and an M.S. and Ph.D.
from the Graduate School of Industrial Administration, Carnegie-Mellon
University.

WILLIAM J. JENKINS, M.D. has served as a member of our board of directors since
November 1999. Since the beginning of 1999, Dr. Jenkins has been a strategic
consultant to the pharmaceutical industry, primarily at Hoffman-La Roche Inc.
Prior to that, he served as Head of Clinical Development & Regulatory Affairs
and a member of the board of directors of Novartis Pharma AG in Basel,
Switzerland since 1992. Dr. Jenkins served as Head of Clinical Research for the
Glaxo Group from 1988 to 1992. Dr. Jenkins received his medical degrees from the
University of Cambridge.

                                       48
<PAGE>
SCIENTIFIC ADVISORS

An important component of our scientific strategy is to establish collaborative
relationships with leading researchers in our fields of interest. Certain of our
scientific advisors attend periodic meetings and provide us with specific
expertise in both research and clinical development. In addition, we have
collaborative research relationships with certain individual advisors. We do not
employ our scientific advisors, and they may have commitments to or consulting
or advisory agreements with other entities that may limit their availability to
us. These companies may also compete with us. Several of our advisors have, from
time to time, devoted significant time and energy to our affairs. In general,
our scientific advisors may hold stock options, own our stock and/or receive
financial remuneration for their services.

Our scientific advisors include:


NAME                                        TITLE AND AFFILIATION
- -------------------------------------------------------------------------
K. Frank Austen, M.D................ Director, Inflammation and Allergic
                                     Diseases
                                     Brigham and Women's Hospital, Boston,
                                     Massachusetts
James Larrick, M.D., Ph.D........... Managing Director and Founder,
                                     Panorama Research, Inc.
Ethan M. Shevach, M.D............... Chief, Cellular Immunology Section,
                                     National Institutes of Health

COMPOSITION OF THE BOARD OF DIRECTORS

Our amended and restated certificate of incorporation and bylaws divide our
board of directors into three classes of equal number: Classes I, II and III.
Our stockholders elected directors comprising Class I to a term of office to
expire at the 2001 annual meeting of stockholders; directors comprising Class II
to a term of office to expire at the 2002 annual meeting of stockholders; and
directors comprising Class III to a term of office to expire at the 2003 annual
meeting of stockholders. At each annual meeting of stockholders beginning in
2001, the stockholders will elect the successors to directors whose terms will
then expire. These directors will serve from the time of election and
qualification until the third annual meeting of stockholders following election
and until a successor is duly elected and qualified.

Our bylaws also authorize the board of directors to fix the number of directors
at not less than one nor more than nine. The board of directors currently has
four members. The board of directors is considering an increase in the number of
directors. Currently, Osama Mikhail is designated as a Class I director, whose
term expires at the 2001 annual meeting of stockholders; Tse Wen Chang is
designated as a Class II director, whose term expires at the 2002 annual meeting
of stockholders; and Nancy T. Chang and William J. Jenkins are designated as
Class III directors, whose terms expire at the 2003 annual meeting of
stockholders. Each officer serves at the discretion of the board of directors.

There are no family relationships among any of our directors, executive officers
or key employees, although Drs. Nancy T. Chang and Tse Wen Chang previously were
married.

Our bylaws also require the affirmative vote of holders of at least two-thirds
of the issued and outstanding shares of common stock to remove any director or
the entire board of directors. Directors can only be removed for "cause."

DIRECTOR COMPENSATION

As compensation for serving on the board of directors, we granted Dr. Mikhail
options to acquire 108,000 shares of common stock, 96,000 of which were granted
at an exercise price per share of $1.04 and are all currently exercisable, 8,000
of which were granted at an exercise price per share of $4.06, none of which are
currently exercisable, and 4,000 of which were granted at an exercise price per
share of $12.50, none of which are currently exercisable. All of his options
vest over a three-year period. On February 17, 2000, we granted Dr. Jenkins
options to acquire 24,000 shares of common stock at an exercise price per share
of $12.50, none of which are currently exercisable, and all of which vest over a
two-year period. Outside directors also receive $1,000 for each board meeting
attended and $500 for each committee meeting attended, except for Dr. Tse Wen
Chang, who is compensated under a consulting agreement with us. Other

                                       49
<PAGE>
than the foregoing, the directors receive no other compensation for their
services as directors. We reimburse our outside directors, on request, for
reasonable out-of-pocket expenses incurred in attending board meetings.

COMMITTEES OF THE BOARD OF DIRECTORS

Our board of directors currently has two committees: an audit committee and a
compensation committee.

The audit committee makes recommendations to our board of directors regarding
the selection of independent auditors, reviews the results and scope of audit
and other services provided by our independent auditors and reviews the
accounting principles and auditing practices and procedures to be used for our
financial statements.

The compensation committee reviews and makes recommendations to our board of
directors regarding the compensation of officers and other managerial employees.
The compensation committee also considers and recommends grants of stock options
under our stock option plans.

COMPENSATION COMMITTEE INTERLOCKS AND INSIDER PARTICIPATION

The compensation committee made decisions concerning the compensation of our
executive officers for the years ended December 31, 1997, 1998 and 1999, during
which time Nancy T. Chang, our Chairman of the Board, President and Chief
Executive Officer has been a member of the committee. No interlocking
relationship exists between any member of our board of directors or our
compensation committee and any member of the board of directors or compensation
committee of any other company.

EXECUTIVE COMPENSATION

                           SUMMARY COMPENSATION TABLE

The following table sets forth the annual compensation we paid during the year
1999 to our chief executive officer and the four highest paid executive officers
whose total annual salary and bonus exceeded $100,000. These individuals are
referred to as the "named executive officers" here and elsewhere in this
prospectus.

<TABLE>
<CAPTION>
                                                    ANNUAL COMPENSATION
                                        -------------------------------------------
                                                                      OTHER ANNUAL
NAME AND PRINCIPAL POSITION               SALARY          BONUS       COMPENSATION
- -------------------------------------   -----------    -----------    -------------
<S>                                     <C>            <C>            <C>
Nancy T. Chang.......................    $ 368,622       $58,000         $12,132(1)
  Chairman of the Board, President
  and Chief Executive Officer
John C. Morris.......................      154,587        10,000          --
  Senior Vice President of Operations
David W. Thomas......................      199,548        10,000          --
  Senior Vice President of Research
  and Development
David Duncan, Jr.....................      164,221         5,000          10,569(2)
  Vice President of Finance and
  Chief Financial Officer
George Y. Wang.......................      134,359        --              --
  Vice President of Process
  Development
  and Production
</TABLE>

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

(1)  Dr. Chang's other annual compensation includes $7,549 of taxable income
     associated with a company car, $4,000 associated with 401(k) matching and
     $583 relating to group term life insurance.

(2)  Mr. Duncan's other annual compensation includes $8,800 related to housing
     expenses, $1,054 associated with 401(k) matching and $715 relating to group
     term life insurance.

                                       50
<PAGE>
                               1999 OPTION GRANTS

The following table sets forth information regarding options granted to each of
our named executive officers during the year 1999. The exercise prices of the
options we granted were the fair market value of our common stock on the date of
grant, as determined by the compensation committee of our board of directors. In
determining the fair market value, the compensation committee considered
information contained in a then current valuation report made for tax purposes,
market conditions, business prospects, and the absence of a market for our
common stock.

The potential realizable value is calculated based on the ten-year term of the
option at the time of grant. Stock price appreciation of 5% and 10% is assumed
pursuant to rules promulgated by the Securities and Exchange Commission and does
not represent our prediction of our stock price performance. The potential
realizable values at 5% and 10% appreciation are calculated by:

  o    multiplying the number of shares of common stock under the option by
       $28.50 per share;

  o    assuming that the aggregate stock value derived from that calculation
       compounds at the annual 5% or 10% rate shown in the table until the
       expiration of the options; and

  o    subtracting from that result the aggregate option exercise price.

The options in this table were granted under our 1997 Stock Plan, have 10-year
terms, will terminate before their expiration dates if the optionee leaves his
employment with us, and, unless otherwise noted, vest over a period of five
years. We have not granted any stock appreciation rights.

The percentage shown below of options granted is based on an aggregate of
117,120 options we granted to employees during 1999.

<TABLE>
<CAPTION>
                                                                                                               POTENTIAL REALIZABLE
                                                                                                                VALUES AT ASSUMED
                                         NUMBER OF      PERCENT OF TOTAL                                      ANNUAL RATES OF STOCK
                                         SECURITIES     OPTIONS GRANTED                                       PRICE APPRECIATION FOR
                                         UNDERLYING       TO EMPLOYEES                                             OPTION TERM
                                          OPTIONS          IN FISCAL         EXERCISE PRICE      EXPIRATION   ----------------------
                NAME                      GRANTED          YEAR 1999           PER SHARE            DATE         5%          10%
- -------------------------------------   ------------    ----------------    ----------------    ------------  ---------  -----------
<S>                                     <C>             <C>                 <C>                 <C>           <C>        <C>
Nancy T. Chang.......................          --           --                  --                   --          --          --
John C. Morris(1)....................      16,000             13.7%              $ 8.13           01/25/09    $ 612,696  $ 1,052,667
                                            8,000              6.8                12.50           12/01/09      271,388      491,373
David W. Thomas(2)...................      16,000             13.7                 8.13           01/25/09      612,696    1,052,667
                                           16,000             13.7                12.50           12/01/09      542,776      982,747
David Duncan, Jr.....................          --           --                  --                   --          --          --
George Y. Wang(3)....................       4,000              3.4                12.50           12/01/09      135,694      245,687
</TABLE>

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

(1)  The vesting start date for the options to purchase 16,000 shares of common
     stock granted to Mr. Morris is January 25, 1999. 9,600 of these options
     vest over a period of three years and the remaining 6,400 of these options
     vest over a period of five years. The vesting start date for the options to
     purchase 8,000 shares of common stock granted to Mr. Morris is December 1,
     1999. 4,800 of these options vest over a period of three years and the
     remaining 3,200 of these options vest over a period of five years.

(2)  The vesting start date for the options to purchase 16,000 shares of common
     stock granted to Mr. Thomas is January 25, 1999. The vesting start date for
     the options to purchase an additional 16,000 shares of common stock granted
     to Mr. Thomas is December 1, 1999.

(3)  The vesting start date for these options is December 1, 1999.

                                       51
<PAGE>
                               1999 OPTION VALUES

The following table describes for the named executive officers the exercisable
and unexercisable options held by them as of December 31, 1999. No options were
exercised by the named executive officers during the fiscal year ended December
31, 1999. The "Value of Unexercised In-the-Money Options at December 31, 1999"
shown in the table is based on an initial public offering price of $28.50 per
share, less the per share exercise price, multiplied by the number of shares
issued upon exercise of the option.

<TABLE>
<CAPTION>
                                             NUMBER OF SECURITIES
                                        UNDERLYING UNEXERCISED OPTIONS         VALUE OF UNEXERCISED
                                                                             IN-THE-MONEY OPTIONS AT
                                             AT DECEMBER 31, 1999               DECEMBER 31, 1999
                                        ------------------------------    ------------------------------
NAME                                    EXERCISABLE     UNEXERCISABLE     EXERCISABLE     UNEXERCISABLE
- -------------------------------------   ------------    --------------    ------------    --------------
<S>                                     <C>             <C>               <C>             <C>
Nancy T. Chang.......................      260,800          391,200        $ 5,476,800      $8,215,200
John C. Morris.......................       51,199          100,801          1,075,179       2,066,821
David W. Thomas......................       80,000          152,000          1,980,000       3,552,000
David Duncan, Jr.....................       38,399          153,601            782,380       3,129,620
George Y. Wang.......................      201,600            4,000          5,523,171          64,000
</TABLE>

EMPLOYMENT AGREEMENTS

We do not have employment agreements with our executive officers, other than
agreements that we maintain with all of our employees and option agreements
under which we issue incentive and non-qualified stock options to employees.

EMPLOYEE BENEFIT PLANS

1987 STOCK OPTION PLAN

Our 1987 Stock Option Plan expired on June 24, 1997. This plan provided for the
grant of incentive stock options, which were intended to qualify for favorable
tax treatment, and non-qualified stock options to eligible parties who were
employees. Our board of directors adopted and our stockholders approved the plan
on June 25, 1987. Under the terms of the plan, as amended in April 1989, we
could issue options on 4,320,000 shares of our common stock.

Special provisions applied to incentive stock options granted under the plan,
including requirements that the exercise price of incentive stock options be at
least equal to the fair market value of the common stock on the date of the
grant. There was also a $100,000 limit on the value of stock, determined at the
time of grant, covered by incentive stock options that first become exercisable
by a holder in any calendar year.

No person could receive an incentive stock option under the plan if, at the time
of grant, the person owned directly or indirectly more than 10% of our total
combined voting power. This restriction did not apply, however, if the option
price was at least 110% of the fair market value of the common stock, and the
exercise period was limited to five years.

If a reorganization, recapitalization, stock dividend, merger, consolidation or
other change in corporate structure affecting the number of issued shares of our
common stock occurred, then our board of directors could make equitable
adjustments to the terms of this plan. In particular, the board could make an
equitable adjustment in the number and type of shares authorized by this plan,
the number and type of shares covered by outstanding awards under this plan and
the exercise prices of these awards. After the adjustments, any incentive stock
options granted under the plan must have continued to constitute incentive stock
options under applicable tax laws. Our board of directors could amend or
terminate this plan at any time, although certain amendments required
stockholder approval and an amendment or termination could not adversely affect
any rights under outstanding stock options without the holder's consent.

                                       52
<PAGE>

For the year ended December 31, 1999, we granted no options, we cancelled
options to purchase 32,000 shares of common stock and the holders of options to
purchase 1,738,320 shares of common stock exercised those options. As of
December 31, 1999, options to purchase an aggregate of 2,242,800 shares of
common stock held by 26 employees were outstanding under the plan at exercise
prices ranging from $0.28 to $7.50 per share with a weighted average exercise
price of $4.03 per share. These options consist of incentive stock options to
purchase 856,264 shares at a weighted average exercise price of $3.78 per share
and non-qualified options to purchase 1,386,536 shares at a weighted average
exercise price of $4.19 per share. At December 31, 1999, no shares were
available for future option grants under the plan. After December 31, 1999,
employees exercised options to purchase 379,800 shares of common stock.

1997 STOCK PLAN

The board of directors adopted our 1997 Stock Plan on September 19, 1997, and
our stockholders approved the plan to be effective as of November 1, 1997. This
plan provides for the grant of incentive stock options to our employees and the
grant of non-qualified stock options, awards of stock, stock appreciation
rights, purchase rights and performance units to all types of eligible parties.
The board of directors or, in the discretion of the board, the compensation
committee or other committee appointed by the board of directors, consisting of
at least two members of the board of directors administers the plan. We may
grant options or other rights under the plan to some of our and our
subsidiaries' directors, employees, consultants and advisors. We may also, with
the consent of the holder, convert a holder's incentive stock options into
on-qualified stock options. We have reserved 8,000,000 shares of authorized but
unissued common stock for issuance under the plan. The plan extends for a
10-year period, beginning November 1, 1997.

Special provisions apply to incentive stock options granted under the plan in a
manner that is similar to those granted under the 1987 Stock Option Plan. The
1997 Stock Plan also contains adjustment provisions that are similar to those in
the 1987 Stock Option Plan.

For the year ended December 31, 1999, we granted options to purchase an
aggregate of 125,120 shares of common stock under the 1997 Stock Plan at a
weighted average exercise price of $10.60 per share. This includes incentive
stock options to purchase 102,720 shares at a weighted average exercise price of
$10.59 per share and non-qualified options to purchase 22,400 shares at a
weighted average exercise price of $10.63 per share. In 1999, options to
purchase 3,200 shares of common stock were exercised and options to purchase
8,000 shares were cancelled.

As of December 31, 1999, 30 individuals held options to purchase a total of
636,320 shares of common stock under the plan at exercise prices ranging from
$3.75 to $12.50 per share with a weighted average exercise price of $7.05 per
share. These options consist of incentive stock options to purchase 206,656
shares at a weighted average exercise price of $9.71 per share and non-qualified
options to purchase 429,664 shares at a weighted average exercise price of $5.77
per share. At December 31, 1999, there were 7,360,480 shares available for
future option grants under the plan. After December 31, 1999 we granted options
to purchase 8,000 shares of common stock, and option holders exercised options
to purchase 13,987 shares of common stock.

1992 NON-EMPLOYEE DIRECTORS STOCK OPTION PLAN

Under our 1992 Non-Employee Directors Stock Option Plan, we may grant
non-qualified stock options to certain directors who are not also our employees.
The plan was adopted by the board of directors and approved by our stockholders
on January 10, 1992. The number of shares of common stock eligible for issuance
under the plan is 480,000. The compensation committee administers the plan. We
may issue options under the plan at any exercise price determined by the
compensation committee. Generally, options granted under the plan expire upon
voluntary resignation of the holder from the board of directors. Upon
termination of a holder's tenure as a director for any other reason, the holder
must exercise the options within 60 days thereafter or within 180 days after the
holder's death or disability, but in no event later than the originally
prescribed term of the option. Unless otherwise determined by the compensation
committee,

                                       53
<PAGE>
options granted under the plan vest over a three-year period. The plan
terminates on January 10, 2002, unless earlier terminated by the board of
directors.

For the year ended December 31, 1999, we granted options to purchase a total of
12,000 shares of common stock under the plan at a weighted average exercise
price of $6.88 per share. As of December 31, 1999, one director held options to
purchase an aggregate of 108,000 shares of common stock under the plan at
exercise prices ranging from $1.04 to $12.50 per share with a weighted average
of $1.69 per share. At December 31, 1999, 372,000 shares were available for
future option grants under the plan. After December 31, 1999, we granted options
to purchase 24,000 shares of common stock to one of our directors at an exercise
price per share of $12.50 that vest over a two-year period. No options issued
under the plan have been exercised.

2000 NON-EMPLOYEE DIRECTORS' STOCK OPTION PLAN

Our board of directors adopted, and our stockholders approved, the 2000
Non-Employee Directors' Stock Option Plan on February 17, 2000. The plan
automatically grants options to purchase shares of our common stock to our
non-employee directors. The board of directors administers the plan. We have
reserved a total of 500,000 shares of our common stock for issuance under the
plan. We have not issued any options under this plan.

After this offering, we will automatically issue options to our non-employee
directors under this plan as follows:

  o    each person who is elected or appointed to be a non-employee director for
       the first time will automatically receive an initial grant to purchase
       15,000 shares; and

  o    each person who is re-elected or re-appointed as a non-employee director
       will automatically receive a grant to purchase 5,000 shares.

Directors serving on the board as of February 17, 2000 may not receive any
options under this plan.

The options are exercisable immediately but vest 1/36th per month for each month
after the date of the grant over three years. As long as the option holder
continues to serve with us or with an affiliate of ours, whether in the capacity
of a director, an employee or a consultant, the option will continue to vest and
be exercisable during its term.

The options have an exercise price equal to 100% of the fair market value of our
common stock on the grant date.

The option term is ten years. Options terminate on the earlier of normal option
termination or three months after the option holder's service terminates. If
this termination is due to the option holder's disability, the post-termination
exercise period is extended to the earlier of normal option termination or 12
months. If termination is due to the option holder's death or if the option
holder dies within three months of the date on which his or her service
terminates, the post-termination exercise period is extended to the earlier of
normal option termination or 18 months following death.

The option holder may transfer the option by gift to immediate family members or
for estate planning purposes. The option holder may also designate a beneficiary
to exercise the option if the option holder dies. If the option holder does not
designate a beneficiary, the option exercise rights will pass by the option
holder's will or by the laws of descent and distribution.

If an option holder does not purchase the shares subject to his or her option
before the option expires or otherwise terminates, the shares that are not
purchased will again become available for issuance under the plan.

Transactions that do not involve our receiving consideration, including a
merger, consolidation, reorganization, stock dividend and stock split, may
trigger a change in the class and number of shares subject to this plan and to
outstanding options. If any of these events occurs, the board of directors will
appropriately adjust the plan as to the class and the maximum number of shares
subject to the plan and the

                                       54
<PAGE>

automatic option grants. It will also adjust outstanding options as to the
class, number and price of shares subject to these options.

If we dissolve or liquidate, outstanding options will terminate immediately
before this event. However, we treat outstanding options differently in the
following situations:

  o    a sale, lease or other disposition of all or substantially all of our
       assets;

  o    a merger or consolidation in which we are not the surviving corporation;
       and

  o    a reverse merger in which we are the surviving corporation but the shares
       of our common stock outstanding immediately before the merger are
       converted by virtue of the merger into other property.

In these situations, any surviving entity will either assume or replace all
outstanding options under the plan. Otherwise, the vesting of the options will
accelerate.

The board of directors may suspend or terminate the plan at any time.

GENERAL

We intend to file a registration statement on Form S-8 upon the completion of
this offering to register the sale of common stock issuable upon exercise of
stock options issued under our stock option plans.

401(K) PLAN

Effective January 1, 1992, we adopted a qualified retirement plan, or 401(k)
plan, covering all of our employees who are at least 21 years of age and have
completed at least one year of service with us. Under the plan, employees may
elect to reduce their current compensation by up to the statutorily prescribed
annual limit ($10,500 in 2000) and have the amount of the reduction contributed
to the plan. The plan is covered by the Employee Retirement Income Security Act
of 1974. We also intend for the plan to satisfy certain federal income tax law
requirements so that the contributions by employees or by us to the plan, and
income earned on plan contributions, are not taxable to employees until they
withdraw from the plan. We also intend for any contributions we make to the plan
to be deductible by us for federal income tax purposes.

The trustee of the plan, at the direction of each participant, invests the
assets of the plan in any of nine investment options. The plan permits, but does
not require, us to make additional matching contributions on behalf of all
participants in the plan. For the year ended December 31, 1998, we contributed
matching contributions of approximately $66,000 to the plan. Our matching
contribution for the year ending December 31, 1999 was 50% of employee
contributions up to a maximum employer contribution of 2.5% of each
participant's total compensation. Our matching contribution for the year ending
December 31, 2000 will be 50% of employee contributions up to a maximum employer
contribution of 2.5% of each participant's total compensation. The compensation
committee of the board of directors administers the plan.

OTHER OPTIONS

We have granted non-qualified stock options not covered by the 1987 Stock Option
Plan, the 1997 Stock Plan or the 1992 Non-Employee Directors Stock Option Plan
to an outside, non-affiliated director and to key advisors and consultants to
purchase an aggregate of 196,800 shares of common stock, net of cancellations,
as of December 31, 1999, at a weighted average exercise price of $1.31 per
share. All of these options are currently exercisable. After December 31, 1999,
consultants exercised options to purchase 74,400 shares of common stock.

                                       55
<PAGE>
                             PRINCIPAL STOCKHOLDERS

The following table sets forth information regarding beneficial ownership of our
common stock as of March 7, 2000 by:

  o    each stockholder known by us to be the beneficial owner of more than 5%
       of the outstanding shares of common stock;

  o    each of our directors;

  o    each of our named executive officers; and

  o    all directors and officers as a group.

Beneficial ownership is determined according to the rules of the Securities and
Exchange Commission, and generally means that person has beneficial ownership of
a security if he or she possesses sole or shared voting or investment power of
that security, and includes options that are currently exercisable or
exercisable within 60 days. Each director, officer or 5% or more stockholder, as
the case may be, has furnished us information with respect to beneficial
ownership. Except as otherwise indicated, we believe that the beneficial owners
of the common stock listed below, based on the information each of them has
given to us, have sole investment and voting power with respect to their shares,
except where community property laws may apply.

This table lists applicable percentage ownership based on 33,788,731 shares of
common stock outstanding as of March 7, 2000, and also lists applicable
percentage ownership based on 40,788,731 shares of common stock outstanding
after completion of this offering. Options to purchase shares of our common
stock that are exercisable within 60 days of March 7, 2000, are deemed to be
beneficially owned by the persons holding these options for the purpose of
computing percentage ownership of that person, but are not treated as
outstanding for the purpose of computing any other person's ownership
percentage. Shares underlying options that are deemed beneficially owned are
listed in this table separately in the column labeled "Shares Subject to
Options." These shares are included in the number of shares listed in the
column labeled "Total Number."

Unless otherwise indicated, the principal address of each stockholder below is:
c/o Tanox, Inc., 10301 Stella Link, Houston, Texas 77025.

<TABLE>
<CAPTION>
                                                          SHARES BENEFICIALLY OWNED
                                        --------------------------------------------------------------
                                          TOTAL      SHARES SUBJECT    PERCENT BEFORE    PERCENT AFTER
NAME AND ADDRESS OF BENEFICIAL OWNER     NUMBER        TO OPTIONS         OFFERING         OFFERING
- -------------------------------------   ---------    --------------    --------------    -------------
<S>                                     <C>          <C>               <C>               <C>
DIRECTORS AND NAMED OFFICERS
Nancy T. Chang(1)....................   6,977,643        260,800            20.5%             17.0%
Tse Wen Chang(2).....................   6,564,239             --            19.4              16.1
George Y. Wang.......................     209,925        142,000           *                 *
Osama I. Mikhail(3)..................     103,467        103,467           *                 *
David W. Thomas(4)...................      86,400         83,200           *                 *
John C. Morris.......................      54,399         54,399           *                 *
David Duncan, Jr. ...................      38,399         26,092           *                 *
William J. Jenkins(5)................          --             --          --                --
All directors and officers as a group
  (10 persons)(6)....................   15,018,760       863,858            43.3              36.1
5% STOCKHOLDERS
Novartis AG(7).......................   6,373,732             --            18.9%             15.6%
Alafi Capital Company(8).............   2,514,724             --             7.4               6.2
</TABLE>

(FOOTNOTES ON FOLLOWING PAGE)

                                       56
<PAGE>
- ---------------------------

  *   Less than 1%

 (1)  Includes 6,429,318 shares of common stock owned by Robinhood Ventures,
      L.P. and 34,816 shares of common stock owned by AMC Ventures, L.P. Apex
      Enterprises, Inc., a corporation wholly-owned by Dr. Chang, is the sole
      general partner of Robinhood Ventures, L.P. and AMC Ventures, L.P. and has
      voting and investment control over the common stock owned by them.
      Includes 248,847 shares held in trust by Dr. Chang for her children.
      Includes 3,862 shares held by Dr. Chang's minor daughter, with respect to
      which Dr. Chang disclaims beneficial ownership.

 (2)  Includes 256,481 shares held in trust by Dr. Chang for his children. Dr.
      Chang's address is College of Life Sciences, National Tsing Hua
      University, Hsinchu, Taiwan, Republic of China.

 (3)  Dr. Mikhail's address is 6720 Bertner Ave., Suite B111, Houston, Texas
      77030.

 (4)  Includes 2,400 shares held by Dr. Thomas' spouse and children, with
      respect to which Dr. Thomas disclaims beneficial ownership.

 (5)  Dr. Jenkins' address is Gelham, Church Road, Waxham, Norfolk NR12 0DY,
      United Kingdom.

 (6)  See footnotes 1, 2 and 4 above. Includes 790,388 shares of common stock
      and 193,900 shares subject to options held by officers not shown in the
      table above.

 (7)  The address of Novartis AG is S-202.502, CH-4002, Basel, Switzerland.

 (8)  Includes 216,000 shares of common stock held by the Alafi Family
      Foundation, a non-profit organization, with respect to which Mr. Alafi
      disclaims beneficial ownership. Mr. Alafi is the General Partner of Alafi
      Capital Company. Alafi Capital Company's address is P.O. Box 7338,
      Berkeley, California 94707.

                                       57
<PAGE>
                              CERTAIN TRANSACTIONS

REGISTRATION RIGHTS

Some of our stockholders, including Nancy T. Chang, our Chairman, President and
Chief Executive Officer, and Novartis and Tse Wen Chang, each of whom owns more
than 10% of our common stock, have certain registration rights, which they may
exercise after this offering. They may request that we register their shares for
sale with the Securities and Exchange Commission, and, if all of the conditions
that are contained in our agreements with them are met, we must register their
shares. We must bear all the expenses of a registration. There are some
restrictions on their rights, including that we are not obligated to effect more
than one registration for their shares, except that we may be required, in
certain circumstances, to register their shares up to three times using a
short-form registration. For a more detailed description see also "Description
of Capital Stock -- Registration Rights."

NOVARTIS NOTE PAYABLE

We have a loan agreement with Novartis, which, prior to this offering, held
18.9% of our common stock. Under the agreement, Novartis loaned us $10.0
million, bearing interest at a rate equal to LIBOR plus two percent (8.1% at
December 31, 1999). Novartis has agreed to forgive interest on the loan through
December 31, 1999. The loan is due December 31, 2005. Tanox and Novartis have
agreed in principle that Novartis may partially or totally forgive the principal
and future interest payments based on our future use of the facility.

COLLABORATION AGREEMENTS WITH NOVARTIS

We also have agreements with Novartis to jointly develop anti-IgE antibody
products. For a more detailed description see "Business -- Collaboration and
License Agreements."

TRANSACTIONS WITH DIRECTORS

In addition to being one of our directors, Tse Wen Chang is a consultant to us.
In 1999, we paid $48,000 to Dr. Chang under the terms of his consulting
agreement. Our Taiwan subsidiary, TanAsia Pharma, Ltd., also entered into a
collaboration agreement, which expired in June 1999, with Dr. Chang and the
National Tsing Hua University. Dr. Chang is a professor, and was formerly Dean
of the College of Life Sciences, at National Tsing Hua University. Under the
agreement with the university, Dr. Chang and other scientists at the university
participated in certain research and development activities for our benefit. In
1999, we paid the university $20,250 under the terms of that agreement.

LOANS TO MANAGEMENT

In April 1999, we loaned John Blickenstaff, our Vice President of
Administration, Secretary and Treasurer, $161,250 to enable him to purchase
258,000 shares of our common stock, pursuant to stock options he held that were
soon to expire. This full recourse loan is secured by shares of our common stock
owned by Mr. Blickenstaff, bears interest at a rate of 8.5% and is due and
payable in full in September 2001.

In October 1999, we loaned David W. Thomas, our Senior Vice President of
Research and Development, $150,000 to enable him to purchase a residence in
Houston. The loan is unsecured, bears interest at a rate of 8.5% per annum and
is due and payable in full on the first to occur of the sale of his
Massachusetts residence or July 15, 2000.

                                       58
<PAGE>
                          DESCRIPTION OF CAPITAL STOCK

We may issue up to 120,000,000 shares of common stock, par value $.01 per share,
and 10,000,000 shares of preferred stock, par value $.01 per share. As of March
7, 2000, there were 33,788,731 shares of common stock outstanding held by 185
stockholders of record, and no shares of preferred stock were outstanding.

We do not intend the following summary description of our capital stock to be
complete and we qualify the description by referring to the provisions of
applicable law and to our amended and restated certificate of incorporation and
our bylaws, filed as exhibits to the registration statement of which this
prospectus is a part.

COMMON STOCK

Based on the number of shares outstanding as of March 7, 2000 and giving effect
to the issuance of the 7,000,000 shares of common stock offered pursuant to this
prospectus there will be 40,788,731 shares of common stock outstanding upon the
completion of this offering. In addition, as of March 7, 2000, there were
outstanding stock options to purchase 2,747,733 shares of common stock.

Each share of common stock entitles the holder to one vote on all matters
submitted to a vote of stockholders, including the election of directors.
Holders of common stock may receive ratably the dividends, if any, declared from
time to time by the board of directors out of legally available funds. Holders
of common stock have no conversion, redemption or preemptive rights to subscribe
to any of our securities. If any liquidation, dissolution or winding-up of our
affairs occurs, holders of common stock may share ratably in our assets
remaining after provision for payment of liabilities to creditors. The rights,
preferences and privileges of holders of common stock are subject to the rights
of the holders of any shares of preferred stock that we may issue in the future.

PREFERRED STOCK

We have no present plans to issue any shares of preferred stock. However, the
board of directors may, without action by the stockholders, designate and issue
preferred stock in one or more series and designate the rights, preferences and
privileges of each series, which may be greater than the rights of the common
stock.

REGISTRATION RIGHTS

After the completion of this offering, the holders of 21,530,980 shares of
common stock will be entitled to rights with respect to the registration of
these shares under the Securities Act. These rights include demand registration
rights.

Under the terms of the agreements providing registration rights, the holders of
a specified minimum number of shares can demand that we register their shares.
We must use our best efforts to effect a registration, subject to conditions and
limitations. We are not required to effect more than three of these
registrations pursuant to these demand registration rights. Under the terms of
the agreements providing registration rights, if we propose to register any of
our securities under the Securities Act, either for our own account or for the
account of other security holders exercising registration rights, these holders
are entitled to notice of the registration and may include shares of common
stock in the registration. The rights are subject to conditions and limitations,
among them the right of the underwriters of an offering subject to the
registration to limit the number of shares included in the registration. These
registration rights have been waived with respect to this offering. Furthermore,
stockholders with demand registration rights may require us to file additional
registration statements on Form S-3, subject to conditions and limitations. We
generally must bear all of the expenses of all of these registrations, except
underwriting discounts and selling commissions. We also have agreed to indemnify
stockholders who include shares in a registration statement from losses arising
from violations by us of applicable securities laws in connection with the
registration. Registration of any of the shares of common stock held by
stockholders with registration rights would

                                       59
<PAGE>

result in shares becoming freely tradable without restriction under the
Securities Act immediately upon effectiveness of the registration.

ANTI-TAKEOVER EFFECTS OF VARIOUS PROVISIONS OF DELAWARE LAW AND OUR AMENDED AND
RESTATED CERTIFICATE OF INCORPORATION AND BYLAWS

SECTION 203 OF THE DELAWARE GENERAL CORPORATION LAW.  We are subject to the
provisions of Section 203 of the Delaware General Corporation Law, which
regulates corporate takeovers. Subject to some exceptions, Section 203 prohibits
a publicly held Delaware corporation from engaging in a "business combination"
with an "interested stockholder" for a period of three years after the date of
the transaction in which the person became an interested stockholder, unless:

  o    before the date of the business combination, the board of directors of
       the corporation approves the transaction;

  o    upon consummation of the transaction that resulted in the stockholder
       becoming an interested stockholder, the interested stockholder owns at
       least 85% of the outstanding stock; or

  o    the board of directors and at least 66 2/3% of the outstanding voting
       stock that is not owned by the interested stockholder approve the
       business combination.

A "business combination" includes mergers, asset sales and other transactions
resulting in a financial benefit to the interested stockholder. Subject to
various exceptions, an "interested stockholder" is a person who, together with
affiliates and associates, owns, or within three years did own, 15% or more of
the corporation's voting stock. This statute could prohibit or delay mergers or
other takeover or change in control attempts with respect to us and,
accordingly, may discourage attempts to acquire us.

In addition, various provisions of our amended and restated certificate of
incorporation and our bylaws, which are summarized in the following paragraphs,
may be deemed to have an anti-takeover effect and may delay, defer or prevent a
tender offer or takeover attempt that a stockholder might consider in its best
interest, including those attempts that might result in a premium over the
market price for the shares held by stockholders.

REMOVAL OF DIRECTORS; VACANCIES.  Our bylaws provide that directors may be
removed from office only for cause and only by the affirmative vote of the
holders of at least two-thirds of our total outstanding voting stock. Vacancies
on our board of directors, including those resulting from an increase in the
number of directors, may be filled only by the remaining directors, not by
stockholders.

CLASSIFIED BOARD OF DIRECTORS.  Our bylaws divide our board into three classes
of directors serving staggered, three year terms. The classification of the
board has the effect of requiring at least two annual stockholder meetings,
instead of one, to replace a majority of the members of the board of directors.

ADVANCE NOTICE REQUIREMENTS FOR STOCKHOLDER PROPOSALS AND DIRECTOR
NOMINATIONS.  Our bylaws provide that stockholders seeking to bring business
before an annual meeting of stockholders must provide timely notice thereof in
writing. To be timely, a stockholder's notice must be delivered to or mailed and
received at our principal executive offices not less than 30 nor more than 60
days prior to the annual meeting. If less than 40 days notice has been given to
the stockholders for the meeting, notice by the stockholder, to be timely, must
be received not later than the close of business on the 10th day following the
date on which notice of the meeting is given to stockholders.

Our bylaws also specify certain requirements as to the form and content of a
stockholder's notice. These provisions may preclude stockholders from bringing
matters before an annual meeting.

SPECIAL MEETINGS OF STOCKHOLDERS.  Our bylaws provide that only a majority of
the board of directors, the chairman of our board of directors or our president
or chief executive officer may call a special meeting of our stockholders.

PREFERRED STOCK.  The board of directors may, without action by the
stockholders, designate and issue preferred stock in one or more series and
designate the rights, preferences and privileges of each series, which may be
greater than the rights of the common stock. We cannot predict the effect of the
issuance of any shares of preferred stock upon the rights of holders of our
common stock until the board of directors

                                       60
<PAGE>
determines the specific rights of the holders of our preferred stock. However,
the effects could include one or more of the following:

  o    restricting dividends on our common stock;

  o    diluting the voting power of our common stock;

  o    impairing the liquidation rights of our common stock; or

  o    discouraging or preventing a change in our control.

LIMITATION OF LIABILITY AND INDEMNIFICATION MATTERS

Section 145 of the Delaware General Corporation Law authorizes a corporation's
board of directors to indemnify directors and officers in terms sufficiently
broad to permit indemnification under certain circumstances for liabilities,
including reimbursement for expenses incurred, arising under the Securities Act.

As permitted by Delaware law, our amended and restated certificate of
incorporation includes a provision that eliminates the personal liability of our
directors for monetary damages for breach of fiduciary duty as a director,
except for liability:

  o    for any breach of the director's duty of loyalty to us or our
       stockholders;

  o    for acts or omissions not in good faith or that involve intentional
       misconduct or a knowing violation of law;

  o    under Section 174 of the Delaware General Corporation Law regarding
       unlawful dividends and stock purchases; or

  o    for any transaction from which the director derived an improper personal
       benefit.

As permitted by Delaware law, our bylaws provide that:

  o    we must indemnify our directors and officers to the fullest extent
       permitted by Delaware law;

  o    we must advance expenses, as incurred, to our directors and officers in
       connection with a legal proceeding, subject to certain limited
       exceptions; and

  o    the rights conferred in the bylaws are not exclusive.

We have entered into indemnification agreements with each of our officers and
directors to give them additional contractual assurances regarding the scope of
the indemnification provided in our amended and restated certificate of
incorporation and bylaws and to provide additional procedural protections. These
agreements, among other things, require us to indemnify each director and
officer to the fullest extent permitted by Delaware law, including
indemnification for expenses such as attorneys' fees, judgments, fines and
settlement amounts incurred by the director or officer in any action or
proceeding, including any action by or in the right of us, arising out of the
person's services as a director or officer of us, any subsidiary of ours or any
other company or enterprise to which the person provides services at our
request. At present, we are not aware of any pending or threatened litigation or
proceeding involving any of our directors, officers, employees or agents where
indemnification would be required or permitted. We believe that the provisions
of our amended and restated articles of incorporation, bylaws and these
indemnification agreements are necessary to attract and retain qualified persons
as directors and officers.

TRANSFER AGENT AND REGISTRAR

The transfer agent and registrar for our common stock is American Stock Transfer
and Trust Company, New York, New York.

LISTING

We have applied for quotation of the common stock on the Nasdaq National Market
under the symbol "TNOX."

                                       61
<PAGE>
                        SHARES ELIGIBLE FOR FUTURE SALE

Immediately prior to this offering, there was no public market for our common
stock. Future sales of substantial amounts of common stock in the public market
or the perception that sales could occur, could adversely affect the market
price of our common stock and our ability to sell equity securities.

When we complete this offering, we will have a total of 40,788,731 shares of
common stock outstanding. The 7,000,000 shares offered by this prospectus will
be freely tradeable unless they are purchased by our "affiliates," as defined
in Rule 144 under the Securities Act. Shares purchased by affiliates may
generally only be sold pursuant to an effective registration statement under the
Securities Act or in compliance with Rule 144 as described below. The remaining
shares are "restricted," which means they were originally sold in offerings
that were not subject to a registration statement filed with the Securities and
Exchange Commission. These restricted shares may be resold only through
registration under the Securities Act or under an available exemption from
registration, such as provided through Rule 144.

Directors and officers and stockholders who together own 32,040,783 shares of
common stock will be subject to lock-up agreements providing that they will not
offer, sell or otherwise dispose of common stock owned by them, other than
shares of common stock acquired in this offering for a period of 180 days after
the date of this prospectus. CIBC World Markets Corp., however, may in its sole
discretion, at any time, without notice, release all or any portion of the
shares subject to lock-up agreements. Upon expiration of the lock-up agreements,
7,724,426 shares will become eligible for sale pursuant to Rule 144(k),
21,334,800 shares will become eligible for sale under Rule 144 and 1,749,884
shares will become eligible for sale under Rule 701.

RULE 144

Generally, Rule 144 as currently in effect provides that, beginning 90 days
after the first date of this prospectus, a person who has beneficially owned
shares of our common stock for at least one year may sell within any three-month
period, a number of shares that does not exceed the greater of:

  o    1% of the number of shares of common stock then outstanding, which, based
       on the shares outstanding as of March 7, 2000 will equal approximately
       337,887 shares; or

  o    the average weekly trading volume of the common stock on the Nasdaq
       National Market during the four calendar weeks preceding the filing of
       the notice on Form 144 with respect to the sale.

Rule 144 provides limitations on the manner of sales and imposes requirements as
to notice and the availability of current public information about us.

RULE 144(K)

Under Rule 144(k), a person who has not been one of our affiliates at any time
during the 90 days preceding a sale, and who has beneficially owned the shares
proposed to be sold for at least two years, may sell his or her shares without
complying with the manner of sale, public information, volume limitation or
notice provisions of Rule 144. Therefore, unless otherwise restricted, a person
who has been a non-affiliate for at least two years may sell his or her shares
in the open market immediately after the lock-up agreements expire.

RULE 701

Rule 701 permits any of our employees, officers, directors, or consultants who
purchased their shares under a compensatory stock or option plan or other
written agreement prior to the effective date of this offering and who are not
an affiliate to sell these shares under Rule 144 without complying with the
holding period, public information, volume limitation or notice requirements of
Rule 144. Rule 701 permits these persons who are our affiliates to sell these
shares without complying with the holding period requirement of Rule 144. All
holders of Rule 701 shares may not sell their Rule 701 shares until 90 days
after the date of this prospectus. However, substantially all shares of our
common stock issued under Rule 701 are subject to lock-up agreements described
above.

                                       62
<PAGE>

Shortly following the date of this prospectus, we intend to file a registration
statement on Form S-8 under the Securities Act covering shares of common stock
reserved for issuance under our stock option plans. Shares registered under this
registration statement will, subject to Rule 144 volume limitations applicable
to our affiliates, be available for sale in the open market immediately after
the lock-up agreements expire. As of March 7, 2000, an aggregate of 2,747,733
shares of common stock were subject to outstanding options.

REGISTRATION RIGHTS

As of March 7, 2000, holders of 21,530,980 shares of common stock will be
entitled to certain rights with respect to the registration of those shares
under the Securities Act. After we register these shares, they will be freely
tradeable. For a description of these rights, see "Description of Capital
Stock."

                                       63
<PAGE>
                                  UNDERWRITING

We have entered into an underwriting agreement with the underwriters named
below. CIBC World Markets Corp., FleetBoston Robertson Stephens Inc., Warburg
Dillon Read LLC, Adams, Harkness & Hill, Inc. and KBC Securities Inc. are acting
as representatives of the underwriters.

The underwriting agreement provides for the purchase of a specific number of
shares of common stock by each of the underwriters. The underwriters'
obligations are several, which means that each underwriter is required to
purchase a specified number of shares, but is not responsible for the commitment
of any other underwriter to purchase shares. Subject to the terms and conditions
of the underwriting agreement, each underwriter has severally agreed to purchase
the number of shares of common stock set forth opposite its name below:


UNDERWRITER                             NUMBER OF SHARES
- -------------------------------------   -----------------
CIBC World Markets Corp..............
FleetBoston Robertson Stephens
Inc..................................
Warburg Dillon Read LLC..............
Adams, Harkness & Hill, Inc..........
KBC Securities Inc...................
                                        -----------------
  Total..............................         7,000,000
                                        =================


The underwriters have agreed to purchase all of the shares offered by this
prospectus (other than those covered by the over-allotment option described
below) if any are purchased. Under the underwriting agreement, if an underwriter
defaults in its commitment to purchase shares, the commitments of non-defaulting
underwriters may be increased or the underwriting agreement may be terminated,
depending on the circumstances.

The shares should be ready for delivery on or about             , 2000, against
payment in immediately available funds. The representatives have advised us that
the underwriters propose to offer the shares directly to the public at the
public offering price that appears on the cover page of this prospectus. In
addition, the representatives may offer some of the shares to other securities
dealers at such price less a concession of $   per share. The underwriters may
also allow, and such dealers may reallow, a concession not in excess of $   per
share to other dealers. After the shares are released for sale to the public,
the representatives may change the offering price and other selling terms at
various times.

We have granted the underwriters an over-allotment option. This option, which is
exercisable for up to 30 days after the date of this prospectus, permits the
underwriters to purchase a maximum of 1,050,000 additional shares from us to
cover over-allotments. If the underwriters exercise all or part of this option,
they will purchase shares covered by the option at the initial public offering
price that appears on the cover page of this prospectus, less the underwriting
discount. If this option is exercised in full, the total price to the public
will be $__________, and the total proceeds to us will be $_________________.
The underwriters have severally agreed that, to the extent the over-allotment
option is exercised, they will each purchase a number of additional shares
proportionate to the underwriter's initial amount reflected in the above table.

The following table provides information regarding the amount of the discount to
be paid to the underwriters by us:

<TABLE>
<CAPTION>
                                                      TOTAL WITHOUT EXERCISE OF     TOTAL WITH FULL EXERCISE OF
                                        PER SHARE       OVER-ALLOTMENT OPTION          OVER-ALLOTMENT OPTION
                                        ----------    --------------------------    ----------------------------
<S>                                     <C>           <C>                           <C>
Tanox................................     $                   $                              $
</TABLE>

We estimate that our total expenses of the offering, excluding the underwriting
discount, will be approximately $__________.

We have agreed to indemnify the underwriters against certain liabilities,
including liabilities under the Securities Act.

                                       64
<PAGE>

KBC Bank, N.V., an affiliate of KBC Securities, Inc., one of our underwriters,
has agreed with John Blickenstaff, our Vice President of Administration,
Secretary and Treasurer, and one of our other employees, to purchase, at the
option of Mr. Blickenstaff and our other employee, shares of common stock having
a value of up to $3.7 million at the initial public offering price or, if the
offering is not completed by May 16, 2000, at $28.50 per share, less a discount
of seven percent. KBC will not resell any shares acquired under the agreement in
this offering. KBC has also agreed not to resell these shares except in limited
circumstances where the resale would be registered with the Securities and
Exchange Commission under the Securities Act or subject to an exemption
therefrom.

We and our officers and directors and substantially all other stockholders have
agreed to a 180-day "lock up" with respect to 32,040,783 shares of common
stock that they beneficially own, including securities that are convertible into
shares of common stock and securities that are exchangeable or exercisable for
shares of common stock. This means that, subject to certain exceptions, for a
period of 180 days following the date of this prospectus, we and such persons
may not offer, sell, pledge or otherwise dispose of these securities without the
prior written consent of CIBC World Markets Corp.

The representatives have informed us that they do not expect discretionary sales
by the underwriters to exceed 5% of the shares offered by this prospectus.

The underwriters have reserved for sale up to 350,000 shares for employees and
their direct family members, directors and other persons associated with us.
These reserved shares will be sold at the initial public offering price that
appears on the cover page of this prospectus. The number of shares available for
sale to the general public in the offering will be reduced to the extent
reserved shares are purchased by such persons. The underwriters and we reserve
the right to terminate this program. The underwriters will offer to the general
public, on the same terms as other shares offered by this prospectus, any
reserved shares that are not purchased by such persons.

There is no established trading market for the shares. The offering price for
the shares has been determined by us and the representatives, based on the
following factors:

  o    prevailing market and general economic conditions;

  o    our financial information;

  o    our history and prospects;

  o    Tanox and the industry in which we compete;

  o    an assessment of our management, its past and present operations, and the
       prospects for, and timing of, our future revenues; and

  o    the present stage of our development and the above factors in relation to
       the market values and various valuation measures of other companies
       engaged in activities similar to ours.

Rules of the Securities and Exchange Commission may limit the ability of the
underwriters to bid for or purchase shares before the distribution of the shares
is completed. However, the underwriters may engage in the following activities
in accordance with the rules:

  o    Stabilizing transactions -- The representatives may make bids or
       purchases for the purpose of pegging, fixing or maintaining the price of
       the shares, so long as stabilizing bids do not exceed a specified
       maximum.

  o    Over-allotment and syndicate covering transactions -- The underwriters
       may create a short position in the shares by selling more shares than are
       set forth on the cover page of this prospectus. If a short position is
       created in connection with the offering, the representatives may engage
       in syndicate covering transactions by purchasing shares in the open
       market. The representatives may also elect to reduce any short position
       by exercising all or part of the over-allotment option.

  o    Penalty bids -- If the representatives purchase shares in the open market
       in a stabilizing transaction or syndicate covering transaction, they may
       reclaim a selling concession from the underwriters and selling group
       members who sold those shares as part of this offering.

                                       65
<PAGE>
Stabilization and syndicate covering transactions may cause the price of the
shares to be higher than it would be in the absence of such transactions. The
imposition of a penalty bid might also have an effect on the price of the shares
if it discourages resales of the shares.

Neither we nor the underwriters make any representation or prediction as to the
effect that the transactions described above may have on the price of the
shares. These transactions may occur on the Nasdaq National Market or otherwise.
If such transactions are commenced, they may be discontinued without notice at
any time.

                                 LEGAL MATTERS

Chamberlain, Hrdlicka, White, Williams & Martin, Houston, Texas will pass upon
certain legal matters with respect to the legality of the issuance of the shares
of common stock offered by this prospectus. Skadden, Arps, Slate, Meagher & Flom
(Illinois), Chicago, Illinois will pass upon certain legal matters in connection
with this offering for the underwriters.

Wilburn O. McDonald, Jr. and seven other shareholders of Chamberlain, Hrdlicka,
White, Williams & Martin, hold an aggregate of 9,266 shares of our common stock.

                                    EXPERTS

The financial statements as of December 31, 1998 and 1999, and for each of the
three years in the period ended December 31, 1999 included in this prospectus
and elsewhere in the Registration Statement have been audited by Arthur Andersen
LLP, independent public accountants, as indicated in their report with respect
thereto, and are included herein in reliance upon the authority of said firm as
experts in giving said report.

Certain legal matters with respect to the statements in this prospectus under
the captions "Risk Factors -- We depend on our patents and proprietary rights.
The validity, enforceability and commercial value of these rights are highly
uncertain," and "Business -- Patents and Proprietary Rights" have been
reviewed and approved by Chamberlain, Hrdlicka, White, Williams & Martin,
Houston, Texas, our patent counsel who are experts in these matters and are
subject to an opinion to be rendered to the underwriters. We are including this
information relying on their review and approval.

                      WHERE YOU CAN FIND MORE INFORMATION

We have filed a registration statement on Form S-1 with the Securities and
Exchange Commission in connection with this offering. In addition, upon
completion of the offering, we must file annual, quarterly and current reports,
proxy statements and other information with the Securities and Exchange
Commission.

You may read and copy the registration statement and any other documents filed
by us at the Securities and Exchange Commission's Public Reference Room at 450
Fifth Street, N.W., Washington, D.C. 20549. Please call the Securities and
Exchange Commission at 1-800-SEC-0330 for further information on the Public
Reference Room. Our Securities and Exchange Commission filings are also
available to the public at the Securities and Exchange Commission's Internet
site at "http://www.sec.gov."

This prospectus is part of the registration statement and does not contain all
of the information included in the registration statement. Whenever a reference
is made in this prospectus to any contract or other document of ours, the
reference may not be complete and you should refer to the exhibits that are a
part of the registration statement for a copy of the contract or document.

After the offering, we intend to provide annual reports to our stockholders that
include financial information examined and reported on by an independent public
accounting firm.

                                       66
<PAGE>
                                   TANOX, INC.
                   INDEX TO CONSOLIDATED FINANCIAL STATEMENTS


                                                                           PAGE
                                                                           -----
Report of Independent Public Accountants...........................         F-2

Consolidated Balance Sheets as of December 31, 1998 and 1999.......         F-3

Consolidated Statements of Operations and Comprehensive Income
  (Loss) for the years ended December 31, 1997, 1998 and 1999......         F-4

Consolidated Statements of Stockholders' Equity for the years
  ended December 31, 1997, 1998 and 1999...........................         F-5

Consolidated Statements of Cash Flows for the years ended
  December 31, 1997, 1998 and 1999.................................         F-6

Notes to Consolidated Financial Statements.........................         F-7

                                       F-1
<PAGE>
                    REPORT OF INDEPENDENT PUBLIC ACCOUNTANTS

To the Board of Directors of
Tanox, Inc.:

We have audited the accompanying consolidated balance sheets of Tanox, Inc., a
Delaware corporation, and subsidiaries as of December 31, 1998 and 1999, and the
related consolidated statements of operations and comprehensive income (loss),
stockholders' equity and cash flows for each of the three years in the period
ended December 31, 1999. These consolidated financial statements are the
responsibility of the Company's management. Our responsibility is to express an
opinion on these consolidated financial statements based on our audits.

We conducted our audits in accordance with generally accepted auditing
standards. Those standards require that we plan and perform the audit to obtain
reasonable assurance about whether the consolidated financial statements are
free of material misstatement. An audit includes examining, on a test basis,
evidence supporting the amounts and disclosures in the consolidated financial
statements. An audit also includes assessing the accounting principles used and
significant estimates made by management, as well as evaluating the overall
financial statement presentation. We believe that our audits provide a
reasonable basis for our opinion.

In our opinion, the consolidated financial statements referred to above present
fairly, in all material respects, the financial position of Tanox, Inc., and
subsidiaries as of December 31, 1998 and 1999, and the results of their
operations and their cash flows for each of the three years in the period
December 31, 1999, in conformity with generally accepted accounting principles.

ARTHUR ANDERSEN LLP

Houston, Texas
February 1, 2000

                                       F-2
<PAGE>
                                  TANOX, INC.
                          CONSOLIDATED BALANCE SHEETS


                                               DECEMBER 31,
                                       ----------------------------
                                           1998           1999
                                       -------------  -------------
               ASSETS
CURRENT ASSETS:
     Cash and cash equivalents.......  $  28,352,000  $  44,242,000
     Short-term investments..........      5,383,000      3,012,000
     Accounts receivable.............         74,000        125,000
     Interest receivable.............        253,000        414,000
     Income taxes receivable.........      2,052,000        132,000
     Prepaid expenses................         91,000        114,000
                                       -------------  -------------
          Total current assets.......     36,205,000     48,039,000
PROPERTY AND EQUIPMENT:
     Laboratory and office
      equipment......................      8,709,000      9,369,000
     Leasehold improvements..........      1,902,000      2,102,000
     Furniture and fixtures..........         92,000        119,000
                                       -------------  -------------
                                          10,703,000     11,590,000
     Less -- Accumulated depreciation
      and amortization...............     (3,577,000)    (4,577,000)
                                       -------------  -------------
          Net property and
             equipment...............      7,126,000      7,013,000
OTHER ASSETS, net of accumulated
     amortization of $25,000 and
     $59,000, respectively...........         91,000        276,000
                                       -------------  -------------
          Total assets...............  $  43,422,000  $  55,328,000
                                       =============  =============

   LIABILITIES AND STOCKHOLDERS' EQUITY
CURRENT LIABILITIES:
     Accounts payable................  $     802,000  $     874,000
     Accrued liabilities.............      1,080,000        947,000
     Accrued arbitration award.......       --            3,500,000
                                       -------------  -------------
          Total current
             liabilities.............      1,882,000      5,321,000
NOTE PAYABLE TO RELATED PARTY........     10,000,000     10,000,000
COMMITMENTS AND CONTINGENCIES
STOCKHOLDERS' EQUITY:
     Preferred stock, $.01 par value;
      10,000,000 shares authorized;
      none outstanding...............       --             --
     Common stock, $.01 par value;
      120,000,000 shares authorized;
      29,310,175 shares in 1998 and
      33,324,402 shares in 1999
      issued and outstanding.........        293,000        333,000
     Additional paid-in capital......     39,266,000     71,701,000
     Deferred compensation...........       (902,000)      (651,000)
     Loans receivable from
      employees......................       --           (1,086,000)
     Other comprehensive income,
      cumulative translation
      adjustment.....................         (2,000)       171,000
     Retained earnings (deficit).....     (7,115,000)   (30,461,000)
                                       -------------  -------------
          Total stockholders'
             equity..................     31,540,000     40,007,000
                                       -------------  -------------
          Total liabilities and
             stockholders' equity....  $  43,422,000     55,328,000
                                       =============  =============

   The accompanying notes are an integral part of these consolidated financial
                                   statements.

                                       F-3
<PAGE>
                                  TANOX, INC.
                     CONSOLIDATED STATEMENTS OF OPERATIONS
                        AND COMPREHENSIVE INCOME (LOSS)

<TABLE>
<CAPTION>
                                             FOR THE YEAR ENDED DECEMBER 31,
                                       --------------------------------------------
                                           1997           1998            1999
                                       ------------  --------------  --------------
<S>                                    <C>           <C>             <C>
REVENUES:
     Development agreement with
       related party.................  $  1,271,000  $    2,369,000  $    1,063,000
     Other development agreements and
       licensing fees................     7,668,000          53,000         342,000
                                       ------------  --------------  --------------
               Total revenues........     8,939,000       2,422,000       1,405,000
OPERATING COSTS AND EXPENSES:
     Research and development........     6,926,000      11,933,000      17,163,000
     General and administrative......     2,230,000       3,431,000       8,582,000
                                       ------------  --------------  --------------
               Total operating costs
                 and expenses........     9,156,000      15,364,000      25,745,000
                                       ------------  --------------  --------------
LOSS FROM OPERATIONS.................      (217,000)    (12,942,000)    (24,340,000)
OTHER INCOME (EXPENSE):
     Interest income.................     1,684,000       2,061,000       1,884,000
     Interest expense................      (639,000)       (825,000)       (741,000)
     Other...........................       --                4,000        (115,000)
                                       ------------  --------------  --------------
               Total other income....     1,045,000       1,240,000       1,028,000
                                       ------------  --------------  --------------
INCOME (LOSS) BEFORE INCOME TAXES....       828,000     (11,702,000)    (23,312,000)
               (Provision) benefit of
                 income taxes........      (198,000)      1,533,000         (34,000)
                                       ------------  --------------  --------------
NET INCOME (LOSS)....................  $    630,000  $  (10,169,000) $  (23,346,000)
                                       ============  ==============  ==============
EARNINGS (LOSS) PER SHARE:
     Basic...........................  $       0.02  $        (0.35) $        (0.75)
                                       ============  ==============  ==============
     Diluted.........................  $       0.02  $        (0.35) $        (0.75)
                                       ============  ==============  ==============
SHARES USED IN COMPUTING EARNINGS
  (LOSS) PER SHARE:
     Basic...........................    27,909,000      29,105,000      31,113,000
                                       ============  ==============  ==============
     Diluted.........................    31,190,000      29,105,000      31,113,000
                                       ============  ==============  ==============
COMPREHENSIVE NET INCOME (LOSS):
     Net income (loss)...............  $    630,000  $  (10,169,000) $  (23,346,000)
     Foreign currency translation
       adjustment....................       --               (2,000)        173,000
                                       ------------  --------------  --------------
TOTAL COMPREHENSIVE NET INCOME
  (LOSS).............................  $    630,000  $  (10,171,000) $  (23,173,000)
                                       ============  ==============  ==============
</TABLE>

   The accompanying notes are an integral part of these consolidated financial
                                   statements.

                                       F-4
<PAGE>
                                  TANOX, INC.
                CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY
              FOR THE YEARS ENDED DECEMBER 31, 1997, 1998 AND 1999
<TABLE>
<CAPTION>
                                                                                                                LOANS
                                           COMMON STOCK                                                       RECEIVABLE
                                       ---------------------      ADDITIONAL         DEFERRED      TREASURY      FROM
                                        SHARES     PAR VALUE    PAID-IN CAPITAL    COMPENSATION     STOCK     EMPLOYEES
                                       ---------   ---------    ---------------    ------------    --------   ----------
<S>                                    <C>         <C>          <C>                <C>             <C>        <C>
BALANCES, December 31, 1996..........  26,215,004  $262,000       $13,125,000       $  --          $ --       $   --
    Issuance of common stock for
      cash, $7.50 per share, net of
      issuance costs.................   2,331,262    23,000        17,308,000          --            --           --
    Purchase of treasury stock,
      10,425 shares..................     --          --             --                --          (78,000)       --
    Issuance of common stock upon
      exercise of stock options......      36,614     1,000            (9,000)         --           78,000        --
    Capital contribution from
      forgiveness of interest by
      related party..................     --          --              639,000          --            --           --
    Deferred compensation related to
      stock options..................     --          --              750,000         (750,000)      --           --
    Amortization of deferred
      compensation related to stock
      options........................     --          --             --                 25,000       --           --
    Net income.......................     --          --             --                --            --           --
                                       ---------   ---------    ---------------    ------------    --------   ----------
BALANCES, December 31, 1997..........  28,582,880   286,000        31,813,000         (725,000)      --           --
    Issuance of common stock for
      cash, $II.25 per share, net of
      issuance costs.................     273,686     3,000         3,046,000          --            --           --
    Issuance of common stock upon
      exercise of stock options......     230,400     2,000           223,000          --            --           --
    Exchange of mature common stock
      to exercise stock options......      (3,200)    --              (36,000)         --            --           --
    Issuance of common stock to
      acquire foreign subsidiary.....     226,409      2,000         2,545,000         --            --           --
    Income tax benefit from stock
      options exercised..............     --          --              400,000          --            --           --
    Capital contribution from
      forgiveness of interest by
      related party..................     --          --              825,000          --            --           --
    Deferred compensation related to
      stock options..................     --          --              450,000         (450,000)      --           --
    Amortization of deferred
      compensation related to stock
      options........................     --          --             --                273,000       --           --
    Exchange translation
      adjustment.....................     --          --             --                --            --           --
    Net loss.........................     --          --             --                --            --           --
                                       ---------   ---------    ---------------    ------------    --------   ----------
BALANCES, December 31, 1998..........  29,310,175   293,000        39,266,000         (902,000)      --           --
    Issuance of common stock for
      cash, $12.50 per share, net of
      issuance costs.................   1,896,000    19,000        22,907,000          --            --           --
    Issuance of common stock upon
      exercise of stock options......   1,789,520    18,000         1,131,000          --            --           --
    Issuance of common stock on a net
      issuance basis upon exercise of
      warrants.......................      86,632     --             --                --            --           --
    Issuance of common stock to
      acquire foreign subsidiary.....     242,075     3,000         3,023,000          --            --           --
    Capital contribution from
      forgiveness of interest by
      related party..................     --          --              738,000          --            --           --
    Deferred compensation related to
      stock options..................     --          --            4,636,000          (60,000)      --           --
    Amortization of deferred
      compensation related to stock
      options........................     --          --             --                311,000       --           --
    Loans receivable from
      employees......................     --          --             --                --            --       (1,086,000)
    Exchange translation
      adjustment.....................     --          --             --                --            --           --
    Net loss.........................     --          --             --                --            --           --
                                       ----------  ---------    ---------------    ------------    --------   -----------
BALANCES, December 31, 1999..........  33,324,402  $333,000       $71,701,000       $ (651,000)    $ --       $(1,086,000)
                                       ==========  =========    ===============    ============    ========   ===========

<CAPTION>
                                           OTHER
                                       COMPREHENSIVE
                                          INCOME-
                                        CUMULATIVE      RETAINED         TOTAL
                                        TRANSLATION     EARNINGS     STOCKHOLDERS'
                                        ADJUSTMENT      (DEFICIT)       EQUITY
                                       -------------   -----------   -------------
<S>                                    <C>             <C>           <C>
BALANCES, December 31, 1996..........    $ --          $ 2,424,000    $15,811,000
    Issuance of common stock for
      cash, $7.50 per share, net of
      issuance costs.................      --              --          17,331,000
    Purchase of treasury stock,
      10,425 shares..................      --              --             (78,000)
    Issuance of common stock upon
      exercise of stock options......      --              --              70,000
    Capital contribution from
      forgiveness of interest by
      related party..................      --              --             639,000
    Deferred compensation related to
      stock options..................      --              --             --
    Amortization of deferred
      compensation related to stock
      options........................      --              --              25,000
    Net income.......................      --              630,000        630,000
                                       -------------   -----------   -------------
BALANCES, December 31, 1997..........      --            3,054,000     34,428,000
    Issuance of common stock for
      cash, $II.25 per share, net of
      issuance costs.................      --              --           3,049,000
    Issuance of common stock upon
      exercise of stock options......      --              --             225,000
    Exchange of mature common stock
      to exercise stock options......      --              --             (36,000)
    Issuance of common stock to
      acquire foreign subsidiary.....      --              --           2,547,000
    Income tax benefit from stock
      options exercised..............      --              --             400,000
    Capital contribution from
      forgiveness of interest by
      related party..................      --              --             825,000
    Deferred compensation related to
      stock options..................      --              --             --
    Amortization of deferred
      compensation related to stock
      options........................      --              --             273,000
    Exchange translation
      adjustment.....................       (2,000)        --              (2,000)
    Net loss.........................      --          (10,169,000)   (10,169,000)
                                       -------------   -----------   -------------
BALANCES, December 31, 1998..........       (2,000)     (7,115,000)    31,540,000
    Issuance of common stock for
      cash, $12.50 per share, net of
      issuance costs.................      --              --          22,926,000
    Issuance of common stock upon
      exercise of stock options......      --              --           1,149,000
    Issuance of common stock on a net
      issuance basis upon exercise of
      warrants.......................      --              --             --
    Issuance of common stock to
      acquire foreign subsidiary.....      --              --           3,026,000
    Capital contribution from
      forgiveness of interest by
      related party..................      --              --             738,000
    Deferred compensation related to
      stock options..................      --              --           4,576,000
    Amortization of deferred
      compensation related to stock
      options........................      --              --             311,000
    Loans receivable from
      employees......................      --              --          (1,086,000)
    Exchange translation
      adjustment.....................      173,000         --             173,000
    Net loss.........................      --          (23,346,000)   (23,346,000)
                                       -------------   ------------   ------------
BALANCES, December 31, 1999..........    $ 171,000     $(30,461,000)  $40,007,000
                                       =============   ============   ============
</TABLE>

   The accompanying notes are an integral part of these consolidated financial
                                   statements.

                                       F-5
<PAGE>
                                  TANOX, INC.
                     CONSOLIDATED STATEMENTS OF CASH FLOWS

<TABLE>
<CAPTION>
                                              FOR THE YEAR ENDED DECEMBER 31,
                                       ---------------------------------------------
                                           1997            1998            1999
                                       -------------  --------------  --------------
<S>                                    <C>            <C>             <C>
CASH FLOWS FROM OPERATING ACTIVITIES:
     Net income (loss)...............  $     630,000  $  (10,169,000) $  (23,346,000)
     Adjustments to reconcile net
       income (loss) to net cash used
       in operating activities --
          Depreciation and
            amortization.............        199,000         934,000       1,034,000
          Interest expense forgiven
            by related party.........        639,000         825,000         738,000
          Amortization of deferred
            compensation related to
            stock options............         25,000         273,000       4,887,000
          In-process research and
            development..............       --             2,798,000       3,359,000
     Changes in operating assets and
       liabilities --
          Increase in accounts and
            interest receivables and
            prepaid expenses.........        (17,000)        (15,000)       (235,000)
          Change in taxes receivable
            or payable...............       (463,000)     (1,576,000)      1,920,000
          (Decrease) increase in
            accounts payable and
            accrued liabilities......       (694,000)        391,000       3,439,000
          Decrease in collaboration
            advances.................       (500,000)       --              --
                                       -------------  --------------  --------------
               Net cash used in
                 operating
                 activities..........       (181,000)     (6,539,000)     (8,204,000)
                                       -------------  --------------  --------------
CASH FLOWS FROM INVESTING ACTIVITIES:
     Purchase of short-term
       investments...................       --            (9,517,000)    (12,082,000)
     Maturity of short-term
       investments...................       --             4,134,000      14,453,000
     Purchases of property and
       equipment.....................       (520,000)       (455,000)       (887,000)
     Increase in other assets........       --              --              (219,000)
     Purchase of wholly owned
       subsidiary (net of cash
       acquired).....................       --              (364,000)       (333,000)
                                       -------------  --------------  --------------
               Net cash provided by
                 (used in) investing
                 activities..........       (520,000)     (6,202,000)        932,000
                                       -------------  --------------  --------------
CASH FLOWS FROM FINANCING ACTIVITIES:
     Proceeds of note payable to
       related party.................      2,000,000       1,000,000        --
     Issuance of employee loans in
       connection with the exercise
       of stock options..............       --              --            (1,086,000)
     Proceeds from issuance of common
       stock.........................     17,401,000       3,238,000      24,075,000
     Purchases of treasury stock.....        (78,000)       --              --
                                       -------------  --------------  --------------
          Net cash provided by
            financing activities.....     19,323,000       4,238,000      22,989,000
IMPACT OF EXCHANGE RATES ON CASH.....       --                (2,000)        173,000
                                       -------------  --------------  --------------
INCREASE (DECREASE) IN CASH AND CASH
  EQUIVALENTS........................     18,622,000      (8,505,000)     15,890,000
CASH AND CASH EQUIVALENTS, beginning
  of year............................     18,235,000      36,857,000      28,352,000
                                       -------------  --------------  --------------
CASH AND CASH EQUIVALENTS, end of
  year...............................  $  36,857,000  $   28,352,000  $   44,242,000
                                       =============  ==============  ==============
SUPPLEMENTAL DISCLOSURE OF CASH FLOW
  INFORMATION:
     Cash paid during the year for
       taxes.........................  $     661,000  $       37,000  $      137,000
     Noncash investing and financing
       activities --
          Capital contribution from
            forgiveness of interest
            by a related party.......        639,000         825,000         738,000
          Receivable related to
            income tax benefit from
            stock options
            exercised................       --               400,000        --
</TABLE>

   The accompanying notes are an integral part of these consolidated financial
                                   statements.

                                       F-6
<PAGE>
                                   TANOX, INC.
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS

1.  ORGANIZATION, BUSINESS AND RISK FACTORS:

Tanox, Inc. (Tanox), was formerly known as Tanox Biosystems, Inc. and was
originally incorporated as a Texas corporation on March 19, 1986. Tanox was
reincorporated in Delaware in January 2000. Tanox is engaged in the discovery
and development of therapeutic products that beneficially influence or are
derived from the immune system. Tanox is focusing its product development
efforts on therapeutics in three broad areas: immunology (asthma/allergy,
autoimmune diseases and inflammation), infectious diseases and oncology.

Tanox has not yet generated any significant revenues from product sales, nor is
there any assurance of significant future revenues from product sales. The
research and development activities engaged in by Tanox involve a high degree of
risk and uncertainty. The ability of Tanox to successfully develop, manufacture
and market its proprietary products is dependent upon many factors. These
factors could include, but are not limited to, the need for additional
financing, the reliance on collaborative arrangements for research and
development, marketing and product commercialization and the ability to develop
or obtain manufacturing, sales and marketing capabilities. Additional factors
could include resolution of ongoing contingencies, including legal proceedings,
changes in the level of sponsored research revenue, uncertainties as to patents
and proprietary technologies, technological change and risk of obsolescence,
development of its products, competition, government regulations and regulatory
approval, and product liability exposure. As a result of the aforementioned
factors and related uncertainties, there can be no assurance of Tanox's future
success.

Tanox entered into a development and licensing agreement with Novartis Pharma AG
(Novartis) in May 1990. Under this agreement, Tanox and Novartis agreed to
jointly develop certain products for IgE-mediated diseases, including asthma and
allergies. Tanox received a contract payment upon signing the agreement and has
received additional contract payments and reimbursement payments upon the
occurrence of specified events. Under a separate agreement (the Stock
Agreement), Tanox and Novartis also agreed to the sale and purchase of shares of
Tanox's common stock. Sales of these shares were completed in May 1990, May 1992
and June 1994. Tanox notified Novartis of the termination of the Stock Agreement
as provided therein, and the termination was effective as of May 10, 1997.
Novartis owned approximately 19.1 percent of Tanox's outstanding common stock at
December 31, 1999.

On December 22, 1993, Tanox sued Genentech, Inc. (Genentech), F. Hoffman-La
Roche, Ltd., Roche Holdings, Inc., Roche Holding Ltd. and Hoffman-La Roche, Inc.
(collectively referred to as "Roche"), in Harris County District Court in
Houston, Texas. The action arose from collaboration discussions between Tanox
and Genentech in 1989 and 1990 relating to Tanox's anti-IgE product as a
treatment for IgE-mediated diseases, including allergy and asthma. In response,
Genentech filed suit against Tanox for patent infringement and subsequently
named Novartis in the suit. Tanox's litigation against Genentech and Roche and
Genentech's litigation against Tanox and Novartis were settled in July 1996.
Contemporaneously with the settlement, Genentech, Novartis and Tanox also
entered into a binding agreement in principle to combine their existing anti-IgE
antibody programs into a cooperative effort to develop and commercialize
selected anti-IgE antibodies (the Three-Party Collaboration). Tanox also entered
into additional licensing arrangements with Genentech. Tanox received an initial
payment upon entering into the Three-Party Collaboration and settling the
litigation. Tanox received and will receive additional milestone and royalty
payments upon the accomplishment of specified subsequent events. Novartis or
Genentech may terminate their participation in the Three-Party Collaboration on
short notice, subject to reversion of product rights to Tanox and the remaining
collaborator.

                                       F-7
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

2.  ACQUISITION OF PANGENETICS B.V.:

In March 1998, Tanox acquired the common stock of Tanox Pharma B.V. (formerly
PanGenetics), a biotechnology company located in Amsterdam, The Netherlands.
Tanox recorded the transaction for accounting purposes as a purchase, and the
consolidated financial statements include the operations of Tanox Pharma
subsequent to the acquisition date. Under the terms of the agreement, Tanox
purchased Tanox Pharma for an initial cash payment of $508,000 and 226,409
shares of common stock, valued at $II.25 per share, for a total initial
consideration of $3,055,000. In addition, Tanox agreed to pay future
consideration, in two installments, totaling up to $667,000 in cash and 484,147
shares of common stock upon occurrence of specified future events. These events
include originating at least three additional reseach projects within a three
year period, retaining the services of two individuals for 36 months and
maintaining a certain level of government grants and subsidies. Any additional
consideration will be paid to all shareholders in proportion to their ownership
at the acquisition date. In September 1999, Tanox made the second installment
payments of $333,000 in cash and 242,075 shares of common stock valued at $12.50
per share, for a total additional consideration of $3,359,000. If the specified
future events take place and the final future payment is made in March 2001,
Tanox will record an additional purchase price amount based on the cash paid and
fair value of the common stock issued at the time of payment.

Tanox engaged an independent firm to perform an appraisal of the assets acquired
in the transaction. The appraisal was completed and the report issued in 1998.
The acquisition of Tanox Pharma was accounted for under the purchase method of
accounting in which the aggregate purchase price was allocated to tangible and
intangible assets acquired based on their relative fair values as of the date of
the transactions. At the time of the acquisition, the total current and future
consideration of the acquisition was valued for accounting purposes at $9.2
million, based on the total of the cash and then fair value of common stock paid
to Tanox Pharma shareholders. Of this amount, we allocated approximately $0.2
million to tangible fixed assets, $0.1 million to intangible assets, $7.2
million to in-process research and development and $1.7 million to goodwill.

The valuation of acquired in-process research and development considered:

  o    the current technological feasibility, scientific and development states
       of the anti-CD40 research project;

  o    the expected amount of time and resources required to complete the
       projects;

  o    the alternative future use of the acquired research and development; and

  o    valuation and allocation approaches.

Tanox's ability to commercialize the acquired anti-CD40 research project is
affected by several risks. These risks include:

  o    Tanox's ability to construct low cost versions of the antibody;

  o    successful preclinical testing;

  o    successful completion of Phase I, II and III clinical testing;

  o    successful filing and acceptance of European and Japanese regulatory
       submissions;

  o    Tanox's ability to successfully commercialize the anti-CD40 monoclonal
       antibody by itself or in connection with a collaborative partner; and

  o    Tanox's ability to manufacture the anti-CD40 monoclonal antibody at a
       competitive cost.

                                       F-8
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

At the time of the acquisition, the anti-CD40 monoclonal antibody required
additional development work and preclinical testing to enter clinical trials. In
order for anti-CD40 to become a marketable product, it was necessary to conduct
several clinical trials and to improve the manufacturing of the product. Tanox
estimated at the time of the acquisition that it would take from seven to nine
years and cost at least $50 million to complete the development of the anti-CD40
product.

The initial purchase price installment was allocated as follows:

               In-process research and development    $2,798,000
               Net working capital ...............       125,000
               Intangible assets .................       100,000
               Tangible fixed assets .............        22,000
               Noncurrent financial assets .......        10,000
                                                      ----------
                    Total ........................    $3,055,000
                                                      ==========

The allocation of the second installment payment of $3,359,000 was assigned to
in-process research and development based on the appraisal. Accordingly, Tanox's
financial statements for the years ended December 31, 1998 and 1999 include a
charge of $2,798,000 in 1998 and $3,359,000 in 1999 for expensing the cost of
the in-process research and development. If the final purchase price payments
are made to the former shareholders of Tanox Pharma in 2001, Tanox anticipates
that the cost of such additional payments will be allocated to acquired
in-process research and development and goodwill based upon the appraisal
obtained as of the date of the acquisition.

If Tanox had acquired PanGenetics prior to January 1, 1997, the unaudited pro
forma financial results would have been as follows:

<TABLE>
<CAPTION>
                                           1997            1998            1999
                                       -------------  --------------  --------------
<S>                                    <C>            <C>             <C>
Total assets.........................  $  44,984,000  $   43,422,000  $   55,328,000
Total revenues.......................      9,384,000       2,439,000       1,405,000
Net income (loss)....................        557,000      (7,473,000)    (19,988,000)
Basic earnings (loss) per share......           0.02           (0.25)          (0.64)
Diluted earnings (loss) per share....           0.02           (0.25)          (0.64)
</TABLE>

3.  SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES:

  PRINCIPLES OF CONSOLIDATION

The accompanying consolidated financial statements include the accounts of Tanox
and its wholly owned subsidiaries, Tanox Pharma International, Inc., Tanox
Pharma B.V. and TanAsia Pharma, Ltd. Intercompany transactions and balances are
eliminated in consolidation.

  USE OF ESTIMATES

The preparation of consolidated financial statements in conformity with
generally accepted accounting principles requires management to make estimates
and assumptions that affect the reported amounts of assets and liabilities and
disclosure of contingent assets and liabilities at the date of the consolidated
financial statements and the reported amounts of expenses during the reporting
period. Actual results could differ from those estimates.

                                       F-9
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

  REVENUE RECOGNITION

Revenues associated with development agreements which include rights to license
or sublicense Tanox's technology or product rights are recognized when payments
are earned. Revenues earned under development agreements include payments for
milestone achievements and payments for sponsored research. Milestone payments
are received under best efforts contracts, and such revenues are not refundable.
Any revenue from milestones is recognized when the milestones are achieved and
there are no remaining performance obligations. Revenues earned in connection
with sponsored research are recognized as Tanox performs its obligations related
to such research. Any revenue contingent upon future performance by Tanox is
deferred and recognized as the performance is completed.

Tanox recognized revenues of $6.3 million, $2.4 million and $1.1 million during
1997, 1998 and 1999, respectively, under the Three-Party Collaboration described
in Note 1. Included in these revenues are milestone payments of $5.0 million,
$1.5 million and $1.0 million, respectively. The remainder of the revenues
related to sponsored research. Expenses incurred related to these agreements
were approximately $1.3 million, $0.2 million and $0.1 million in 1997, 1998 and
1999, respectively. Revenues from collaborative agreements with Novartis
accounted for 15 percent of 1997 revenues, 98 percent of 1998 revenues and 76
percent of 1999 revenues. Revenues from collaborative agreements with Genentech
accounted for 56 percent of 1997 revenues and none of 1998 and 1999 revenues. At
December 31, 1998, Tanox had accounts receivable of approximately $14,000 for
estimated costs reimbursable under the Three-Party Collaboration. There were no
receivables under the Three-Party Collaboration at December 31, 1999.

Tanox recognized revenues of $2.5 million in 1997 from advances for sponsored
research made to Tanox in 1996 and 1997 under a development and licensing
agreement (the Takara Agreement) with Takara Shuzo, Ltd.. The Takara Agreement
was concluded effective November 30, 1997. The Takara Agreement accounted for 28
percent of 1997 revenues.

  CASH EQUIVALENTS

Cash equivalents consist of highly liquid investments with an original maturity
of three months or less when purchased.

  SHORT-TERM INVESTMENTS

Short-term investments consist of U.S. Government agency debt obligations and
investment grade commercial paper with an original maturity greater than three
months but less than one year. Tanox's policy is to hold short-term investments
until maturity. Short-term investments are recorded at cost, which approximates
fair value. Tanox has no available-for-sale or trading securities.

  PROPERTY AND EQUIPMENT

Property and equipment is carried at cost and depreciated on a straight-line
basis over the estimated useful economic lives of the assets or, in the case of
leasehold improvements, over the remaining term of the lease. The estimated
useful lives employed in computing depreciation are three to seven years for
laboratory and office equipment, five to seven years for furniture and fixtures,
and the lesser of nine years or the remaining lease term for leasehold
improvements. When property is retired or otherwise disposed of, the cost and
accumulated depreciation are removed from the accounts and any resulting gain or
loss is included in income. Maintenance and repairs are charged to expense when
incurred.

                                      F-10
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

Tanox has adopted Statement of Financial Accounting Standards (SFAS) No. 121,
"Accounting for the Impairment of Long-Lived Assets and for Long-Lived Assets
to Be Disposed Of." Management periodically reviews long-lived assets and
certain identifiable intangibles whenever events or changes in circumstances
indicate that the carrying amount of an asset may not be recoverable. If factors
indicate that an asset should be evaluated for possible impairment, management
compares estimated undiscounted future operating cash flow from the related
asset to the carrying amount of the asset. If the carrying amount of the asset
were greater than undiscounted future operation cash flow, an impairment loss
would be recognized. Any impairment loss would be computed as the excess of the
carrying amount of the asset over the estimated fair value of the asset
(calculated based on discounting estimated future operating cash flows). The
application of SFAS No. 121 has had no material impact on Tanox's financial
position or results of operations.

  RESEARCH AND DEVELOPMENT

Research and development costs, including incidental patent costs, are expensed
as incurred.

  ACCRUED LIABILITIES

Accrued liabilities at December 31, 1998 and 1999, consist of the following:

                                             1998          1999
                                          ----------    ----------
             Accrued payroll .........    $  413,000    $  282,000
             Accrued vacation ........       178,000       153,000
             Accrued taxes ...........       122,000        55,000
             Accrued rent ............         7,000        24,000
             Accrued professional fees       270,000       264,000
             Other ...................        90,000       169,000
                                          ----------    ----------
                                          $1,080,000    $  947,000
                                          ==========    ==========

See Note 9 for discussion of accrued arbitration award as of December 31, 1999.

  INCOME TAXES

Tanox accounts for income taxes using the liability method prescribed by SFAS
No. 109, "Accounting for Income Taxes." Under this method, deferred income tax
assets and liabilities reflect the impact of temporary differences between the
financial accounting and tax bases of assets and liabilities and are measured
using the enacted tax rates and laws that will be in effect when the differences
are expected to reverse.

  FOREIGN CURRENCY TRANSACTIONS AND TRANSLATIONS

The balance sheet accounts of Tanox are translated into U.S. dollars at exchange
rates in effect on reporting dates. These amounts are reflected in other
comprehensive income. Income statement items are translated at average exchange
rates in effect during the financial statement period. Gains and losses
resulting from foreign currency transactions denominated in currency other than
the functional currency are classified as other income (expense).

                                      F-11
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

  EARNINGS PER SHARE

SFAS No. 128, "Earnings Per Share," requires dual presentation of basic and
diluted earnings per share (EPS). Basic EPS is computed by dividing net income
by the weighted average number of shares of common stock outstanding during the
year. Diluted EPS is computed in the same manner as basic EPS, except that
diluted EPS reflects the potential dilution that would occur if outstanding
options and warrants were exercised.

The following table reconciles basic and diluted EPS for the year ended December
31, 1997. Since Tanox incurred net losses for the years ended December 31, 1998
and 1999, basic and diluted EPS are the same.

                                          NET                     PER SHARE
                                         INCOME       SHARES       AMOUNT
                                       ----------  ------------   ---------
For the year ended December 31,
  1997 --
     Basic EPS.......................  $  630,000    27,909,000     $0.02
                                                                  =========
     Effect of dilutive securities --
       Options and warrants
       outstanding...................      --         3,281,000
                                       ----------  ------------
     Diluted EPS.....................  $  630,000    31,190,000     $0.02
                                       ==========  ============   =========

  OTHER COMPREHENSIVE INCOME (LOSS)

SFAS No. 130, "Reporting Comprehensive Income," establishes standards for
reporting and displaying comprehensive income and its components in an entity's
financial statements, and is effective for fiscal years beginning after December
15, 1997. The standard requires that all items that meet the definition of
components of comprehensive income be reported in Tanox's financial statements.
Tanox has included comprehensive income in its consolidated statements of
operations and comprehensive income (loss).

  CONCENTRATION OF CREDIT RISK

Tanox's receivables are primarily associated with research collaborations with
pharmaceutical and biotechnology companies and grants from foreign government
entities. Tanox does not believe this concentration of credit risk presents a
material risk to Tanox. Tanox does not require collateral from these entities.
Tanox has invested its excess cash generally in high-quality commercial paper
and U.S. Government agency debt obligations. As of December 31, 1999, these
investments mature within 68 days of year-end and, therefore, management
believes that they bear minimal risk. Tanox has not experienced any losses on
its investments.

                                      F-12
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

4.  GEOGRAPHIC AREAS:

Tanox operates in a single business segment. Tanox's operations by geographic
area for the years ended December 31, 1997, 1998 and 1999, are presented below:

<TABLE>
<CAPTION>

                                      TOTAL          NET INCOME      IDENTIFIABLE
                                     REVENUES          (LOSS)           ASSETS
                                   ------------     ------------     ------------
<S>                                <C>              <C>                <C>
Year ended December 31, 1997 --
     North America ............    $  8,939,000     $    630,000       43,330,000
     Asia .....................            --               --          1,501,000
                                   ------------     ------------     ------------
                                   $  8,939,000     $    630,000     $ 44,831,000
                                   ============     ============     ============
Year ended December 31, 1998 --
     North America ............    $  3,089,000     $ (8,983,000)    $ 42,794,000
     Europe ...................          39,000       (1,192,000)         282,000
     Asia .....................            --              6,000        1,507,000
     Interarea eliminations ...        (706,000)            --         (1,161,000)
                                   ------------     ------------     ------------
                                   $  2,422,000     $(10,169,000)    $ 43,422,000
                                   ============     ============     ============
Year ended December 31, 1999 --
     North America ............    $  3,865,000     $(19,242,000)    $ 58,522,000
     Europe ...................         244,000       (4,039,000)         754,000
     Asia .....................            --            (65,000)       1,534,000
     Interarea eliminations ...      (2,704,000)            --         (5,482,000)
                                   ------------     ------------     ------------
                                   $  1,405,000     $(23,346,000)    $ 55,328,000
                                   ============     ============     ============
</TABLE>

5.  NOTE PAYABLE TO RELATED PARTY:

Novartis has advanced Tanox $10.0 million pursuant to a loan agreement to
finance a new clinical manufacturing facility. The loan bears interest at LIBOR
plus two percent (7.3 percent and 8.1 percent at December 31, 1998 and 1999,
respectively). Through December 31, 1999, Novartis has agreed to forgive
interest on the loan. For the years ended December 31, 1997, 1998 and 1999, the
interest forgiven by Novartis has been reflected as interest expense and a
capital contribution. These amounts totalled $639,000, $825,000 and $738,000,
respectively for the years 1997 through 1999. The loan is due December 31, 2005.
Subject to modifications agreed to in principle concurrent with completion of
the Three-Party Collaboration, the principal and future interest payments may be
partially or totally forgiven by Novartis based on the future use of the
facility.

6.  INCOME TAXES:

Tanox's pretax income (loss) consists of the following:


                                       1997           1998            1999
                                   ------------  --------------  --------------
U.S. ............................  $  1,253,000  $   (7,973,000) $  (16,505,000)
Foreign..........................      (425,000)     (3,729,000)     (6,807,000)
                                   ------------  --------------  --------------
                                   $    828,000  $  (11,702,000) $  (23,312,000)
                                   ============  ==============  ==============

                                      F-13
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

The components of the provision (benefit) for income taxes are as follows:

                                          1997         1998         1999
                                       ----------  -------------  ---------
Current..............................  $  198,000  $  (1,533,000) $  34,000
Deferred.............................      --           --           --
                                       ----------  -------------  ---------
                                       $  198,000  $  (1,533,000) $  34,000
                                       ==========  =============  =========

Tanox's effective income tax rate for 1997 was less than the statutory federal
income tax rate of 35 percent primarily due to benefits from franchise tax
adjustments and research and developments tax credits that were partially offset
by increases in the valuation allowance. For 1998 and 1999, the benefit was less
than that computed at the statutory rate primarily due to an increase in the
valuation allowance and nondeductible foreign losses. At December 31, 1998,
Tanox recorded a $2.1 million income tax receivable related to the carryback of
1998 losses to prior periods. Of this receivable $2.0 million was collected in
1999 and the remaining $0.1 million will be received in 2000.

Significant components of Tanox's deferred tax assets are as follows:

                                           1998            1999
                                       -------------  --------------
Federal net operating loss
  carryforward.......................  $    --        $    1,821,000
In-process research and
  development........................        979,000       2,038,000
Foreign net operating loss
  carryforwards......................        417,000       1,831,000
Deferred compensation related to
  stock options......................        360,000       1,909,000
Research and development tax
  credits............................        278,000       1,019,000
Alternative minimum tax credit.......        171,000         248,000
Differences in book and tax
  depreciation.......................        211,000        --
Capitalized interest.................        308,000         300,000
Accruals not currently deductible....        157,000       1,291,000
Other, net...........................          4,000          28,000
                                       -------------  --------------
     Total deferred tax assets.......      2,885,000      10,485,000
Differences in book and tax
  depreciation.......................       --              (130,000)
Deferred tax valuation allowance.....     (2,885,000)    (10,355,000)
                                       -------------  --------------
     Net deferred taxes..............  $    --        $     --
                                       =============  ==============

At December 31, 1999, Tanox has a net operating loss of approximately $6,560,000
for federal income tax reporting purposes. Tanox's intent is to carry back
approximately $1,360,000 of the net operating loss to prior years to obtain a
refund of approximately $132,000. The remaining net operating loss will begin to
expire in 2019. Tanox also has a foreign net operating loss carryforward of
approximately $5,230,000 which will be available to offset the separate company
taxable incomes of certain foreign subsidiaries. Additionally, Tanox has an
unused U.S. research and development tax credit carryforward at December 31,
1999, of approximately $1,019,000 which will begin to expire in 20II. Tanox also
has alternative minimum tax credit carryforwards of approximately $248,000 as of
December 31, 1999. As Tanox has incurred cumulative losses to date and there is
no assurance of future taxable income, a valuation allowance has been
established to fully offset the deferred tax asset at December 31, 1998 and
1999. Tanox's valuation allowance increased from $2,885,000 at December 31,
1998, to $10,355,000 at December 31, 1999, primarily due to Tanox's increase in
net operating loss carryforwards, tax credit carryforwards and deferred
compensation.

                                      F-14
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

7.  LEASE OBLIGATIONS:

Tanox leases its facilities pursuant to various operating leases that expire at
various dates through March 2002. Future minimum lease obligations under
noncancelable leases at December 31, 1999, are as follows:


                     Year ending December 31 --
                          2000 ................    $305,000
                          2001 ................     258,000
                          2002 ................      65,000
                                                   --------
                               Total ..........    $628,000
                                                   ========

Tanox incurred rent expense of $287,000, $344,000 and $331,000 in 1997, 1998 and
1999, respectively. Certain of the facility leases include escalation clauses
for operating expenses and real estate taxes.

At December 31, 1999, Tanox had outstanding an unsecured, irrevocable letter of
credit for $20,000 related to a lease agreement.

8.  CAPITAL STOCK:

  PREFERRED STOCK

Tanox is authorized to issue up to 10,000,000 shares of $.01 par value preferred
stock. The board of directors has the authority to issue these shares in one or
more series and to establish the rights, preferences and dividends. No shares of
preferred stock have been issued.

  STOCK SPLIT

On March 31, 1997, Tanox declared a stock dividend to effect a stock split that
provided two shares of Tanox's common stock for every one share of Tanox's
common stock held by stockholders of record as of March 21, 1997. On February 1,
2000, Tanox declared a stock dividend to effect a stock split that provided 1.6
shares of Tanox's common stock for every one share of Tanox's common stock held
by stockholders of record as of January 31, 2000. In both cases the aggregate
par value of the dividend was transferred from additional paid-in capital to
common stock. The stock splits have been retroactively reflected in the
accompanying consolidated financial statements.

  STOCK OPTIONS

During 1987, Tanox established the 1987 Stock Option Plan (the 1987 Plan)
covering key employees, officers and directors of Tanox. Under the terms of the
1987 Plan, as amended, the number of shares of common stock eligible for
issuance was 4,320,000. Options issued under the 1987 Plan were generally
granted at a purchase price equal to the fair market value at the date of grant
and are generally exercisable beginning two years after the date of grant for 40
percent of the shares, with the balance to become exercisable cumulatively in
three installments of 20 percent each year thereafter. Options expire ten years
after the date of grant. At December 31, 1999, options to purchase 2,242,800
shares of Tanox's common stock were outstanding under the 1987 Plan. The 1987
Plan expired June 24, 1997, and no more shares may be granted under this plan.

Tanox established the 1997 Stock Plan (the 1997 Plan) in November 1997. Under
the terms of the 1997 Plan, Tanox may grant options to purchase up to 8,000,000
shares of Tanox's common stock to employees,

                                      F-15
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

directors, advisors and consultants. The 1997 Plan also provides for several
types of grants including incentive stock options, non-qualified stock options,
stock appreciation rights, stock awards, stock purchases and performance units.
Incentive stock options provide the right to purchase common stock at a price
not less than 100 percent of the fair value of common stock on the date of the
grant. Non-qualified stock options provide the right to purchase common stock at
a price not less than 50 percent of the fair value of the common stock on the
date of the grant. The options granted under the 1997 Plan generally expire ten
years after date of grant and are generally completely exercisable five years
after the grant date. At December 31, 1999, options to purchase 636,320 shares
of common stock were outstanding and 7,360,480 were available for future grants.
The 1997 Plan will expire on October 31, 2007.

In January 1992, Tanox established the 1992 Non-Employee Directors Stock Option
Plan (the Directors Plan) and reserved 480,000 shares of common stock for
issuance upon the exercise of options granted pursuant to the Directors Plan.
Unless otherwise provided, options granted under the Directors Plan will vest
one-third annually from the date of grant. The exercise price of the options
granted will be determined by a committee appointed by Tanox's board of
directors. At December 31, 1999, options to purchase 108,000 shares of Tanox's
common stock were outstanding under the Directors Plan and options to purchase
372,000 shares were available for future grants.

In addition to the plans discussed above, Tanox has entered into various stock
option agreements with certain outside consultants and advisors. At December 31,
1999, options to purchase 196,800 shares of Tanox's common stock were
outstanding under such agreements. All of the outstanding options issued under
these agreements were issued prior to 1996, are currently exercisable and expire
in either 2002 or 2003.

At December 31, 1999, options to purchase 3,183,920 shares were outstanding with
a weighted average exercise price of $4.39 per share, of which options to
purchase 2,106,318 shares were exercisable at a weighted average exercise price
of $2.79 per share. The following table summarizes stock option transactions
since December 31, 1996:


                                                                     WEIGHTED
                                     NUMBER OF      EXERCISE         AVERAGE
                                       SHARES         PRICE       EXERCISE PRICE
                                    ------------  -------------   --------------
Outstanding, December 31, 1996....     3,680,400  $  0.21- 5.28       $ 0.95
     Granted......................     1,337,600     3.75- 7.50         6.94
     Exercised....................       (47,040)    0.63- 2.50         1.50
     Canceled.....................       (54,240)    0.21- 3.02         1.47
                                    ------------  -------------   --------------
Outstanding, December 31, 1997....     4,916,720     0.21- 7.50         2.57
     Granted......................       322,400     5.63-II.25         7.81
     Exercised....................      (230,400)    0.21- 2.29         0.98
     Canceled.....................      (127,600)    2.29- 7.50         6.86
                                    ------------  -------------   --------------
Outstanding, December 31, 1998....     4,881,120     0.21-II.25         2.88
     Granted......................       137,120     4.06-12.50        10.27
     Exercised....................    (1,789,520)    0.63- 5.63         0.64
     Canceled.....................       (44,800)    2.29-II.25         7.61
                                    ------------  -------------   --------------
     Outstanding, December 31,
       1999.......................     3,183,920  $  0.21-12.50       $ 4.39
                                    ============  =============   ==============
     Exercisable, December 31,
       1999.......................     2,106,318  $  0.21-12.50       $ 2.79
                                    ============  =============   ==============

                                      F-16
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

Tanox follows SFAS No. 123 which permits one of two methods of accounting for
stock options. Tanox adopted Accounting Principles Board Opinion (APB) No. 25,
"Accounting for Stock Issued to Employees," and related interpretations in
accounting for its employee stock options. SFAS No. 123, however, requires Tanox
to disclose the income statement effect of the alternative fair value method
assuming Tanox was required to record compensation expense for stock options
equal to the assumed fair value on the grant date.

Under APB No. 25, Tanox recognizes as compensation expense the excess of the
estimated fair value of the common stock issuable upon exercise of such options
over the aggregate exercise price of such options on the date of grant. This
compensation expense is amortized ratably over the vesting period of each
option. The compensation expense of such options, net of reversals for
terminations, was $25,000, $273,000 and $311,000 during the years ended December
31, 1997, 1998 and 1999, respectively.

The fair value of each option grant is estimated using the Black-Scholes option
pricing model. The Black-Scholes model uses grant price, as stated in the option
agreements, market price as established by stock sales in 1997, 1998 and 1999
and deemed market prices established by the Compensation Committee of Tanox's
board of directors. The following assumptions were used for options granted in
1997: risk-free interest rate of six percent, expected option life of ten years,
no expected dividends, no expected turnover and a 20 percent volatility factor.
The following assumptions were used for options granted in 1998: risk-free
interest rate of five percent, expected option life of six years, no expected
dividends, expected turnover of 20 percent and a volatility factor of 20
percent. The assumptions used for options granted in 1999 were the same as those
used in 1998, with the exception of the risk-free interest rate which was six
percent and the volatility factor which was 42 percent.

Assuming the compensation cost for these plans had been determined pursuant to
the fair value method under SFAS No. 123, Tanox's pro forma net income (loss)
would have been as follows:


                                       1997          1998            1999
                                    ----------  --------------  --------------
Net income (loss) --
     As reported..................  $  630,000  $  (10,169,000) $  (23,346,000)
     Pro forma....................     176,000     (11,260,000)    (24,551,000)
Basic EPS --
     As reported..................        0.02           (0.35)          (0.75)
     Pro forma....................        0.01           (0.39)          (0.79)
Diluted EPS --
     As reported..................        0.02           (0.35)          (0.75)
     Pro forma....................        0.01           (0.39)          (0.79)

Because the SFAS No. 123 method of accounting has not been applied to options
granted prior to January 1, 1996, the resulting pro forma compensation cost may
not be representative of that to be expected in future years.

                                      F-17
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

The number and weighted average fair value of options granted in 1997, 1998 and
1999 is as follows:

<TABLE>
<CAPTION>
                                                 1997                       1998                       1999
                                       ------------------------   ------------------------   ------------------------
                                                     WEIGHTED                   WEIGHTED                   WEIGHTED
                                                      AVERAGE                    AVERAGE                    AVERAGE
                                         SHARES     FAIR VALUE      SHARES     FAIR VALUE      SHARES     FAIR VALUE
                                       ----------   -----------   ----------   -----------   ----------   -----------
<S>                                    <C>          <C>           <C>          <C>           <C>          <C>
Option price equals fair market
  value..............................   1,137,600      $3.69         234,400      $2.71         129,120      $5.24
Option price greater than fair market
  value..............................      --          --              8,000       1.18          --          --
Option price less than fair market
  value..............................     200,000       5.46          80,000       5.63           8,000       5.65
</TABLE>

The following table summarizes information about fixed-price stock options
outstanding at December 31, 1999:

<TABLE>
<CAPTION>
                                     OPTIONS OUTSTANDING
- ---------------------------------------------------------------------------------------------
                                                              WEIGHTED                                    OPTIONS EXERCISABLE
                                                               AVERAGE                             ---------------------------------
                                          OUTSTANDING         REMAINING                            EXERCISABLE
                                             AS OF           CONTRACTUAL         WEIGHTED             AS OF             WEIGHTED
              RANGE OF                    DECEMBER 31,          LIFE              AVERAGE          DECEMBER 31,          AVERAGE
           EXERCISE PRICES                    1999           (IN YEARS)       EXERCISE PRICE           1999          EXERCISE PRICE
- -------------------------------------     ------------      -------------     ---------------      ------------      ---------------
<S>                                       <C>               <C>               <C>                  <C>               <C>
$ 0.21 - $ 2.50                            1,386,000             2.3              $  1.06           1,386,000            $  1.06
  2.50 -   5.00                              354,400             5.7                 3.46             226,400               3.28
  5.00 -   7.50                            1,080,000             7.1                 7.35             443,839               7.30
  7.50 -  10.00                              264,800             8.8                 8.13              42,079               8.13
 10.00 -  12.50                               98,720             8.9                12.20               8,000              12.50
                                          ------------                                             ------------
$ 0.21 -  12.50                            3,183,920             5.0              $  4.39           2,106,318            $  2.79
                                          ============                                             ============
</TABLE>

In April 1999, Tanox loaned 12 employees approximately $1,086,000 to enable the
employees to exercise 1,738,320 options to purchase shares of Tanox's common
stock, pursuant to stock options held by such employees. All of the loans are
full-recourse, secured by shares of Tanox's common stock owned by the employees,
bear interest at a rate of 8.5 percent, and are due and payable in full in
September 2001. The loans have been reflected as a contra equity in the
accompanying financial statements.

Also during 1999, Tanox agreed to extend, for an additional three-year period,
the term of 524,400 stock options held by certain employees and consultants that
were scheduled to expire. In connection with this extension, Tanox expensed, for
accounting purposes, approximately $4,576,000 of compensation expense
representing the fair value of the options as of the remeasurement date. For
employees, compensation expense was recorded for the difference between the fair
value of the underlying stock on the date of the extension and the exercise
price of the option. For consultants, compensation expense was calculated using
the Black-Scholes valuation model on the date of the extension.

  WARRANTS

In connection with the issuance of notes payable to an unrelated company for
equipment financing in 1989, Tanox issued warrants to purchase 93,053 shares of
Tanox's common stock at $0.86 per share. All of the

                                      F-18
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

warrants were exercised on a net issuance basis in 1999 and converted into
86,632 shares of common stock. As of December 31, 1999, there were no
outstanding warrants.

9.  COMMITMENTS AND CONTINGENCIES:

  ARBITRATIONS

Following settlement of Tanox's lawsuit against Genentech and Roche referred to
in Note 1, Tanox filed a demand for arbitration against the attorneys who
represented Tanox in the litigation in order to resolve a dispute over the
amount of attorneys' fees due by Tanox. On September 29, 1999, the arbitration
panel issued an award entitling the attorneys to receive approximately $3.5
million, including interest, payments ranging from 33 1/3 percent to 40 percent
of the future payments that Tanox may receive from Genentech following product
approval and ten percent of the royalties that Tanox may receive on sales of
anti-IgE products. At December 31, 1999, Tanox has reflected an accrued expense
of $3.5 million for the arbitration award in its consolidated financial
statements.

Tanox sought a court order vacating this arbitration award. However, a judgment
was entered confirming the award. Tanox intends to pursue all available
remedies, including appealing the decision. If Tanox is ultimately required to
pay all or part of the award to the attorneys, Tanox could be required to pay up
to $3.5 million, plus accrued interest would become due, and the award would
effectively reduce certain future milestone payments from Genentech by up to 40
percent and reduce future royalties from the Three-Party Collaboration by ten
percent. Tanox's future revenues, results of operations, cash flows and
financial condition could be materially adversely affected. During the appeals
process we will either post a bond or place cash in escrow to secure payment of
the award.

Tanox is also engaged in a dispute with Novartis and Genentech over its right to
independently develop certain of its anti-IgE monoclonal antibodies, which are
not being developed in connection with the Three-Party Collaboration. Tanox is
attempting to resolve the dispute in separate arbitrations with each of Novartis
and Genentech and they are attempting to resolve the dispute in federal court.
If Tanox ultimately loses its right to independently develop these anti-IgE
monoclonal antibodies, Tanox may be required to discontinue development of
Hu-901.

  LITIGATION

From time to time, Tanox is a defendant in lawsuits incidental to its business.
Management believes that the outcome of these lawsuits will not be material to
Tanox's financial statements.

  MILESTONES AND ROYALTIES

Tanox has agreements with several institutions that call for payments upon the
achievement of milestones by Tanox and royalty payments based upon a percentage
of product sales. No milestone or royalty expense has been incurred related to
these agreements.

  LOAN COMMITMENTS

Tanox has agreed that under certain conditions, it will lend certain of its
employees up to $1,500,000 in April 2000 for payment of their tax obligations
pursuant to the exercise of their stock options.

                                      F-19
<PAGE>
                                   TANOX, INC.
            NOTES TO CONSOLIDATED FINANCIAL STATEMENTS -- (CONTINUED)

  REGISTRATION RIGHTS

Some of Tanox's stockholders including founders, some early investors and
persons who hold 15 percent or more of our stock, have certain registration
rights.

  401(K) PLAN

Effective January 1, 1992, Tanox adopted a qualified retirement plan (the 401(k)
Plan) covering all of Tanox's employees who are at least 21 years of age and
have completed at least one year of service with Tanox. Pursuant to the 401(k)
Plan, employees may elect to reduce their current compensation by up to the
statutorily prescribed annual limit and have the amount of such reduction
contributed to the 401(k) Plan. The 401(k) Plan permits, but does not require,
additional matching contributions by Tanox on behalf of all participants in the
401(k) Plan. Tanox's contributions totaled approximately $81,000, $54,000 and
$66,000 in 1997, 1998 and 1999, respectively, representing matching 50 percent
of employee contributions, including those made by executive officers. Tanox's
matching contribution only applies to the first five percent of each employee's
total compensation.

                                      F-20
<PAGE>
                 (This page has been intentionally left blank)
<PAGE>
- --------------------------------------------------------------------------------

                                   [LOGO]

                                 TANOX, INC.
                              7,000,000 SHARES
                                COMMON STOCK

                       ------------------------------
                                 PROSPECTUS
                       ------------------------------

                                          , 2000

                             CIBC WORLD MARKETS
                             ROBERTSON STEPHENS
                           WARBURG DILLON READ LLC
                        ADAMS, HARKNESS & HILL, INC.
                               KBC SECURITIES

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

YOU SHOULD RELY ONLY ON THE INFORMATION CONTAINED IN THIS PROSPECTUS. NO DEALER,
SALESPERSON OR OTHER PERSON IS AUTHORIZED TO GIVE INFORMATION THAT IS NOT
CONTAINED IN THIS PROSPECTUS. THIS PROSPECTUS IS NOT AN OFFER TO SELL NOR IS IT
SEEKING AN OFFER TO BUY THESE SECURITIES IN ANY JURISDICTION WHERE THE OFFER OR
SALE IS NOT PERMITTED. THE INFORMATION CONTAINED IN THIS PROSPECTUS IS CORRECT
ONLY AS OF THE DATE OF THIS PROSPECTUS, REGARDLESS OF THE TIME OF THE DELIVERY
OF THIS PROSPECTUS OR ANY SALE OF THESE SECURITIES.

UNTIL                , 2000 (25 DAYS AFTER THE COMMENCEMENT OF THE OFFERING),
ALL DEALERS THAT EFFECT TRANSACTIONS IN THESE SECURITIES, WHETHER OR NOT
PARTICIPATING IN THIS OFFERING, MAY BE REQUIRED TO DELIVER A PROSPECTUS. THIS IS
IN ADDITION TO THE DEALERS' OBLIGATION TO DELIVER A PROSPECTUS WHEN ACTING AS
UNDERWRITERS AND WITH RESPECT TO THEIR UNSOLD ALLOTMENTS OR SUBSCRIPTIONS.


<PAGE>
                                    PART II
                     INFORMATION NOT REQUIRED IN PROSPECTUS

ITEM 13.  OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.

The following table sets forth all expenses, other than underwriting discounts
and commissions payable by the Registrant in connection with the sale of the
common stock being registered. All the amounts shown are estimates except for
the registration fee, NASD filing fee and the Nasdaq National Market listing
fee.


Registration fee.....................  $     63,756
NASD filing fee......................        24,650
Nasdaq National Market listing fee...        95,000
Printing and engraving expenses......       170,000*
Legal fees and expenses..............       300,000*
Accounting fees and expenses.........       200,000*
Blue Sky fees and expenses...........        15,000*
Transfer agent and registrar fees....        15,500*
Premium for directors and officers
insurance............................       150,000*
Miscellaneous........................        51,094*
                                       ------------
     Total...........................     1,085,000*
                                       ============


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

*  Estimated

ITEM 14.  INDEMNIFICATION OF DIRECTORS AND OFFICERS.

Our amended and restated certificate of incorporation requires us to indemnify
our directors and officers against liabilities they may incur in these
capacities, including liabilities under the Securities Act, as amended, to the
fullest extent permitted by Section 145 of the Delaware General Corporation Law.

Our bylaws require that we indemnify each of our directors and officers for the
expenses, judgments, fines and amounts paid in settlement actually and
reasonably incurred in connection with any type of threatened, pending or
completed action, suit or proceeding (other than actions by us or on our behalf)
if he or she:

  o    acted in good faith and in a manner he or she reasonably believed to be
       in or not opposed to our best interests; and

  o    in the case of a criminal proceeding (including preliminary), had no
       reason to believe his or her conduct was unlawful.

Under our bylaws we also must indemnify a director and officer for expenses of
an action brought by us or on our behalf if he or she acted in good faith and in
a manner he or she reasonably believed to be in or not opposed to our best
interests. We may not indemnify a director or officer for expenses of an action
brought by us or on our behalf if the director or officer is adjudged liable to
the corporation, unless a court determines that, despite such adjudication but
in view of all of the circumstances, he or she is entitled to indemnification of
such expenses.

We have a duty to indemnify only if the director or officer has met the
applicable standard of conduct described above. This is determined by:

  o    a majority vote of the disinterested directors; or

  o    a majority vote of a committee of disinterested directors designated by a
       majority vote of the disinterested directors; or

                                      II-1
<PAGE>
  o    if there are no disinterested directors, or if the disinterested
       directors so direct, independent legal counsel in a written opinion; or

  o    the stockholders.

We must indemnify a director or officer for all expenses of litigation or other
legal proceedings actually and reasonably incurred when he or she is successful
on the merits or otherwise in defense of the litigation or proceeding or in
defense of any claim, issue or matter therein.

We must advance to a director or officer the actually and reasonably incurred
expenses incurred in defending an action before the action is finally disposed
if the director undertakes to repay these expenses if the director is ultimately
determined not entitled to be indemnified in connection with the action to which
the expenses relate. We may advance to an officer the expenses incurred in
defending an action before the action is finally disposed if the board of
directors authorizes the advance, and the officer undertakes to repay these
expenses if the officer is ultimately determined not entitled to be indemnified
in connection with the action to which the expenses relate. The board of
directors may not consider the officer's financial ability to repay these
advances in determining whether to authorize the advancement.

We may purchase and maintain insurance on behalf of any director and officer to
the extent permitted by Section 145. We intend to purchase liability insurance
policies covering our directors and officers in certain circumstances.

In addition to the indemnification rights described above, our amended and
restated certificate of incorporation eliminates, in certain circumstances, our
directors' liability for monetary damages for breach of fiduciary duty as a
director. These provisions do not eliminate a director's liability:

  o    for any breach of the director's duty of loyalty to us or our
       stockholders;

  o    for acts or omissions not in good faith or which involve intentional
       misconduct or a knowing violation of law;

  o    under Section 174 of the Delaware General Corporation Law, which relates
       to the declaration of dividends and purchase or redemption of shares in
       violation of this law; or

  o    for any transaction from which the director derived an improper personal
       benefit.

Prior to the completion of this offering, we have entered into indemnification
agreements with each of our directors and officers to give them additional
contractual assurances regarding the scope of the indemnification provided in
our amended and restated certificate of incorporation and bylaws and to provide
additional procedural protections.

The underwriting agreement (exhibit 1.1 to this Registration Statement) provides
that the underwriters must, under some circumstances, indemnify our directors,
officers and controlling persons against specified liabilities, including
liabilities under the Securities Act.

ITEM 15.  RECENT SALES OF UNREGISTERED SECURITIES.

Since January 1, 1997, the Registrant has sold and issued the following
unregistered securities:

(a)   Between February 21, 1997 and September 4, 1997, the Registrant issued and
      sold an aggregate of 2,331,260 shares of its common stock to 23 non-U.S.
      persons in reliance on Regulation S promulgated under the Securities Act
      for an aggregate consideration of $17,484,468.

(b)   Between February 16, 1998 and April 6, 1998, the Registrant issued and
      sold an aggregate of 273,684 shares of its common stock to 8 non-U.S.
      persons in reliance on Regulation S promulgated under the Securities Act
      for an aggregate consideration of $3,078,972.

(c)   Between August 31, 1999 and December 30, 1999, the Registrant issued and
      sold an aggregate of 880,000 shares of its common stock to 14 non-U.S.
      persons in reliance on Regulation S promulgated under the Securities Act
      for an aggregate consideration of $11,000,000. The placement agent was KBC
      Securities, NV. The Registrant paid to KBC Securities, N.V. a placement
      agent fee of $715,000.

                                      II-2
<PAGE>
(d)   Between October 25, 1999 and December 10, 1999, the Registrant issued and
      sold an aggregate of 1,016,000 shares of its common stock to 12 non-U.S.
      persons in reliance on Regulation S promulgated under the Securities Act
      for an aggregate consideration of $12,700,000.

(e)   Between March 12, 1998 and September 12, 1999, the Registrant issued
      468,726 shares of its common stock, at a price per share (as of March 12,
      1998) of $18, with an aggregate value of $4,395,150, to 3 United States
      residents and 5 non-U.S. persons in connection with the Registrant's
      acquisition of Tanox Pharma, formerly PanGenetics B.V. The sale and
      issuance were exempt from registration under Section 4(2) of the
      Securities Act.

(f)   From time to time since January 1, 1997, the Registrant has granted stock
      options to purchase shares of its common stock to various employees,
      directors and consultants pursuant to its 1997 Stock Plan and its 1992
      Non-Employee Directors Stock Option Plan. With respect to all grants of
      options, exemption from registration was unnecessary in that the
      transactions did not involve a "sale" of securities as that term is used
      in Section 2(a)(3) of the Securities Act.

(g)   As of March 8, 2000, the Registrant had issued and sold, in the aggregate,
      2,292,747 shares of its common stock for per share exercise prices ranging
      from $0.28 to $8.13 to employees and one consultant pursuant to their
      exercise of stock options granted under the Registrant's 1987 Stock Option
      Plan and 1997 Stock Plan. The Registrant relied on the exemption provided
      by Rule 701 under the Securities Act.

(h)   Between June 24, 1997 and February 9, 2000, the Registrant issued and sold
      242,000 shares of its common stock for per share exercise prices ranging
      from $0.21 to $2.29, to consultants of the Registrant, pursuant to their
      exercise of stock options. The Registrant relied on the exemption provided
      by Rule 701 under the Securities Act.

(i)   On October 25, 1999, the Registrant issued and sold to Phoenix Leasing
      Incorporated 86,632 shares of its common stock, on a net issuance basis,
      at a price per share of $0.86, for an aggregate consideration of 6,421
      shares of common stock, pursuant to the exercise by Phoenix Leasing
      Incorporated of warrants.

All sales of common stock made pursuant to the exercise of stock options were
made in reliance on Section 701 under the Securities Act or Section 4(2) of the
Securities Act. All other sales were made in reliance on Section 4(2) of the
Securities Act and Regulation S promulgated under the Securities Act. These
sales were made without general solicitation or advertising, to investors who
were sophisticated and had access to all relevant information necessary to
evaluate the investment, and who represented to the Registrant that they were
acquiring the securities for investment and appropriate legends were affixed to
the share certificates issued in such transactions.

ITEM 16.  EXHIBITS AND FINANCIAL STATEMENT SCHEDULES.

(a)  Exhibits

           1.1+      -- Form of Underwriting Agreement.
           3.1*      -- Amended and Restated Certificate of
                        Incorporation of the Registrant, as
                        amended, as currently in effect.
           3.2*      -- Bylaws of the Registrant, as
                        currently in effect.
           4.1*      -- Specimen of Common Stock Certificate,
                        $.01 par value, of the Registrant.
           4.2*      -- Warrant to Purchase 6,462 Shares of
                        Common Stock, dated November 1989, by
                        and between the Registrant and
                        Phoenix Venture Incorporated.
           5.1+      -- Opinion of Chamberlain, Hrdlicka,
                        White, Williams & Martin.

                                      II-3
<PAGE>

          10.1*      -- Form of Indemnification Agreement
                        between the Registrant and its
                        officers and directors.
          10.2*      -- 1987 Stock Option Plan of the
                        Registrant, as amended.
          10.3*      -- 1992 Non-employee Directors Stock
                        Option Plan of the Registrant.
          10.4*      -- 1997 Stock Plan of the Registrant.
          10.5*      -- Lease of premises at 10301 Stella
                        Link, Suite 110, Houston, Texas,
                        dated December 3, 1986, as amended.
          10.6*      -- Stock Purchase Agreement, dated July
                        14, 1987, by and among the Registrant
                        and Tse Wen Chang, Nancy T. Chang,
                        Alafi Capital Company, Shireen Alafi,
                        Joseph Heskel, Trustee for
                        Christopher Alafi, and Invitron
                        Corporation.
          10.7++*   --  License for Winter Patent, dated June
                        26, 1989, by and between Medical
                        Research Council and the Registrant.
          10.8++*   --  Amendment to the License for Winter
                        Patent, dated February 9, 1990, by
                        and between Medical Research Council
                        and the Registrant.
          10.9++*   --  Development and Licensing Agreement,
                        dated May 11, 1990, by and between
                        the Registrant and Ciba-Geigy
                        Limited.
          10.10*     -- Term Sheet for Secured Loan, dated
                        December 14, 1994, by and between the
                        Registrant and Ciba-Geigy Limited.
          10.11++*  --  Chiron-PanGenetics Research and
                        Development License and Options for
                        Commercial License, dated September
                        25, 1995, by and between PanGenetics,
                        B.V., Panorama Research, Inc. and
                        Chiron Corporation.
          10.12*     -- Stock Purchase Agreement, dated as of
                        March 12, 1998, by and between the
                        Registrant and the holders of shares
                        of PanGenetics, B.V.
          10.13++*  --  License Agreement, dated June 1,
                        1998, by and between Biogen, Inc. and
                        the Registrant.
          10.14++*  --  Outline of Terms for Settlement of
                        the Litigations Among Genentech,
                        Inc., Genentech International, Ltd.,
                        the Registrant and Ciba-Geigy Limited
                        Relating to Anti-IgE Inhibiting
                        Monoclonal Antibodies, dated July 8,
                        1996.
          10.15++*  --  Supplemental Agreement between the
                        Registrant and Ciba-Geigy Limited,
                        dated July 8, 1996.
          10.16++*  --  Settlement and Cross-Licensing
                        Agreement, dated July 8, 1996, by and
                        between the Registrant and Genentech,
                        Inc. and Genentech International
                        Limited.
          10.17++*  --  Settlement and Participation
                        Agreement, dated July 8, 1996, by and
                        between the Registrant and F.
                        Hoffman-La Roche, Ltd., Hoffman-La
                        Roche, Inc., Roche Holding Ltd. and
                        Roche Holdings, Inc.
          10.18++*  --  Patent License Agreement, dated June
                        30, 1998, by and between Protein
                        Design Labs, Inc. and the Registrant.
          10.19++*  --  Amendment to Patent License
                        Agreement, dated June 28, 1999, by
                        and between Protein Design Labs, Inc.
                        and the Registrant.
          10.20*     -- Research and License Agreement, dated
                        April 21, 1999, by and between the
                        Registrant and Biovation Limited.
          10.21*     -- Material Transfer and Commercial
                        Evaluation Agreement, dated March 9,
                        1999, by and between Tanox, Inc. and
                        Biovation Limited.
          10.22++*  --  G-CSF Receptor Non-exclusive License
                        Agreement, dated January 11, 2000, by
                        and between Immunex Corporation and
                        the Registrant.
          10.23*     -- 2000 Non-Employee Directors' Stock
                        Option Plan.
          10.24++    -- Master Agreement, dated March 17, 2000,
                        by and between the Registrant and Protein
                        Design Labs, Inc.
          21.1*      -- List of Subsidiaries of the
                        Registrant.
          23.1*      -- Consent of Arthur Andersen LLP dated
                        March 6, 2000.
          23.2+      -- Consent of Chamberlain, Hrdlicka,
                        White, Williams & Martin (included in
                        Exhibit 5.1).
          23.3*      -- Consent of KPMG LLP, dated March 5,
                        2000.
          24.1*      -- Power of Attorney.
          27*        -- Financial Data Schedule.

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

 +  To be filed by amendment.

++  Certain confidential material contained in the document has been omitted and
    filed separately with the Securities and Exchange Commission pursuant to
    Rule 406 of the Securities Act.

 *  Previously filed

                                      II-4
<PAGE>
(b)  Financial Statement Schedules

All schedules are omitted because they are not applicable or because the
required information is contained in the Financial Statements or Notes thereto.

ITEM 17.  UNDERTAKINGS.

Insofar as indemnification for liabilities arising under the Securities Act may
be permitted to directors, officers and controlling persons of the registrant
pursuant to the foregoing provisions, or otherwise, the registrant has been
advised that in the opinion of the Securities and Exchange Commission such
indemnification is against public policy as expressed in the Act, and is,
therefore, unenforceable. In the event that a claim for indemnification against
such liabilities (other than the payment by the registrant of expenses incurred
or paid by a director, officer or controlling person of the registrant in the
successful defense of any action, suit or proceeding) is asserted by such
director, officer or controlling person in connection with the securities being
registered, the registrant will, unless in the opinion of its counsel the matter
has been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question whether such indemnification by it is against public
policy as expressed in the Act and will be governed by the final adjudication of
such issue.

The undersigned registrant hereby undertakes:

  (1)  That for purposes of determining any liability under the Securities Act,
       the information omitted from the form of prospectus filed as part of this
       Registration Statement in reliance upon Rule 430A and contained in a form
       of prospectus filed by the registrant pursuant to Rule 424(b)(1) or (4)
       or 497(h) under the Securities Act shall be deemed to be part of this
       Registration Statement as of the time it was declared effective.

  (2)  That for the purposes of determining any liability under the Securities
       Act, each posteffective amendment that contains a form of prospectus
       shall be deemed to be a new registration statement relating to the
       securities offered therein, and the offering of such securities at that
       time shall be deemed to be the initial bona fide offering thereof.

  (3)  To provide to the Underwriters at the closing specified in the
       underwriting agreement certificates in such denominations and registered
       in such names as required by the underwriters to permit prompt delivery
       to each purchaser.

                                      II-5
<PAGE>
                                   SIGNATURES

PURSUANT TO THE REQUIREMENTS OF THE SECURITIES ACT, THE REGISTRANT HAS DULY
CAUSED THIS AMENDMENT NO. 3 TO THE REGISTRATION STATEMENT TO BE SIGNED ON ITS
BEHALF BY THE UNDERSIGNED, THEREUNTO DULY AUTHORIZED, IN THE CITY OF HOUSTON,
STATE OF TEXAS ON MARCH 27, 2000.

                                          TANOX, INC.
                                          By: /s/ JOHN BLICKENSTAFF
                                                  John Blickenstaff
                                                  Secretary

PURSUANT TO THE REQUIREMENTS OF THE SECURITIES ACT, THIS AMENDMENT NO. 3 TO THE
REGISTRATION STATEMENT HAS BEEN SIGNED BY THE FOLLOWING PERSONS IN THE
CAPACITIES AND ON MARCH 27, 2000.

                                    By: /s/ JOHN BLICKENSTAFF
                                            John Blickenstaff
                                            Secretary


                                                  Chairman of the Board,
          NANCY T. CHANG, PH.D.                   President, and Chief
                                                  Executive Officer

                                                  Vice President of Finance
            DAVID DUNCAN, JR.                     and Chief Financial Officer

                                                  Director
           TSE WEN CHANG, PH.D.

                                                  Director
           OSAMA MIKHAIL, PH.D.

                                                  Director
         WILLIAM J. JENKINS, M.D.

                                      II-6

                                                                   EXHIBIT 10.24

     CERTAIN INFORMATION IN THIS EXHIBIT IS SUBJECT TO A REQUEST FOR
CONFIDENTIAL TREATMENT. IN ACCORDANCE WITH RULE 406 UNDER THE SECURITIES ACT OF
1933, AS AMENDED, SUCH INFORMATION HAS BEEN OMITTED AND FILED SEPARATELY WITH
THE SECURITIES AND EXCHANGE COMMISSION. THE LOCATION OF SUCH OMITTED INFORMATION
HAS BEEN INDICATED WITH AN ASTERISK (*).

                                MASTER AGREEMENT

     This MASTER AGREEMENT (the "Agreement") is entered into as of March 17,
2000 (the "Effective Date") by and between PROTEIN DESIGN LABS, INC., a
Delaware corporation having its principal office at 34801 Campus Drive, Fremont,
California 94555 U.S.A. ("PDL"), and TANOX, INC., a Delaware corporation
having offices at 10301 Stella Link, Houston, TX 77025 U.S.A. ("TANOX").

                                    RECITALS

     WHEREAS, PDL owns certain patents and related patent applications related
to humanized antibodies and antibody humanization technology that are sometimes
referred to as the "Queen et al. Patents."

     WHEREAS, PDL and TANOX are parties to a Patent License Agreement dated June
30, 1998, as amended by an amendment dated June 28, 1999 (as amended, the "Prior
Agreement").

     WHEREAS, PDL and TANOX desire to terminate the Prior Agreement and to enter
into this Agreement to provide TANOX the right to obtain certain nonexclusive
license rights under the Queen et al. Patents for the development, manufacture
and commercialization of antibody products directed against up to four antigens,
under the terms and conditions set forth below.

                                    AGREEMENT

     NOW, THEREFORE, the parties agree to follow:

1.  DEFINITIONS

     All references to particular Exhibits, Articles and Sections shall mean the
Exhibits to, and Articles and Sections of, this Agreement, unless otherwise
specified. References to this "Agreement" include the Exhibits. For the purposes
of this Agreement the following words and phrases shall have the following
meanings:

     1.1 "AFFILIATE" means, with respect to a party hereto, any corporate or
other entity which, directly or indirectly controls, is controlled by, or is
under common control with such party during the term of this Agreement, where
"control" means the ownership of not less than fifty percent (50%) of the voting
shares of a corporation, or decision-making authority as to an unincorporated
entity, provided that such entity shall be an Affiliate only so long as such
control exists.

     1.2 "ANTIBODY" means any antibody directed against an Antigen and shall
include, without limitation, monospecific and bispecific antibodies (but a
separate license shall be required for each Antigen targeted by a bispecific
antibody); less than full-length antibody forms such as Fv, Fab, Fab' and
F(ab')2; single-chain antibodies; and antibody conjugates bound to a toxin,
label or other moiety.

     1.3 "ANTIGEN" means a target molecule to which an Antibody specifically
binds and includes all epitopes on that target molecule.

     1.4 "LICENSED PRODUCT" means an Antibody with respect to which TANOX has
either significant marketing rights or has done significant development
(including without limitation created, humanized or conducted preclinical or
clinical development), the manufacture, import, use, offer to sell or sale of
which would infringe, if not licensed under this Agreement, a Valid Claim. *

     1.5 "PDL LICENSE AGREEMENT" means the form of PDL License Agreement
attached as Exhibit B.
<PAGE>
     1.6 "PDL PATENT RIGHTS" means the patent applications or patents (as well
as any foreign counterparts thereto identified on Exhibit A. PDL Patent Rights
shall include U.S. or foreign patents or patent applications which claim
priority to any application to which a listed U.S. Patent also claims priority.
PDL Patent Rights shall also include any foreign equivalents, including any
additions, continuations, continuations-in-part or divisions of such patents or
patent applications, or any substitute applications therefor; any patents issued
with respect to such patent applications, any reissues, extensions or patent
term extensions of any such patent, and any confirmation patents or registration
patents or patents of addition based on any such patents.

     1.7  "TANOX NAMED ANTIGENS"  shall mean: *

     1.8 "VALID CLAIM" means (a) any claim in any issued patent included in the
PDL Patent Rights which would be infringed but for the license granted under
Section 2.01 and which claim has not been disclaimed or held unenforceable or
invalid by a governmental agency or court of competent jurisdiction by a
decision from which no appeal has been taken within the time period permitted
for filing an appeal or may be taken; and (b) any pending claims under PDL
Patent Rights which, if granted, would be infringed but for the license granted
under Section 2.01 and which pending claim would be a Valid Claim if the pending
claim were treated as granted, provided that examination has been timely
requested for such pending claims and they are otherwise being diligently
prosecuted in an effort to have them allowed and granted in an issued patent.

2.  RIGHTS TO LICENSES

     2.1 ELECTION. Subject to the terms and conditions of this Agreement, PDL
hereby grants to TANOX through the * anniversary of the Effective Date
(or until such earlier time as TANOX has exercised its rights under this Section
2.1 with respect to four (4) Antigens), the rights upon written notice to
receive licenses under the PDL Patent Rights for up to four (4) Antigens
designated by TANOX. Each license shall be a nonexclusive, worldwide license
under the PDL Patent Rights to make, have made, use, import, market, promote,
offer for sale and sell or otherwise dispose of Antibodies pursuant to a PDL
License Agreements. The rights of TANOX under the PDL License Agreements shall
include the right to grant sublicenses for Antibodies in accordance with the
terms of the applicable PDL License Agreement. Each license elected by TANOX
hereunder shall be pursuant to a separate PDL License Agreement and effective as
of the date when the election for such license by TANOX becomes irrevocable,
pursuant to Section 2.2.

     2.2 PROCEDURE FOR EXERCISE OF LICENSE RIGHTS. TANOX shall provide PDL with
written notice identifying the Antigen for which TANOX desires to enter into a
PDL License Agreement pursuant to the provisions of Section 2.1. A separate
notice shall be provided with respect to each Antigen for which a license is
requested. Within fifteen (15) business days of the written notice or upon
execution and return of the relevant PDL License Agreement by PDL, whichever is
later, TANOX shall pay the applicable License Exercise Fee specified in Section
3.2. PDL shall promptly review and respond in writing to the request by TANOX
for a license within ten (10) business days of receipt of the written request.
PDL may refuse to grant TANOX a license only if PDL has previously granted an
exclusive or co-exclusive license or an unexpired option for an exclusive or
co-exclusive license with respect to an Antibody to the identical Antigen, can
demonstrate that it is then actively engaged in bona fide negotiations for such
an exclusive or co-exclusive license or option for an exclusive or co-exclusive
license, or then has development, marketing or co-promotion rights, or an option
to obtain such rights, to a product or a potential product directed against the
Antigen and which is on the market or in research or development and reasonably
expected to enter human clinical trials within twenty-four (24) months;
provided, however, that with respect to each of the Tanox Named Antigens, PDL
shall provide Tanox with written notice prior to entering into an exclusive or
co-exclusive license, development, marketing or co-promotion rights, or option
for such license or rights, with any third party with respect to that Tanox
Named Antigen and shall permit Tanox the opportunity to exercise its rights
under Section 2.1 for a period not to exceed fifteen (15) days for a

                                        2
<PAGE>
license for that Tanox Named Antigen prior to conclusion of an agreement with
such third party for a license or option; * In the event that PDL validly
refuses to grant TANOX a license under the PDL Patent Rights, TANOX' right under
Section 2.1 shall not be considered exercised. If PDL affirms TANOX' request or
has not responded by notice in writing with ten (10) business days of receipt of
TANOX' request under this Section 2.2, then TANOX' election shall be deemed
irrevocable and TANOX and PDL shall enter into a PDL License Agreement with
respect to that Antigen.

3.  PAYMENTS

     3.1 INITIAL FEES. Within thirty (30) days after the Effective Date, TANOX
shall pay an initial nonrefundable, noncreditable fee of * for the right to
obtain nonexclusive licenses pursuant to Section 2.1.

     3.2 LICENSE EXERCISE FEES. Within fifteen (15) business days after the
delivery of a written notice to PDL for a nonexclusive license for Antibodies
for one (1) Antigen under Section 2.1 or PDL's execution of a PDL license
Agreement for the same, whichever is later, TANOX shall pay to PDL an exercise
fee ("License Exercise Fee") of * provided that such amount shall be increased
annually beginning April 1, 2001 and on each April 1 thereafter by an amount
equal to the Consumer Price Index-U (or its successor) published by the U.S.
Bureau of Labor Statistics ("CPI-U") for the prior calendar year; provided,
however, that if the CPI-U for the prior year has not been published by April 1
of the applicable year, any adjustments hereunder shall be payable within
fifteen (15) days of the publication of the CPI-U for the applicable year.

4.  TERMINATION OF PRIOR AGREEMENT

     In partial consideration of the rights granted to TANOX in Article 2, the
parties agree that upon execution and delivery of this Agreement, the Prior
Agreement shall immediately terminate and be of no further force or effect.

5.  REPRESENTATIONS, DISCLAIMERS

     5.1 VALID AGREEMENT. Each party represents and warrants to the other that
it knows of no legal reason to, and is not a party to any agreement which would,
prevent it from entering into and performing its obligations under this
Agreement, and the the signatory hereto is duly authorized to execute and
deliver this Agreement.

     5.2 CURRENT PDL PATENT RIGHTS. The PDL Patent Rights constitute all of the
patents and patent applications owned by PDL as of the Effective Date which
relate generally on the humanization of antibodies.

     5.3 NO WARRANTY OF VALIDITY, NON-INFRINGEMENT. Nothing in this Agreement
shall be construed as (a) a warranty or representation by PDL as to the validity
or scope of any PDL Patent Rights; or (b) a warranty or representation that any
Licensed Product made, used, sold or otherwise disposed of under any PDL License
Agreement is or will be free from infringement of patents, copyrights,
trademarks, trade secrets or other rights of third parties.

     5.4 DISCLAIMER OF WARRANTIES. EXCEPT AS SPECIFICALLY SET FORTH IN SECTIONS
5.1 AND 5.2, NEITHER PDL NOR TANOX MAKE TO THE OTHER ANY REPRESENTATIONS OR
EXTEND ANY WARRANTIES OF ANY KIND, EITHER EXPRESS OR IMPLIED. FURTHER, PDL DOES
NOT MAKE ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
PURPOSE OR THAT PRACTICE UNDER THE PDL PATENT RIGHTS UNDER A PDL LICENSE
AGREEMENT WILL NOT INFRINGE ANY THIRD PARTY RIGHTS.

                                        3
<PAGE>
6.  CONFIDENTIALITY

     6.1 PRIOR AGREEMENTS. This Agreement supersedes the Prior Agreement;
provided, however, that with respect to all Confidential Information disclosed
thereunder and/or within the definition of "Confidential Information" in the
Prior Agreement, or information otherwise subject to the terms and conditions of
Section 6.02 of the Prior Agreement, such Confidential Information shall be
subject to the terms and conditions of Section 6.2 of this Agreement.

     6.2 CONFIDENTIALITY. PDL and TANOX acknowledge that in the course of
negotiations of this Agreement or other discussions between the parties of
prospective agreements or business relationships and/or in furtherance of the
interests of the parties hereunder that it ("Recipient") has received and/or may
receive confidential information of the other party ("Provider"). "Confidential
Information" means any and all data and information which (a) has been reduced
to tangible form and marked clearly and conspicuously with a legend identifying
its confidential or proprietary nature; or (b) with respect to any oral
presentation or communication, is or was designated as confidential immediately
before, during, or within a reasonable time after the oral presentation or
communication and such designation is subsequently confirmed in writing; (c) is
or was otherwise characterized by Provider as confidential information; or (d)
is subject to the terms and conditions of this Section 6.2 as provided in
Section 6.1.

     Each party shall keep and protect as confidential, and shall not use for
any purpose (other than TANOX' use for the development and commercial
exploitation of Licensed Products or either party's use for specific purposes
otherwise contemplated by a business relationship entered into between them),
during the term of this Agreement and for five (5) years after termination
hereof, all Confidential Information heretofore and hereafter supplied by the
other, provided however, that the foregoing obligations of confidentiality shall
not apply to the extent that any Confidential Information (a) is already known
to the Recipient at the time of disclosure, as evidenced by written record, or
is developed by Recipient thereafter in the course of work entirely independent
of any disclosure by the other party; (b) is publicly known prior to or becomes
publicly known after disclosure other than through acts of omissions of the
Recipient; (c) is disclosed in good faith to Recipient by a third party who is
not under obligations of non-disclosure to the Provider; (d) is required to be
disclosed pursuant to an order of a court of law or regulation of a governmental
agency; provided that the disclosing party shall advise the other party promptly
of any such disclosure requirement (other than a filing of information or
materials with the U.S. Securities and Exchange Commission made with a request
for confidential treatment for portions of such material for which such
treatment may reasonably be expected to be granted) in order to permit such
other party to undertake efforts to restrict or limit the required disclosure
including by cooperating in drafting a protective order; or (e) disclosure is
made to a government regulatory agency as part of such agency's biological
license approval process.

7.  TERM AND TERMINATION

     7.1 TERM. Unless earlier terminated in accordance with this Article 7, this
Agreement shall remain in effect until the expiration of the last PDL License
Agreement.

     7.2  TERMINATION.

     (a) If either party shall at any time default in the making of any payment
hereunder, or shall commit any material breach of any covenant or agreement
herein contained, and shall fail to have initiated and actively pursued remedy
of any such default or breach within sixty (60) days after receipt of written
notice thereof by the other party, the other party may, at its option, cancel
this Agreement and revoke any rights herein granted by notice in writing to such
effect, but such act shall not prejudice the right of the party giving notice to
recover any sums due at the time of such cancellation, it being understood,
however, that if within sixty (60) days after receipt of any such notice the
receiving party shall have initiated and actively pursued remedy of its default,
then the rights herein

                                        4
<PAGE>
granted shall remain in force as if no breach or default had occurred on the
part of the receiving party, unless such breach or default is not in fact
remedied within a reasonable period of time.

     (b) This Agreement may be terminated by either party upon the occurrence of
any of the following which is not stayed or vacated within ninety (90) days of
such occurrence: (i) petition in bankruptcy filed by or against the other party;
(ii) adjudication of the other party as bankrupt or insolvent; (iii) appointment
of a liquidator, receiver or trustee for all or a substantial part of the other
party's property; or (iv) an assignment for the benefit of creditors of the
other party.

     (c) All right and licenses granted under or pursuant to this Agreement by
PDL to TANOX, are, and shall otherwise be deemed to be, for purposes of Section
365(n) of the U.S. Bankruptcy Code, licenses of rights to "intellectual
property" as defined under Section 101(52) of the U.S. Bankruptcy Code. The
parties agree that TANOX, as a licensee of such rights under this Agreement,
shall retain and may fully exercise all of its right and elections under the
U.S. Bankruptcy Code, subject to performance by TANOX of its preexisting
obligations under this Agreement.

     (d) To the extent permitted under applicable law, the rights granted under
this agreement may be terminated as to any country by PDL upon thirty (30) days
prior written notice in the event that TANOX challenges the PDL Patent Rights in
that country.

     7.3 NO WAIVER. The right of either party to terminate this Agreement as
provided herein shall not be affected in any way by its waiver of, or failure to
take action with respect to, any previous failure to perform hereunder.

     7.4 RIGHTS AND OBLIGATIONS UPON TERMINATION OR EXPIRATION. Upon expiration
or termination of this Agreement for any reason, nothing herein shall be
construed to release either party from any obligation that matured prior to the
effective date of such termination. In addition, the obligations set forth in
Articles 6 and 7 shall survive the expiration or termination of this Agreement.
Upon termination of this Agreement, each party shall return to the other party
any confidential information disclosed by the other party under this Agreement.

8.  MISCELLANEOUS

     8.1 ASSIGNMENT. This Agreement may not be assigned by either party without
the prior written consent of the other, except to Affiliates upon prior written
notice to the other party, and except that either party may assign this
Agreement to a party which acquires all or substantially all of its business,
whether by merger, sale of assets or otherwise.

     8.2 ENTIRE AGREEMENT. This Agreement constitutes the entire Agreement
between the parties hereto with respect to the within subject matter and
supersedes all previous Agreements (except as provided in Section 6.1), whether
written or oral. This Agreement shall not be changed or modified orally, but
only by an instrument in writing signed by both parties.

     8.3 SEVERABILITY. If any provision of this Agreement is declared invalid by
a court of last resort or by any court, the decision of which appeal is not
taken within the time provided by law, then and in such event, this Agreement
will be deemed to have been terminated only as to the portion thereof which
relates to the provision invalidated by that decision and only in the relevant
jurisdiction, but this Agreement, in all other respects and all other
jurisdictions, will remain in force; provided, however, that if the provision so
invalidated is essential to the Agreement as a whole, then the parties shall
negotiate in good faith to amend the terms hereof as nearly as practical to
carry out the original interest of the parties, and, failing such amendment,
either party may submit the matter to a court of competent jurisdiction for
resolution.

     8.4 NOTICES. Any notice or report required or permitted to be given under
this Agreement shall be in writing and shall be sent by expedited delivery or
telecopied and confirmed by mailing, as follows and shall be considered received
three (3) days after such delivery:

                                        5
<PAGE>
If to PDL:                 Protein Design Labs, Inc.
                           34801 Campus Drive
                           Fremont, California 94555 USA
                           Attention: Chief Executive Officer
                           Fax No: 510-574-1500

Copy to:                   Protein Design Labs, Inc.
                           34801 Campus Drive
                           Fremont, California 94555 USA
                           Attention: General Counsel
                           Fax No: 510-574-1473

If to TANOX:               Tanox, Inc.
                           10301 Stella Link
                           Houston, TX 77025 USA
                           Attention: Nancy T. Chang, President and CEO
                           Fax No: (713) 664-8914

Copy to:                   Tanox, Inc.
                           10301 Stella Link
                           Houston, TX 77025 USA
                           Attention: Head of Legal Affairs

     8.5  DISPUTE RESOLUTION.

     (A) NEGOTIATIONS. Any dispute, controversy or claim arising out of or
relating to any provision of this Agreement or a PDL License Agreement or the
interpretation, enforceability, performance, breach, termination or validity
hereof or thereof, including without limitation, this dispute resolution
provision, shall be subject to the procedures set forth in this Section 8.5. A
designated representative of PDL and TANOX will meet as reasonably requested by
either party to review any dispute, controversy or claim arising out of or
relating to any provision of this Agreement or a PDL License Agreement. If the
disagreement is not resolved by the designated representatives by mutual
agreement within thirty (30) days after a meeting to discuss the disagreement,
either party may at any time thereafter provide the other written notice
specifying the terms of such disagreement in reasonable detail. Upon receipt of
such notice, the chief executive officers of PDL and TANOX shall meet at a
mutually agreed upon time and location for the purpose of resolving such
disagreement. They will discuss the problems and/or negotiate for a period of up
to sixty (60) days in an effort to resolve the disagreement or negotiate an
acceptable interpretation or revision of the applicable portion of this
Agreement or a PDL License Agreement, as applicable, mutually agreeable to both
parties, without the necessity of formal procedures relating thereto. During the
course of such negotiations, the parties will reasonably cooperate and provide
information that is not materially confidential in order that each of the
parties may be fully informed with respect to the issues in dispute. The
institution of a formal legal proceeding under Section 8.5(b) or (c) to resolve
the disagreement may occur by written notice to the other party only after the
earlier of: (a) the chief executive officers mutually agreeing that resolution
of the disagreement through continued negotiation is not likely to occur, or (b)
following expiration of the sixty (60) day negotiation period.

     (B) ARBITRATION. Subject to Section 8.5(a), any dispute, controversy or
claim arising out of or in connection with or relating to this Agreement or a
PDL License Agreement or the breach or alleged breach thereof, but not including
any dispute, controversy or claim concerning the validity of any PDL Licensed
Patent, shall be submitted by the parties to arbitration in Denver, Colorado in
accordance with the then-current JAMS Comprehensive Arbitration Rules and
Procedures under the auspices of JAMS ("JAMS") except as otherwise provided
herein. If the dispute, controversy or claim concerns the validity of any PDL
Licensed Patent, all matters subject to dispute, controversy or claim hereunder
shall be removed to Federal District Court as provided in Section 8.5(c).

     Any arbitration proceeding hereunder shall be held in English and a
transcribed record prepared in English. The parties shall choose, by mutual
agreement, one (1) neutral arbitrator within thirty (30)

                                        6
<PAGE>
days of receipt of notice of the intent to arbitrate. If no arbitrator is
appointed within the times herein provided or any extension of time which is
mutually agreed upon, JAMS shall make such appointment of a person who shall
devote substantial time to arbitrating within thirty (30) days of such failure.
Discovery permitted by the arbitrator shall be pursuant to the Federal Rules of
Civil Procedure, provided that all discovery shall be completed within sixty
(60) days of the appointment of such arbitrator and the decision rendered by
such arbitrator shall thereafter be delivered in writing setting forth the basis
therefor within thirty (30) days after the completion of discovery. The award
rendered by the arbitrator shall include costs of arbitration, reasonable
attorneys' fees and reasonable costs for expert and other witnesses, and
judgment on such award may be entered in any court having jurisdiction thereon.
Nothing in this Agreement shall be deemed as preventing either party from
seeking injunctive relief (or any other provisional remedy) from any court
having jurisdiction over the parties and the subject matter of the dispute but
only to the extent necessary to protect either party's name, proprietary
information, trade secrets, know-how or any other similar proprietary rights. If
the issues in dispute involve scientific or technical matters related to
monoclonal antibody technology, any arbitrator chosen hereunder shall have not
less than five (5) years of educational training and/or experience sufficient to
demonstrate a reasonable level of relevant scientific and/or technical knowledge
related to monoclonal antibody technology or such arbitrator shall have the
right to appoint an expert with such experience to assist such arbitrator as
necessary. If the issues in dispute involve patent matters (other than validity
of a PDL Licensed Patent), then such arbitrator shall also be knowledgeable
about patent law matters and to the extent possible, with monoclonal antibody
technology. The decision of the arbitrator shall be in writing and shall set
forth the basis therefor. The arbitrator shall have the authority to award such
remedies as he or she believes appropriate in the circumstances, including, but
not limited to, compensatory damages subject to the Three and Three-Quarters
Percent (3.75%) royalty maximum set forth in the PDL License Agreement,
consequential and incidental damages, interest, tort damages (but not punitive
or similar damages) and specific performance and other equitable relief.

     (C) PATENT VALIDITY. Subject to Section 8.5(a), any dispute, controversy or
claim (i) which involves the validity of a PDL Licensed Patent issued in the
United States shall be subject to actions before the United States Patent and
Trademark Office and/or adjudicated in Federal District Court, Central District,
Santa Clara County in the State of California and (ii) which involves the
validity of a PDL Licensed Patent issued in any other country shall be brought
before an appropriate regulatory or administrative body or court in that
country. The prevailing party shall be entitled to recover from the other party,
the reasonable attorneys' fees, costs and expenses incurred by such prevailing
party in connection with any action or proceeding under this Section 8.5(c).

     8.6 CHOICE OF LAW. The validity, performance, construction, and effect of
this Agreement shall be governed by the laws of the State of California which
are applicable to contracts between California residents to be performed wholly
within California.

     8.7 WAIVER. None of the terms, covenants and conditions of this Agreement
can be waived except by the written consent of the party waiving compliance.

     8.8 FORCE MAJEURE. Neither party shall be responsible to the other for
failure or delay in performing any of its obligations under this Agreement or
for other non-performance hereof provided that such delay or non-performance is
occasioned by a cause beyond the reasonable control and without fault or
negligence of such party, including, but not limited to war, terrorism,
earthquake, fire, flood, explosion, discontinuity in the supply of power, court
order or governmental interference, act of God, strike or other labor trouble
and provided that such party will inform the other party as soon as is
reasonably practicable and that it will entirely perform its obligations within
a reasonable time after the relevant cause has ceased its effect.

     8.9 PUBLICITY. PDL will issue a press release concerning the parties' entry
into this Agreement and the amount of the initial fees to be paid hereunder,
with the content of such release to be

                                        7
<PAGE>
approved in advance by TANOX, which approval shall not be unreasonably withheld.
Except as required by law or regulation, neither party shall publicly disclose
the other terms and conditions of this Agreement unless expressly authorized to
do so by the other party, which authorization shall not be unreasonably
withheld; provided, however that the parties may disclose such terms and
conditions to their respective investors, prospective investors, advisors, and
consultants under a written confidentiality agreement protecting the
confidentiality of the terms hereof as provided under Article 6, and to their
accountants and attorneys provided that such accountants and attorneys agree to
maintain the confidentiality of the terms hereof as provided under Article 6,
and either party may disclose such terms and conditions as deemed required in
connection with any securities offering it elects to make.

     8.10 HEADINGS. The captions used herein are inserted for convenience of
reference only and shall not be construed to create obligations, benefits, or
limitations.

     8.11 COUNTERPARTS. This Agreement may be executed in any number of
counterparts, each of which shall be deemed to be an original, and such
counterparts together shall constitute one agreement. Execution and delivery of
this Agreement by facsimile copies bearing the facsimile signatures of the
parties shall constitute a valid and binding execution and delivery of this
Agreement by the signing party, and such facsimile copies shall constitute
original documents.

     IN WITNESS WHEREOF, the parties hereto have duly executed this Agreement as
of the Effective Date.


PDL:                               TANOX:
PROTEIN DESIGN LABS, INC.          TANOX, INC.

By:  /s/ LAURENCE JAY KORN         By:  /s/ NANCY T. CHANG
    ----------------------             --------------------
     Laurence Jay Korn                  Nancy T. Chang
     CEO                                President and CEO

                                        8
<PAGE>
                                    EXHIBIT A

                                PDL PATENT RIGHTS

The following are patents and patent applications (also known as the "Queen et
al. patents") issued and filed in certain countries in the world and licensed as
part of the PDL Patent Rights under the Agreement. (Updated as of January 27,
2000.)

1.  The following issued U.S. patents and U.S. patent applications:

        No. 5,585,089, "Humanized Immunoglobulins," issued December 17, 1996.

        No. 5,693,761, "Polynucleotides Encoding Improved Humanized
        Immunoglobulins," issued December 2, 1997.

        No. 5,693,762, "Humanized Immunoglobulins," issued December 2, 1997.

        *

2.  The following patents and patent applications outside the U.S.:

<TABLE>
<CAPTION>
PATENT NO.                          COUNTRY                           TITLE*
<S>             <C>                 <C>              <C>
                                                          "Novel Immunoglobulins, Their

Issued          647383              Australia                  Production and Use"
Issued          671949              Australia                           "
Issued          AT E133452          Austria                             "
Issued          0451216             Belgium                             "
Issued          61095               Bulgaria                            "
Issued          98389               Chile                               "
Issued          P920500A            Croatia                             "
Issued          0451216B1           Europe1                             "
Issued          0682040 B1          Europe1                             "
Issued          FR0451216           France                              "
Issued          DE 68925536         Germany                             "
Issued          DE 68929061         Germany                             "
Issued          DD 296 964          East Germany                        "
Issued          GB 0451216          Great Britain                       "
Issued          1001050             Greece                              "
Issued          211174              Hungary                             "
Issued          IT 0451216          Italy                               "
Issued          2828340             Japan                               "
Issued          LU 0451216          Luxembourg                          "
Issued          92.2146             Monaco                              "
Issued          NL 0451216          Netherlands                         "
Issued          231984              New Zealand                         "
Issued          132068              Pakistan                            "
Issued          29729               Philippines                         "
Issued          92758               Portugal                            "
Issued          2126046             Russia                              "
Issued          SG 0451216          Singapore                           "
Issued          8912489             Slovenia                            "
Issued          89/9956             South Africa                        "
Issued          178385              South Korea                         "
Issued          0231090             South Korea                         "
Issued          2081974 T3          Spain                               "
Issued          2136760             Spain
</TABLE>

                                        9

LEGAL COMMON/CONTRACTS/TANOX/LIC OPTN/MSTR000316                          000316
<PAGE>
<TABLE>
<S>             <C>                 <C>              <C>
Issued          SE 0451216          Sweden                              "
Issued          CH 0451216          Switzerland                         "
Issued          50034               Taiwan                              "
Issued          13349               Uruguay                             "
Issued          56455               Venezuela                           "
Issued          48700               Yugoslavia                          "
<CAPTION>
                COUNTRY             APPLICATION NO.                   TITLE*

Pending            *                      *                             *
Pending
Pending
Pending
Pending
Pending
Pending
Pending
Pending
Pending
Pending
Pending
Pending
Pending
</TABLE>

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

* Exact titles may differ in different countries.

1 and corresponding European national patents issued therefrom.

                                       10
<PAGE>

                                    EXHIBIT B

                          FORM OF PDL LICENSE AGREEMENT

                                       11
<PAGE>
                                    EXHIBIT B

                            PATENT LICENSE AGREEMENT

     This Agreement ("Agreement"), effective as of

("Effective Date"), is made by and between PROTEIN DESIGN LABS, INC., a
Delaware corporation having offices at 34801 Campus Drive, Fremont, CA 94555,
U.S.A. (hereinafter "PDL") and TANOX, INC., a Delaware corporation having
offices at 10301 Stella Link, Houston, TX 77025, U.S.A. (hereinafter "TANOX").

                                    RECITALS

     A. TANOX and PDL have entered in a Master Agreement effective March 17,
2000 (the "Master Agreement"), pursuant to which TANOX may enter into this
Agreement with respect to a license under the PDL Patent Rights (as defined
below) for TANOX'S antibody products.

     B. The Master Agreement provides TANOX with the right to obtain a
nonexclusive, worldwide, royalty-bearing license under the PDL Patent Rights
under the terms and conditions of this Agreement.

                                    AGREEMENT

     NOW THEREFORE, in consideration of the mutual covenants herein contained
and intending to be legally bound, the parties agree as follows:

1.  DEFINITIONS

     All references to Exhibits, Articles and Sections shall be references to
Exhibits, Articles and Sections of this Agreement. In addition, except as
otherwise expressly provided herein, the following terms in this Agreement shall
have the following meanings:

     1.01 "AFFILIATE" means with respect to a party hereto, any corporate or
other entity which, directly or indirectly, controls, is controlled by, or is
under common control with such party where "control" means the ownership of not
less than 50% of the voting shares of a corporation, or decision-making
authority as to an unincorporated entity.

     1.02 "ANTIBODY" means any antibody directed against an Antigen, including,
without limitation, monospecific and bispecific antibodies; less than
full-length antibody forms such as Fv, Fab, and F(ab')2; single-chain
antibodies; and antibody conjugates bound to a toxin, label or other moiety.

     1.03  "ANTIGEN" means the target molecule:
                                    .

     1.04 "COMBINATION PRODUCT(S)" shall mean any product containing both a
pharmaceutically active agent or ingredient which constitutes a Licensed Product
and one or more other pharmaceutically active agents or ingredients which do not
constitute Licensed Products.

     1.05 "LICENSED PRODUCT(S)" shall mean therapeutic products of TANOX or
TANOX'S sublicensees or Affiliates that include an Antibody developed by or for
TANOX or its Affiliates, jointly or otherwise, binding to the Antigen whose
development, manufacture, use or sale would, but for a license under this
Agreement, infringe a Valid Claim.

     1.06 "NET SALES" shall mean the aggregate gross revenues, whether in cash
or in kind, derived by or payable from or on account of the sale of Licensed
Products, less actual amounts incurred for (a) credits or allowances, if any,
actually granted on account of price adjustments, recalls, rejection or return
of items previously sold, (b) excise and sales taxes, duties or other taxes
imposed on and paid with respect to such sales (excluding income or franchise
taxes of any kind) and (c) outer packing, freight and freight insurance costs.
All amounts deducted under this Section 1.06 shall be properly documented and
specified in any summary report provided to PDL under Section 3.07. If TANOX or
any of its Affiliates or sublicensees receive non-cash consideration for any
Licensed Product sold or

                                        1
<PAGE>
otherwise transferred to an independent third party not an Affiliate of the
seller or transferor, the fair market value of such non-cash consideration on
the date of such transfer as known to TANOX, or as reasonably estimated by TANOX
if unknown, shall be included in the definition of Net Sales.

     1.07 "PDL PATENT RIGHTS" means the patent applications or patents (as well
as any foreign counterparts thereto) identified on EXHIBIT A. PDL Patent Rights
shall include U.S. or foreign patents or patent applications which claim
priority to any application to which a listed U.S. Patent also claims priority.
PDL Patent Rights shall also include any foreign equivalents, additions,
continuations, continuations-in-part or divisions of such patents or patent
applications or any substitute applications therefor; any patents issued with
respect to such patent applications, any reissues, extensions or patent term
extensions of any such patent, and any confirmation patents or registration
patents or patents of addition based on any such patents.

     1.08 "VALID CLAIM" means (a) any claim in any issued patent included in the
PDL Patent Rights which would be infringed but for the license granted under
Section 2.01 and which claim has not been disclaimed or held unenforceable or
invalid by a governmental agency or court of competent jurisdiction from which
no appeal has been taken within the time period permitted for filing an appeal
or may be taken; and (b) any pending claims under PDL Patent Rights which, if
granted, would be infringed but for the license granted under Section 2.01 and
which pending claim would be a Valid Claim if the pending claim were treated as
granted, provided that examination has been timely requested for such pending
claims and they are otherwise being diligently prosecuted in an effort to have
them allowed and granted in an issued patent.

2.  LICENSE

     2.01 LICENSE GRANT. Subject to the terms and conditions of this Agreement,
PDL hereby grants and TANOX hereby accepts a worldwide nonexclusive license
under the PDL Patent Rights, including the right to grant sublicenses in
accordance with Section 2.02 to make, import, have made, use, offer to sell or
sell Licensed Products.

     2.02 LIMITATION ON SUBLICENSES; NOTIFICATION. TANOX shall have the right to
grant sublicenses of its rights with respect to the Licensed Product under
Section 2.01 only to its Affiliates or in connection with the assignment or
license by TANOX of a Licensed Product to a third party. Notwithstanding the
assignment or grant of a sublicense by TANOX hereunder, TANOX shall remain
obligated to pay all royalties due to PDL with respect to the sale of Licensed
Products by its assignee or sublicensee. In addition, the grant of any
sublicenses under Section 2.01 shall be on terms and conditions which are
subject to and subordinate to the terms of this Agreement and TANOX shall remain
fully responsible to PDL for the performance of any and all such terms by its
sublicensees. Promptly following execution of any sublicense hereunder, but in
any event not less than ten (10) days thereafter, TANOX shall notify PDL of the
identity of the sublicensee and the scope of the sublicense.

     2.03 NO OTHER LICENSE RIGHTS. TANOX expressly acknowledges and agrees that,
except for the license expressly granted under Section 2.01, no rights to any
other PDL patents, patent applications, know-how or licenses are included in
this Agreement and that any royalties or payments that may be due to third
parties in order for TANOX to make, have made, use or sell Licensed Products
shall be the sole responsibility of TANOX.

     2.04 UPDATES TO LIST OF PDL PATENT RIGHTS. Upon written request of TANOX,
which request shall not be made more than once per calendar year, PDL agrees to
provide a written update listing the patents and patent applications under the
PDL Patent Rights, and such update shall constitute an amendment to Exhibit "A."
PDL may, at its option, furnish such update to TANOX from time to time during
the terms of this Agreement as part of an update to the Master Agreement.

                                        2
<PAGE>
3.  PAYMENT; ROYALTIES, REPORTS

     3.01 PAYMENTS.  In consideration for the licensed granted by PDL under
Article 2 of this Agreement TANOX shall pay to PDL a nonrefundable signing and
licensing fee by the later of fifteen (15) business days after Tanox' exercise
of its right under the Master Agreement to enter into this Agreement or the
Effective Date, the sum of                         United States Dollars
(US$            [DRAFTING NOTE: ENTER AMOUNT CALCULATED UNDER SECTION 3.2 OF THE
MASTER AGREEMENT]

     3.02 ROYALTIES TO PDL. Subject to reduction for any offset as provided in
Section 3.05, or withholding under Section 3.09(b), and in further consideration
of the rights and licenses granted under Article 2, TANOX shall pay to PDL a
royalty of * of the Net Sales of all Licensed Products sold by TANOX or its
Affiliates or sublicensees in each country until the last date on which there is
a Valid Claim that, but for a license granted to TANOX under this Agreement,
would be infringed by the making, using, having made or sale of that Licensed
Product in such country or by the manufacture of Licensed Product in the country
of manufacture.

     3.03 SALES AMONG AFFILIATES. Sales between and among TANOX and its
Affiliates or sublicensees of Licensed Products which are subsequently resold or
to be resold by such Affiliates or sublicensees shall not be subject to royalty,
but in such cases royalties shall accrue and be calculated on the sale of such
Licensed Products to a third party.

     3.04 COMBINATION PRODUCTS. Net Sales in a particular country, in the case
of the Combination Products for which the pharmaceutically active agent or
ingredient constituting a Licensed Product and each of the other
pharmaceutically active agents or ingredients not constituting Licensed Products
have established market prices in that country in the Territory when sold
separately, shall be determined by multiplying the Net Sales for each such
Combination Product by a fraction, the numerator of which shall be the
established market price for the Licensed Product(s) contained in the
Combination Product and the denominator of which shall be the sum of the
established market prices for the Licensed Product(s) plus the established
market prices for the other pharmaceutically active agents or ingredients
contained in the Combination Product. When such separate market prices are not
established in that country, then the parties shall negotiate in good faith to
determine a fair and equitable method of calculating Net Sales in that country
for the Combination Product in question.

     3.05 ANNUAL MAINTENANCE FEE. In further consideration of the licenses
granted under Article 2, following the second (2nd) anniversary of the Effective
Date and not later than each anniversary thereafter within thirty (30) days
after receipt of an invoice from PDL, TANOX shall pay PDL a nonrefundable annual
maintenance fee in the amount of *. An amount of fifty percent (50%) of the
annual maintenance fee paid by TANOX hereunder shall be creditable against
royalties payable to PDL pursuant to Section 3.02 beginning in the year in which
a Biologist License Application ("BLA") or foreign equivalent thereof is filed
with the U.S. Food and Drug Administration (or any successor thereto, or a
foreign counterpart agency), with respect to a Licensed Product; provided that
in no event shall any portion of the annual maintenance fees paid prior to the
year in which a BLA is filed with respect to the Licensed Product be creditable
against royalties hereunder.

     3.06 CURRENCY CONVERSION. All amounts payable to PDL under this Agreement
shall be payable in U.S. Dollars by wire transfer to a bank account designated
by PDL. In the case of royalties on Net Sales, all amounts payable shall first
be calculated in the currency of sale and then converted into U.S. Dollars using
the average of the daily exchange rates for such currency quoted by Citibank,
N.A. for each of the last fifteen (15) banking days of each calendar quarter.

     3.7  ROYALTY REPORTS.

     (A)  CURRENT REPORTS.  TANOX agrees to make written reports and royalty
payments to PDL within sixty (60) days after the close of each calendar quarter
during the terms of this Agreement,

                                        3
<PAGE>
beginning with the calendar quarter in which the date of first sale of Licensed
Product by TANOX, its Affiliates or sublicensees, following regulatory approval
occurs. These reports shall show for the calendar quarter in question Net Sales
by TANOX, its Affiliates and sublicensees of the Licensed Products in the
Territory on a country-by-country basis, details of the quantities of Licensed
Products sold in each country and the country of manufacture, if different,
applicable offsets, withholding taxes and the net royalty due to PDL thereon
pursuant to Article 3. No later than at the time of the making of each such
report, TANOX shall make any payment due to PDL of royalties for the period
covered by such report.

     (B) TERMINATION REPORT. For each Licensed Product, TANOX also agrees to
make a written report to PDL within ninety (90) days after the date on which
TANOX, its Affiliates or sublicensees last sell that Licensed Product stating in
such report the same information required by quarterly reports for all such
Licensed Products made, sold or otherwise disposed of which were not previously
reported to PDL.

     3.08 INSPECTION. TANOX agrees to keep clear, accurate and complete records
for a period of at least three (3) years after each reporting period in which
Net Sales occur showing the manufacturing, sales and other disposition of
Licensed Products in the Territory in sufficient detail to enable the royalties
payable hereunder to be determined. TANOX further agrees to permit its books and
records to be examined by an independent accounting firm selected by PDL and
reasonably satisfactory to TANOX from time-to-time but not more than once a
year. Such examination is to be made at the expense of PDL, except in the event
that the results of the audit reveal that TANOX underpaid PDL by * or more, then
the audit fees shall be paid by TANOX. Any such discrepancies will be promptly
corrected by a payment or refund, as appropriate.

     3.09  WITHHOLDING.

     (A) PAYMENTS. All amounts payable under Section 3.01 and 3.05 shall
represent the actual proceeds to be received by PDL, net of any withholding or
other taxes or levies that may be applicable to such payments. PDL agrees to
reasonably cooperate with TANOX in obtaining a refund of any withholding taxes
or levies paid by TANOX if any, with respect to any payments to PDL hereunder.
In the event that PDL is successful in obtaining any refund of tax withholding
amounts paid by TANOX under this Agreement, PDL agrees to promptly remit such
refund amount to TANOX.

     (B) ROYALTY PAYMENTS. TANOX may withhold from royalties due to PDL amounts
for payment of any withholding tax that TANOX has actually paid to any taxing
authority with respect to the royalty payments due to PDL hereunder, provided
that such amounts are properly documented and specific in any summary report to
PDL hereunder. TANOX agrees to reasonably cooperate with PDL in obtaining a
foreign tax credit in the U.S. with respect to royalties due to PDL on the sale
or manufacture of Licensed Products at the sole expense of PDL.

4.  PATENT PROSECUTION AND MAINTENANCE.

     4.01  PROSECUTION.  PDL hereby agrees that, at its own expense it will use
commercially reasonable efforts to:

     (a) prosecute any patent applications included in the PDL Patent Rights and
     to secure the most extensive protection reasonably obtainable under the PDL
     Patent Rights; and

     (b) maintain the claims in patents included in the PDL Patent Rights as
     valid and enforceable claims for the full term thereof.

     4.02 UPDATES. Upon the written request of TANOX (which request shall not be
made more than once per calendar year), PDL agrees to provide a written update
of the information relating to the PDL Patent Rights as set forth on EXHIBIT A.

                                        4
<PAGE>
     4.03 DEFENSE OF PDL PATENT RIGHTS. With respect to the PDL Patent Rights
licensed under this Agreement, PDL at its sole cost and expense agrees to take
all steps and proceedings and to undertake such other acts as PDL may, in its
sole discretion, deem necessary or advisable to restrain any infringement or
improper or unlawful use of the PDL Patent Rights and to recover any actual or
punitive compensation therefor. TANOX shall provide reasonable assistance to PDL
and permit PDL to have the sole right to take such steps, conduct any such
proceedings or undertake any such action to restrain any infringement or
improper or unlawful use of the PDL Patent Rights, whether or not TANOX is a
party to such steps, proceedings or actions, all of which shall be at PDL's
expense. Any monies recovered from alleged infringers shall be retained by PDL.

5.  REPRESENTATIONS AND WARRANTIES; INDEMNIFICATION

     5.01 VALID AGREEMENT. Each party represents and warrants to the other that
it knows of no legal reason to, and is not a party to any agreement which would,
prevent it from entering into and performing its obligations under this
Agreement and that the signatory hereto is duly authorized to execute and
deliver this Agreement.

     5.02 NO WARRANTY OF VALIDITY, NON-INFRINGEMENT. Nothing in this Agreement
shall be construed as (a) a warranty or representation by PDL as to the validity
or scope of any PDL Patent Rights; or (b) a warranty or representation that any
Licensed Product made, used, sold or otherwise disposed of under the license
granted in this Agreement is or will be free from infringement of patents,
copyrights, trademarks, trade secrets or other rights of third parties.

     5.03 NO OTHER WARRANTIES. EXCEPT AS SPECIFICALLY SET FORTH IN ARTICLE 5,
PDL MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND, EITHER EXPRESS OR
IMPLIED, WITH RESPECT TO ANY CELL LINES, ANTIBODIES OR LICENSED PRODUCTS
DEVELOPED BY TANOX UNDER THE LICENSE SET FORTH IN THIS AGREEMENT AND PDL FURTHER
MAKES NO EXPRESS OR IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE, OR THAT THE USE OF ANY CELL LINES, ANTIBODIES, LICENSED
PRODUCTS OR OTHER MATERIALS DEVELOPED BY TANOX UNDER THE LICENSE SET FORTH IN
THIS AGREEMENT WILL NOT INFRINGE ANY THIRD PARTY RIGHTS.

     5.04 INDEMNIFICATION. TANOX shall at all times, during the term of this
Agreement and thereafter, indemnify and hold harmless PDL and its Affiliates,
sublicensees, directors, officers, agents and employees from any claim,
proceeding, loss, expense, and liability of any kind whatsoever (including but
not limited to those resulting from death, personal injury, illness or property
damage and including legal expenses and reasonable attorneys' fees) arising out
of or resulting from (a) any claim of patent infringement (direct or
contributory) or including patent infringement with respect to the activities of
TANOX and (b) the development, manufacture, holding, use, testing,
advertisement, sale or other disposition by TANOX, its Affiliates or
sublicensees, or any distributor, customer or representative of TANOX or any one
in privity therewith, of any Licensed Product; provided, however, that PDL shall
promptly notify TANOX of such claim, proceeding, loss, expense or liability and
TANOX, at TANOX's cost, shall have sole control over the defense, including
settlement of any claim or action, with full cooperation from PDL.

6. CONFIDENTIALITY. PDL and TANOX acknowledge that in the course of negotiations
of this Agreement or other discussions between the parties of prospective
agreements or business relationships and/or in furtherance of the interests of
the parties hereunder that each has received and/or may receive confidential
information of the other party. To the extent that such information is
"Confidential Information" as defined in Section 6.2 of the Master Agreement,
each party is subject to the obligations of such Section 6.2 with respect to the
Confidential Information.

                                        5
<PAGE>
7.  TERM AND TERMINATION

     7.01 TERM. Unless earlier terminated as provided in this Article 7, this
Agreement shall come into force on the Effective Date and shall continue until
the expiration of the last obligation of TANOX to pay royalties to PDL in
accordance with Article 3 above. Thereafter, with the exception of the surviving
clauses specified in Section 7.04, this Agreement shall terminate and all
licenses or sublicenses granted hereunder shall become fully paid-up,
irrevocable licenses.

     7.02  TERMINATION.

        (A) This Agreement may be terminated in its entirety on sixty (60) days
prior written notice by TANOX.

        (B) If TANOX shall at any time default in the payment of any royalty, or
the making of any report hereunder, or shall commit any material breach of any
covenant or agreement herein contained or shall make any false report, and shall
fail to have initiated and actively pursued remedy of any such default or breach
within sixty (60) days after receipt of written notice thereof by the other
party, PDL may, at its option, cancel this Agreement and revoke any rights and
licenses herein granted and directly affected by the default or breach by notice
in writing to such effect, but such act shall not prejudice PDL's rights to
recover any royalty or other sums due at the time of such cancellation, it being
understood, however, that if within sixty (60) days days after receipt of any
such notice TANOX shall have initiated and actively pursued remedy of its
default, then the rights and licenses herein granted shall remain in force as if
no breach or default had occurred on the part of TANOX, unless such breach or
default is not in fact remedied within a reasonable period of time. If TANOX
disputes the existence of a default or material breach or making a false report
or the failure to pursue a remedy or to remedy the default or breach, the
provisions for resolution of a default shall be limited to those set forth in
Section 8.5 of the Master Agreement.

        (C) This Agreement may be terminated by either party upon the occurrence
of any of the following which is not stayed or vacated within ninety (90) days
of such occurrence: (i) petition in bankruptcy filed by or against the other
party; (ii) adjudication of the other party as bankrupt or insolvent; (iii)
appointment of a liquidator, receiver or trustee for all or a substantial part
of the other party's property; or (iv) an assignment for the benefit of
creditors of the other party.

        (D) All rights and licenses granted under or pursuant to this Agreement
by PDL to TANOX, are and shall otherwise be deemed to be, for purposes of
Section 365(n) of the U.S. Bankruptcy Code, licenses of rights to "intellectual
property" as defined under Section 101(52) of the U.S. Bankruptcy Code. The
Parties agree that TANOX, as a licensee of such rights under this Agreement,
shall retain and may fully exercise all of its rights and elections under the
U.S. Bankruptcy Code, subject to performance by TANOX of its preexisting
obligations under this Agreement.

        (E) To the extent permitted under applicable law, the license granted
under this Agreement may be terminated as to any country by PDL upon thirty (30)
days prior written notice in the event that TANOX challenges any PDL Patent
Rights in that country.

     7.03 NO WAIVER. The right of either party to terminate this Agreement as
provided herein shall not be affected in any way by its waiver of, or failure to
take action with respect to, any previous failure to perform hereunder.

     7.04  SURVIVAL.

        (A) Termination for any reason hereunder shall not affect any accrued
rights or obligations of the parties arising in any manner under this Agreement
as of the date of termination. In any event, the confidentiality and indemnity
obligations and any accrued payment and reporting obligations under Articles 3,
5 and 6 shall survive any termination of this Agreement.

                                        6
<PAGE>
        (b) Upon termination of this Agreement under Section 7.02(c), PDL hereby
grants TANOX a license to sell within three (3) months of such termination any
Licensed Products in TANOX' or TANOX' Affiliate's or sublicensee's inventory on
the date of such termination, which have not previously been sold ("Inventory");
provided, however, that TANOX shall pay the royalties due on such inventory,
provide related reports and allow a final audit in the amounts and manner
provided for in Sections 7.07 and 3.08.

     7.05 DISPUTES. The provisions of Section 8.5 of the Master Agreement are
incorporated by reference as if set forth in their entirety herein.

8.  MISCELLANEOUS

     8.01 ASSIGNMENT. This Agreement may not be assigned by either party without
the prior written consent of the other, except to Affiliates upon prior written
notice to the other party, and except that either party may assign this
Agreement to a party which acquires all or substantially all of its business,
whether by merger, sale of assets or otherwise.

     8.02 ENTIRE AGREEMENT. This Agreement and the Master Agreement constitute
the entire agreement between the parties hereto with respect to the within
subject matter and together they supersede all previous agreements (except as
otherwise provided in the Master Agreement), whether written or oral. This
Agreement shall not be changed or modified orally, but only by an instrument in
writing signed by both parties.

     8.03 SEVERABILITY. If any provision of this Agreement is declared invalid
by a court of last resort or by any court, the decision of which appeal is not
taken within the time provided by law, then and in such event, this Agreement
will be deemed to have been terminated only as to the portion thereof which
relates to the provision invalidated by that decision and only in the relevant
jurisdiction, but this Agreement, in all other respects and all other
jurisdictions, will remain in force; provided, however, that if the provision so
invalidated is essential to the Agreement as a whole, then the parties shall
negotiate in good faith to amend the terms hereof as nearly as practical to
carry out the original interest of the parties, and failing such amendment,
either party may submit the matter to a court of competent jurisdiction for
resolution.

     8.04 NOTICES. Any notice or report required or permitted to be given under
this Agreement shall be in writing and shall be sent by expedited delivery or
telecopied and confirmed by mailing, as follows and shall be considered received
three (3) days after such delivery.

          If to PDL:             Protein Design Labs, Inc.
                                 34801 Campus Drive
                                 Fremont, California 94555 USA
                                 Attention: Chief Executive Officer
                                 Fax No: 510-574-1500

          Copy to:               Protein Design Labs, Inc.
                                 34801 Campus Drive
                                 Fremont, California 94555 USA
                                 Attention: General Counsel
                                 Fax No: 510-574-1473

          If to TANOX:           Tanox, Inc.
                                 10301 Stella Link
                                 Houston, TX 77025 USA
                                 Attention: Nancy T. Chang, President and CEO
                                 Fax No: (713) 664-8914

          Copy to:               Tanox, Inc.
                                 10301 Stella Link
                                 Houston, TX 77025 USA
                                 Attention: Head of Legal Affairs

                                        7
<PAGE>
     8.05 CHOICE OF LAW. The validity, performance, construction, and effect of
this Agreement shall be governed by the laws of the State of California which
are applicable to contracts between California residents to be performed wholly
within California.

     8.06 WAIVER. None of the terms, covenants and conditions of this Agreement
can be waived except by the written consent of the party waiving compliance.

     8.07 FORCE MAJEURE. Neither party shall be responsible to the other for
failure or delay in performing any of its obligations under this Agreement or
for other non-performance hereof provided that such delay or non-performance is
occasioned by a cause beyond the reasonable control and without fault or
negligence of such party, including, but not limited to war, terrorism,
earthquake, fire, flood, explosion, discontinuity in the supply of power, court
order or governmental interference, act of God, strike or other labor trouble
and provided that such party will inform the other party as soon as is
reasonably practicable and that it will entirely perform its obligations within
a reasonable time after the relevant cause has ceased its effect.

     8.08 PUBLICITY. PDL will issue a press release concerning the parties'
entry into this Agreement and the amount of signing and licensing fees paid
hereunder, with the content of such release to be approved in advance by TANOX,
which approval shall not be unreasonably withheld. Except as required by law or
regulation, neither party shall publicly disclose the other terms and conditions
of this Agreement unless expressly authorized to do so by the other party, which
authorization shall not be unreasonably withheld, and both parties agree that
PDL shall not disclose the antigen which is the subject of this Agreement,
without TANOX' prior written consent; provided, however, that the parties may
disclose such terms and conditions to their respective investors, prospective
investors, advisors, and consultants under a written confidentiality agreement
protecting the confidentiality of the terms hereof in a manner at least as
restrictive as provided under Article 6 of the Master Agreement, and to their
accountants and attorneys provided that such accountants and attorneys agree to
maintain the confidentiality of the terms hereof in a manner at least as
restrictive as provided under Article 6 of the Master Agreement, and either
party may disclose such terms and conditions as deemed required in connection
with any securities offering it elects to make.

     8.09 HEADINGS. The captions used herein are inserted for convenience of
reference only and shall not be construed to create obligations, benefits, or
limitations.

     8.10 COUNTERPARTS. This Agreement may be executed in any number of
counterparts, each of which shall be deemed to be an original, and such
counterparts together shall constitute one agreement. Execution and delivery of
this Agreement by facsimile copies bearing the facsimile signatures of the
parties shall constitute a valid and binding execution and delivery of this
Agreement by the signing party, and such facsimile copies shall constitute
original documents.

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

PDL:                                                    TANOX:
PROTEIN DESIGN LABS, INC.                               TANOX, INC.

By:______________________                               By:___________________
     Laurence Jay Korn                                  Nancy T. Chang
     CEO                                                President and CEO

                                        8
<PAGE>
                                    EXHIBIT A

                                PDL PATENT RIGHTS

The following are patents and patent applications (also known as the "Queen et
al. patents") issued and filed in certain countries in the world and licensed as
part of the PDL Patent Rights under the Agreement. (Updated as of January 27,
2000.)

1.  The following issued U.S. patents and U.S. patent applications:

        No. 5,585,089, "Humanized Immunoglobulins," issued December 17, 1996.

        No. 5,693,761, "Polynucleotides Encoding Improved Humanized
        Immunoglobulins," issued December 2, 1997.

        No. 5,693,762, "Humanized Immunoglobulins," issued December 2, 1997.

        *

2.  The following patents and patent applications outside the U.S.:

<TABLE>
<CAPTION>
PATENT NO.                          COUNTRY                           TITLE*
<S>             <C>                 <C>              <C>
                                                          "Novel Immunoglobulins, Their

Issued          647383              Australia                  Production and Use"
Issued          671949              Australia                           "
Issued          AT E133452          Austria                             "
Issued          0451216             Belgium                             "
Issued          61095               Bulgaria                            "
Issued          98389               Chile                               "
Issued          P920500A            Croatia                             "
Issued          0451216B1           Europe1                             "
Issued          0682040 B1          Europe1                             "
Issued          FR0451216           France                              "
Issued          DE 68925536         Germany                             "
Issued          DE 68929061         Germany                             "
Issued          DD 296 964          East Germany                        "
Issued          GB 0451216          Great Britain                       "
Issued          1001050             Greece                              "
Issued          211174              Hungary                             "
Issued          IT 0451216          Italy                               "
Issued          2828340             Japan                               "
Issued          LU 0451216          Luxembourg                          "
Issued          92.2146             Monaco                              "
Issued          NL 0451216          Netherlands                         "
Issued          231984              New Zealand                         "
Issued          132068              Pakistan                            "
Issued          29729               Philippines                         "
Issued          92758               Portugal                            "
Issued          2126046             Russia                              "
Issued          SG 0451216          Singapore                           "
Issued          8912489             Slovenia                            "
Issued          89/9956             South Africa                        "
Issued          178385              South Korea                         "
Issued          0231090             South Korea                         "
Issued          2081974 T3          Spain                               "
Issued          2136760             Spain
</TABLE>

                                        9

LEGAL COMMON/CONTRACTS/TANOX/LIC OPTN/MSTR000316                          000316
<PAGE>
<TABLE>
<S>             <C>                 <C>              <C>
Issued          SE 0451216          Sweden                              "
Issued          CH 0451216          Switzerland                         "
Issued          50034               Taiwan                              "
Issued          13349               Uruguay                             "
Issued          56455               Venezuela                           "
Issued          48700               Yugoslavia                          "
<CAPTION>
                COUNTRY             APPLICATION NO.                   TITLE*

Pending            *                      *                             *
Pending
Pending
Pending
Pending
Pending
Pending
Pending
Pending
Pending
Pending
Pending
Pending
Pending
</TABLE>

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

* Exact titles may differ in different countries.

1 and corresponding European national patents issued therefrom.

                                       10




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