Exhibit 4.3  AMENDMENT TO RIGHTS AGREEMENT  THIS AMENDMENT TO RIGHTS AGREEMENT (Amendment),
dated as of November 6, 2007 (Amendment Effective Date), is between Medarex, Inc.,
a New Jersey corporation, and Continental Stock Transfer & Trust
Company.  RECITALS  A.  The Company
previously entered into a Rights Agreement, dated as of May 23, 2001, with
Continental Stock Transfer & Trust Company, as Rights Agent (the Rights
Agreement).  B.  The Company
now wishes to amend the Rights Agreement as set forth herein.  AGREEMENT  Accordingly, in consideration of the premises and the
mutual agreements herein set forth, the Rights Agreement is hereby amended as
of the Amendment Effective Date as follows:  1.The definition of Acquiring Person in Section 1(a) of
the Rights Agreement is amended and restated to read in its entirety as
follows:  (a)           Acquiring Person shall mean any
Person who or which, together with all Affiliates and Associates of such
Person, shall be the Beneficial Owner of twenty percent (20%) or more of the
shares of Common Stock then outstanding, but shall not include (i) the
Company, (ii) any Subsidiary of the Company, (iii) any employee
benefit plan of the Company, or of any Subsidiary of the Company, or any Person
or entity organized, appointed or established by the Company for or pursuant to
the terms of any such plan, or (iv) any Person, together with all
Affiliates and Associates of such Person, who or which would be an Acquiring
Person solely by reason of being the Beneficial Owner of shares of Common
Stock, the Beneficial Ownership of which was acquired by such Person pursuant
to any action or transaction or series of related actions or transactions
approved by the Board of Directors before such Person otherwise became an
Acquiring Person;provided, however, that no Person shall become an Acquiring
Person:  (1) as the result of an
acquisition of shares of Common Stock by the Company which, by reducing the
number of shares of Common Stock issued and outstanding, increases the proportionate
number of shares of Common Stock beneficially owned by such Person to 20% or
more of the shares of Common Stock then outstanding (provided that if a Person
shall become the Beneficial Owner of 20% or more of the shares of Common Stock
then outstanding as the  1


result of an acquisition of shares of Common Stock by
the Company and shall, following written notice from, or public disclosure by,
the Company of such acquisition of shares by the Company, become the Beneficial
Owner of any additional shares of Common Stock without the prior written
consent of the Company and shall then Beneficially Own 20% or more of the
shares of Common Stock then outstanding, then such Person shall be deemed to be
an Acquiring Person); (2) as the result of the acquisition of shares of
Common Stock directly from the Company (provided that if a Person shall become
the Beneficial Owner of 20% or more of the shares of Common Stock then
outstanding as the result of the acquisition of shares of Common Stock directly
from the Company and shall, after that date, become the Beneficial Owner of any
additional shares of Common Stock without the prior written consent of the
Company and shall then Beneficially Own 20% or more of the shares of Common
Stock then outstanding, then such Person shall be deemed to be an Acquiring
Person); or (3) if the Board of Directors determines in good faith that a
Person who would otherwise be an Acquiring Person, as defined pursuant to the
foregoing provisions of this Section 1(a), has inadvertently become the
Beneficial Owner of 20% or more of the shares of Common Stock then outstanding,
and such Person divests, as promptly as practicable (as determined in good
faith by the Board of Directors), following receipt of written notice from the
Company of such event, of Beneficial Ownership of a sufficient number of shares
of Common Stock so that such Person would no longer be an Acquiring Person, as
defined pursuant to the foregoing provisions of this Section 1(a), then
such Person shall not be deemed to be an Acquiring Person for any purposes of
this Agreement, provided that if such Person shall again become the Beneficial
Owner of 20% or more of the shares of Common Stock then outstanding, such Person
shall be deemed an Acquiring Person, subject to the exceptions set forth in
this Section 1(a).  2.A new defined term Interested
Stockholder is added to the Rights Agreement as Section 1(r), to read in
its entirety as follows:  (r)            Interested Stockholder shall mean
any Acquiring Person or any Affiliate or Associate of an Acquiring Person or
any other Person in which any such Acquiring Person, Affiliate or Associate has
an interest, or any other Person acting directly or indirectly on behalf of or
in concert with any such Acquiring Person, Affiliate or Associate.  3.The definition of Purchase Price
previously set forth in Section 1(u) of the Rights Agreement is
renumbered as Section 1(v) and is amended and restated to read in its
entirety as follows:  (v)           Purchase Price shall have the
meaning set forth in Section 4(a) hereof.  2


4.Section 3(b) of the Rights
Agreement is amended and restated to read in its entirety as follows:  (b)           The Company will make available, as
promptly as practicable following the Record Date, a copy of a summary of the
Rights, in substantially the form attached hereto as Exhibit C (the Summary
of Rights) to any holder of Rights who may so request from time to time prior to
the Expiration Date. With respect to certificates for the Common Stock
outstanding as of the Record Date, until the Distribution Date, the Rights will
be evidenced by such certificates for the Common Stock and the registered
holders of the Common Stock shall also be the registered holders of the
associated Rights.  Until the earlier of
the Distribution Date or the Expiration Date (as such term is defined in Section 7(a) hereof),
the transfer of any certificates representing shares of Common Stock in respect
of which Rights have been issued shall also constitute the transfer of the
Rights associated with such shares of Common Stock.  5.Section 5(a) of the Rights
Agreement is amended and restated to read in its entirety as follows:  (a)           The Rights Certificates shall be
executed on behalf of the Company by its Chairman of the Board, its President
or any Vice President, either manually or by facsimile signature, and shall
have affixed thereto the Companys seal or a facsimile thereof which shall be
attested by the Secretary or an Assistant Secretary of the Company, either
manually or by facsimile signature.  The
Rights Certificates shall be countersigned by the Rights Agent, either manually
or by facsimile signature, and shall not be valid for any purpose unless so
countersigned.  In case any officer of
the Company who shall have signed any of the Rights Certificates shall cease to
be such officer of the Company before countersignature by the Rights Agent and
issuance and delivery by the Company, such Rights Certificates, nevertheless,
may be countersigned by the Rights Agent and issued and delivered by the
Company with the same force and effect as though the person who signed such
Rights Certificates had not ceased to be such officer of the Company; and any Rights
Certificates may be signed on behalf of the Company by any person who, at the
actual date of the execution of such Rights Certificate, shall be a proper
officer of the Company to sign such Rights Certificate, although at the date of
the execution of this Agreement any such person was not such an officer.  6.Section 11(d)(ii) of the Rights
Agreement is amended and restated to read in its entirety as follows:  (ii)           For the purpose of any computation
hereunder, the Current Market Price per share of Preferred Stock shall be
determined in the same manner as set forth above for the Common Stock in clause
(i) of this Section 11(d) (other than the last sentence
thereof).  If the Current Market Price
per share of Preferred Stock cannot be determined in the manner provided above
or if the Preferred Stock is not publicly held or listed or traded in a manner
described in clause (i) of  3


this Section 11(d), the Current Market Price per
share of Preferred Stock shall be conclusively deemed to be an amount equal to
one thousand (1000) (as such number may be appropriately adjusted for such
events as stock splits, stock dividends and recapitalizations with respect to
the Common Stock occurring after the date of this Agreement) multiplied by the
Current Market Price per share of the Common Stock.  If neither the Common Stock nor the Preferred
Stock is publicly held or so listed or traded, Current Market Price per share
of the Preferred Stock shall mean the fair value per share as determined in
good faith by the Board, whose determination shall be described in a statement
filed with the Rights Agent and shall be conclusive for all purposes.  7.Section 11(o) of the Rights
Agreement is amended and restated to read in its entirety as follows:  (o)           The Company covenants and agrees
that, after the Distribution Date, it will not, except as permitted by Section 23
hereof, take (or permit any Subsidiary to take) any action if at the time such
action is taken it is reasonably foreseeable that such action will diminish
substantially or otherwise eliminate the benefits intended to be afforded by
the Rights.  8.Section 13(a) of the Rights
Agreement is amended and restated to read in its entirety as follows:  (a)           In the event that, following the
Stock Acquisition Date, directly or indirectly, (x) the Company shall
consolidate with, or merge with and into, any Interested Stockholder, or if in
such merger or consolidation all holders of Common Stock are not treated alike,
any other Person (other than a Subsidiary of the Company in a transaction which
complies with Section 11(o) hereof), and the Company shall not be the
continuing or surviving corporation of such consolidation or merger, (y) any
Interested Stockholder, or if in such merger or consolidation all holders of
Common Stock are not treated alike, any other Person (other than a Subsidiary
of the Company in a transaction which complies with Section 11(o) hereof)
shall consolidate with, or merge with or into, the Company, and the Company
shall be the continuing or surviving corporation of such consolidation or
merger and, in connection with such consolidation or merger, all or part of the
outstanding shares of Common Stock shall be changed into or exchanged for stock
or other securities of any other Person or cash or any other property (other
than, in the case of either transaction described in (x) or (y), a merger
or consolidation that would result in all of the voting power represented by
the securities of the Company outstanding immediately prior thereto continuing
to represent (either by remaining outstanding or by being converted into
securities of the surviving entity) all of the voting power represented by the
securities of the Company or such surviving entity outstanding immediately
after such merger or consolidation and the holders of such securities not
having changed as a result of such merger or consolidation), or (z) the
Company shall sell or otherwise transfer (or one or more of its Subsidiaries
shall sell or otherwise transfer), in one transaction or a series of related
transactions, assets, cash flow or  4


earning power aggregating more than fifty percent
(50%) of the assets, cash flow or earning power of the Company and its
Subsidiaries (taken as a whole) to any Interested Stockholder or Stockholders,
or if in such transaction all holders of Common Stock are not treated alike,
any other Person (other than the Company or any Subsidiary of the Company in
one or more transactions each of which complies with Section 11(o) hereof),
then, and in each such case (except as may be contemplated by Section 13(d) hereof),
proper provision shall be made so that: (i) each holder of a Right, except
as provided in Section 7(e) hereof, shall thereafter have the right
to receive, upon the exercise thereof at the then current Purchase Price in
accordance with the terms of this Agreement, such number of validly authorized
and issued, fully paid, non-assessable and freely tradable shares of Common
Stock of the Principal Party (as such term is hereinafter defined), not subject
to any liens, encumbrances, rights of first refusal or other adverse claims, as
shall be equal to the result obtained by (1) multiplying the then current
Purchase Price by the number of one one-thousandths of a share of Preferred
Stock for which a Right is exercisable immediately prior to the first
occurrence of a Section 13 Event (or, if a Section 11(a)(ii) Event
has occurred prior to the first occurrence of a Section 13 Event,
multiplying the number of such one one-thousandths of a share for which a Right
was exercisable immediately prior to the first occurrence of a Section 11(a)(ii) Event
by the Purchase Price in effect immediately prior to such first occurrence),
and dividing that product (which, following the first occurrence of a Section 13
Event, shall be referred to as the Purchase Price for each Right and for all
purposes of this Agreement) by (2) fifty percent (50%) of the Current
Market Price (determined pursuant to Section 11(d)(i) hereof) per
share of the Common Stock of such Principal Party on the date of consummation
of such Section 13 Event; (ii) such Principal Party shall thereafter
be liable for, and shall assume, by virtue of such Section 13 Event, all
the obligations and duties of the Company pursuant to this Agreement; (iii) the
term Company shall thereafter be deemed to refer to such Principal Party, it
being specifically intended that the provisions of Section 11 hereof shall
apply only to such Principal Party following the first occurrence of a Section 13
Event; (iv) such Principal Party shall take such steps (including, but not
limited to, the reservation of a sufficient number of shares of its Common
Stock) in connection with the consummation of any such transaction as may be
necessary to assure that the provisions hereof shall thereafter be applicable,
as nearly as reasonably may be, in relation to its shares of Common Stock
thereafter deliverable upon the exercise of the Rights; and (v) the
provisions of Section 11(a)(ii) hereof shall be of no effect
following the first occurrence of any Section 13 Event.  9.The sixth paragraph of the form of Rights
Certificate attached as Exhibit B to the Rights Agreement is amended and
restated to read in its entirety as follows:  Subject
to the provisions of the Rights Agreement, the Rights evidenced by this
Certificate may be redeemed by the Company at its option at a redemption price
of $.001 per Right at any time prior to the earlier of the close of business on
(i) the tenth Business Day following the Stock Acquisition Date, and (ii) the
Final Expiration Date.  In addition,
under certain circumstances following the Stock  5


Acquisition Date, the Rights may be exchanged, in
whole or in part, for shares of the Common Stock, or shares of preferred stock
of the Company having essentially the same value or economic rights as such
shares.  Immediately upon the action of
the Board of Directors of the Company authorizing any such exchange, and
without any further action or any notice, the Rights (other than Rights which
are not subject to such exchange) will terminate and the Rights will only
enable holders to receive the shares issuable upon such exchange.  10.The first paragraph of the Summary of
Rights to Purchase Preferred Stock attached as Exhibit C to the Rights
Agreement is amended and restated to read in its entirety as follows:  On
May 23, 2001, the Board of Directors of Medarex, Inc. (the Company)
declared a dividend distribution of one Right for each outstanding share of
Company Common Stock to stockholders of record at the close of business on July 6,
2001 (the Record Date).  Each Right
entitles the registered holder to purchase from the Company a unit consisting
of one one-thousandth of a share (a Unit) of Series A Junior
Participating Preferred Stock, par value $1 per share (the Series A
Preferred Stock) at a Purchase Price of one hundred and fifty dollars ($150)
per Unit, subject to adjustment.  The description
and terms of the Rights are set forth in a Rights Agreement (the Rights
Agreement) between the Company and Continental Stock Transfer & Trust
Company, as Rights Agent.  11.The sixth paragraph of the Summary of
Rights to Purchase Preferred Stock attached as Exhibit C to the Rights
Agreement is amended and restated to read in its entirety as follows:  For
example, at an exercise price of  one
hundred and fifty dollars ($150) per Right, each Right not owned by an
Acquiring Person (or by certain related parties) following an event set forth
in the preceding paragraph would entitle its holder to purchase three hundred
dollars ($300) worth of Common Stock (or other consideration, as noted above)
for one hundred and fifty dollars ($150). 
Assuming that the Common Stock had a per share value of thirty dollars
($30) at such time, the holder of each valid Right would be entitled to
purchase ten (10) shares of Common Stock for one hundred and fifty dollars
($150).  12.Except as amended pursuant to this Amendment,
the Rights Agreement shall remain in force and effect in accordance with its
terms.  6


IN WITNESS WHEREOF, the
parties to this Amendment have caused this Amendment to be duly executed, all
as of the day and year first above written.   ATTEST:
  MEDAREX, INC./s/ W. Bradford
  Middlekauff
  
  By:
  /s/ Howard H. PienW. Bradford
  Middlekauff, Secretary
  
  Howard H. PienTitle:
  President and CEOCountersigned:CONTINENTAL STOCK
  TRANSFER & TRUST COMPANYBy:
  /s/ William F.
  SeegraberAuthorized
  Signature:William F. SeegraberVice President  7