UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of Earliest Event Reported): August 25, 2011

NABI BIOPHARMACEUTICALS

(Exact Name of Registrant as specified in its charter)

12276 Wilkins Avenue, Rockville, Maryland 20852

(Address of principal executive offices) (Zip Code)

Registrant’s telephone number, including area code: (301) 770-3099

Not Applicable

(Registrant’s name or former address, if change since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

On August 25, 2011, Nabi Biopharmaceuticals, a Delaware corporation (the “Company”), entered into a Rights Agreement with American Stock Transfer & Trust Company, LLC, as Rights Agent, dated as of August 25, 2011 (the “Rights Agreement”). By adopting the Rights Agreement, the Board of Directors is seeking to protect the Company’s ability to carry forward its net operating losses and certain other tax attributes (collectively, “NOLs”). The Company has experienced and may continue to experience substantial operating losses, and for federal and state income tax purposes, the Company may “carry forward” net operating losses in certain circumstances to offset current and future taxable income, which will reduce federal and state income tax liability, subject to certain requirements and restrictions. These federal and state NOLs are a valuable asset of the Company, which may inure to the benefit of the Company and its stockholders. However, if the Company experiences an “ownership change,” as defined in Section 382 of the Internal Revenue Code (the “Code”), its ability to use the NOLs could be substantially limited, and the timing of the usage of the NOLs could be substantially delayed, which could significantly impair the value of the Company’s NOL asset. Generally, an “ownership change” occurs if the percentage of the Company’s stock owned by one or more “five percent stockholders” increases by more than fifty percentage points over the lowest percentage of stock owned by such stockholders at any time during the prior three-year period or, if sooner, since the last “ownership change” experienced by the Company. An NOL rights agreement like the Rights Agreement with a 4.99% “trigger” threshold is intended to act as a deterrent to any person acquiring 4.99% or more of the outstanding shares of Common Stock without the approval of the Board of Directors. This would protect the Company’s NOL asset because changes in ownership by a person owning less than 4.99% of the Common Stock are not included in the calculation of “ownership change” for purposes of Section 382 of the Code.

In connection with the entry into the Rights Agreement, the Board of Directors declared a dividend of one preferred share purchase right (the “Rights”) for each outstanding share of Common Stock under the terms of the Rights Agreement. The dividend is payable on August 25, 2011 to the stockholders of record on August 25, 2011 (the “Record Date”). Each Right entitles the registered holder to purchase from the Company one one-thousandth of a share of Series A Junior Participating Preferred Stock, par value $.10 per share, of the Company (the “Preferred Stock”) at a price of $7.00 per one one-thousandth of a share of Preferred Stock (the “Purchase Price”), subject to adjustment.

As a result of declaring a dividend of the Rights, until the earlier to occur of (i) the tenth business day following a public announcement that a person or group of affiliated or associated persons has acquired beneficial ownership of 4.99% or more of the Common Stock (an “Acquiring Person”) or (ii) ten Business Days (or such later date as may be determined by action of the Board of Directors prior to such time as any person or group of affiliated persons becomes an Acquiring Person) following the commencement or announcement of an intention to make a tender offer or exchange offer the consummation of which would result in the beneficial ownership by a person or group of 4.99% or more of the Common Stock (the earlier of (i) and

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(ii) being called the “Distribution Date”), the Rights will be evidenced, with respect to any of the Common Stock certificates outstanding as of the Record Date, by such Common Stock certificate (or, with respect to any shares of Common Stock held in book entry form, by the notation in book entry) together with the summary of rights. The Rights Agreement provides that any person who beneficially owned 4.99% or more of the Common Stock on the date the Rights Agreement was adopted, together with any affiliates and associates of that person (each an “Existing Holder”), shall not be deemed to be an “Acquiring Person” for purposes of the Rights Agreement unless the Existing Holder becomes the beneficial owner of (x) a percentage of the Common Stock then outstanding that is more than the aggregate percentage of the outstanding Common Stock that such Existing Holder beneficially owns as of the date of the Rights Agreement or (y) less than 4.99% of the Common Stock then outstanding (after which, if the Existing Holder becomes the beneficial owner of 4.99% or more of the Common Stock then outstanding, the Existing Holder shall be deemed to be an “Acquiring Person”). The Rights Agreement includes a procedure whereby the Board of Directors will consider requests to exempt certain proposed acquisitions of Common Stock from the applicable ownership trigger if the Board determines that the requested acquisition will not jeopardize or endanger the availability of the NOLs to the Company.

The Rights. The Rights Agreement provides that until the Distribution Date (or earlier redemption, exchange, termination or expiration of the Rights), the Rights will be transferred with and only with the Common Stock. Until the Distribution Date (or earlier redemption, exchange, termination or expiration of the Rights), new Common Stock certificates issued after the close of business on the Record Date upon transfer or new issuance of the Common Stock will contain a notation incorporating the Rights Agreement by reference, and the Company will deliver a notice to that effect upon the transfer or new issuance of book entry shares. Until the Distribution Date (or earlier redemption, exchange, termination or expiration of the Rights), the surrender for transfer of any certificates for Common Stock or any book entry shares, with or without such notation, notice or the summary of rights, will also constitute the transfer of the Rights associated with the Common Stock represented by such certificate or the book entry shares. As soon as practicable following the Distribution Date, separate certificates evidencing the Rights (“Right Certificates”) will be mailed to holders of record of the Common Stock as of the close of business on the Distribution Date and such separate Right Certificates alone will evidence the Rights.

Exercisability and Expiration. The Rights are not exercisable until the Distribution Date. The Rights will expire on August 24, 2014, subject to the Company’s right to extend such date (the “Final Expiration Date”), unless earlier redeemed or exchanged by the Company or terminated. Until a Right is exercised, the holder thereof, as such, will have no rights as a stockholder of the Company beyond those as an existing stockholder, including, without limitation, the right to vote or to receive dividends. Although the Rights Agreement has a three-year term, the Company’s Board of Directors has determined to review the plan periodically in light of developments at the Company, including in connection with the use and value of the NOLs, to assess whether the Rights Agreement should be maintained.

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Consequences of a Person or Group Becoming an Acquiring Person. In the event that a Person becomes an Acquiring Person or if the Company were the surviving corporation in a merger with an Acquiring Person or any affiliate or associate of an Acquiring Person and the Common Stock were not changed or exchanged, each holder of a Right, other than Rights that are or were acquired or beneficially owned by the Acquiring Person (which Rights will thereafter be void), will thereafter have the right to receive upon exercise that number of shares of Common Stock having a market value of two times the then current Purchase Price of the Right.

Exchange. At any time after a Person becomes an Acquiring Person and prior to the acquisition by such Acquiring Person of 50% or more of the outstanding Common Stock, the Board of Directors may cause the Company to exchange the Rights (other than Rights owned by an Acquiring Person which will have become void), in whole or in part, for Common Stock at an exchange ratio of one share of Common Stock per Right (subject to adjustment).

Redemption. The Rights may be redeemed in whole, but not in part, at a price of $0.0001 per Right (the “Redemption Price”) by the Board of Directors at any time prior to the time that an Acquiring Person has become such. The redemption of the Rights may be made effective at such time, on such basis and with such conditions as the Board of Directors in its sole discretion may establish. Immediately upon any redemption of the Rights, the right to exercise the Rights will terminate and the only right of the holders of Rights will be to receive the Redemption Price.

Preferred Share Provisions. Shares of Preferred Stock purchasable upon exercise of the Rights will not be redeemable. Each share of Preferred Stock will be entitled, when, as and if declared, to a minimum preferential quarterly dividend payment of the greater of (a) $1.00 per share, and (b) an amount equal to 1,000 times the dividend declared per share of Common Stock. In the event of liquidation, dissolution or winding up of the Company, the holders of the Preferred Stock will be entitled to a minimum preferential payment of the greater of (a) $10.00 per share (plus any accrued but unpaid dividends), and (b) an amount equal to 1,000 times the payment made per share of Common Stock. Each share of Preferred Stock will have 1,000 votes, voting together with the Common Stock. Finally, in the event of any merger, consolidation or other transaction in which outstanding shares of Common Stock are converted or exchanged, each share of Preferred Stock will be entitled to receive 1,000 times the amount received per share of Common Stock. These rights are protected by customary antidilution provisions.

Because of the nature of the Preferred Stock’s dividend, liquidation and voting rights, the value of the one one-thousandth interest in a share of Preferred Stock purchasable upon exercise of each Right should approximate the value of one share of Common Stock.

Adjustment. The Purchase Price payable, and the number of shares of Preferred Stock or other securities or property issuable, upon exercise of the Rights are subject to adjustment from time to time to prevent dilution (i) in the event of a stock dividend on, or a subdivision, combination or reclassification of the Preferred Stock, (ii) upon the grant to holders of the Preferred Stock of

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certain rights or warrants to subscribe for or purchase Preferred Stock or convertible securities at less than the current market price of the Preferred Stock or (iii) upon the distribution to holders of the Preferred Stock of evidences of indebtedness, cash, securities or assets (excluding regular periodic cash dividends at a rate not in excess of 125% of the rate of the last regular periodic cash dividend theretofore paid or, in case regular periodic cash dividends have not theretofore been paid, at a rate not in excess of 50% of the average net income per share of the Company for the four quarters ended immediately prior to the payment of such dividend, or dividends payable in Preferred Stock (which dividends will be subject to the adjustment described in clause (i) above)) or of subscription rights or warrants (other than those referred to above). No adjustment in the Purchase Price will be required until cumulative adjustments require an adjustment of at least 1% in such Purchase Price. No fractional Preferred Stock or Common Stock will be issued (other than fractions of Preferred Stock which are integral multiples of one one-thousandth of a share of Preferred Stock, which may, at the election of the Company, be evidenced by depository receipts), and in lieu thereof, a payment in cash will be made based on the market price of the Preferred Stock or Common Stock on the last trading date prior to the date of exercise.

This summary description of the Rights Agreement does not purport to be complete and is qualified in its entirety by reference to the Rights Agreement, which is incorporated herein by reference to Exhibit 4.1 of this Current Report on Form 8-K.

See the disclosures in Items 1.01 and 5.03 of this Current Report on Form 8-K, which are incorporated herein by reference.

In connection with the adoption of the Rights Agreement, the Board of Directors approved the Certificate of Designations establishing the Preferred Stock and the rights, preferences and privileges thereof, which Certificate of Designations was filed with the Secretary of State of the State of Delaware on August 25, 2011. The Certificate of Designations is attached hereto as Exhibit 3.1 and is incorporated herein by reference. See the description set out under Item 1.01 for a more complete description of the rights and preferences of the Series A Junior Participating Preferred Stock, which is incorporated herein by reference.

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SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.

Date: August 25, 2011

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