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Exhibit 10.1

December 4, 2007

Biotest Pharmaceuticals Corporation

c/o Biotest AG

Landsteinerstr. 5

63303 Dreieich

Germany

Biotest AG

Landsteinerstr. 5

63303 Dreieich

Germany

RE: Letter Agreement Regarding Interim Operations and Regulatory Arrangements

Ladies and Gentlemen:

Reference is made to that certain Asset Purchase Agreement by and among Nabi Biopharmaceuticals (“Nabi”), Biotest Pharmaceuticals Corporation (“Biotest”) and Biotest AG (“Parent”; and together with Nabi and Biotest, the “Parties”), dated as of September 11, 2007, as amended from time to time (the “Purchase Agreement”). Capitalized terms used in this letter agreement that are not defined herein have the meanings assigned to such terms in the Purchase Agreement. The Parties hereby agree as follows:

1. Operations Pending Issuance of Florida Licenses. The manufacture, marketing, distribution, and sale of Nabi-HB^® at the current establishment (the “Licensed Business”) shall continue to be operated under Nabi’s oversight and control in full accordance with and pursuant to Nabi’s Florida manufacturing and wholesale distributor licenses (the “Nabi Florida Licenses”) during the period (the “Interim Period”) from and after the Effective Time until Biotest is issued a Florida manufacturing license and prescription drug wholesale distributor license (collectively, the “Biotest Florida Licenses”) and consistent in all material respects with Nabi’s past practices and procedures.

2. Interim Oversight With Respect to Federal Registrations. From the Closing until the transfers of Nabi’s federal Registrations to Biotest are authorized and all federal licenses issued or transferred to Biotest, Nabi shall have oversight and control authority over the elements of Biotest’s post-Closing operations that are subject to such Registrations (to the extent that authorization for transfer has not been received) as necessary or as mandated by FDA in connection with the compliance of such operations with the requirements of such Registrations. Biotest shall comply in all respects with requests and/or instructions of Nabi in the exercise of Nabi’s oversight and control authority hereunder.

3. Indemnification. Biotest shall indemnify and defend the Seller Indemnitees against, and agrees to hold them harmless from, any Losses sustained or incurred (payable promptly upon written request by any Seller Indemnitee), to the extent arising from, in connection with, or otherwise with respect to acts or omissions by or on behalf of Nabi relating to this Agreement and the activities contemplated hereunder, provided, however, that Biotest shall not be required to indemnify any Person, and shall not have any liability under this Section 3 to the extent the liability or obligation is directly caused by the gross negligence or willful misconduct of any Seller Indemnitee. Except as expressly provided herein, indemnification of Seller Indemnitees hereunder shall be governed by Article XI and other applicable provisions of the Purchase Agreement as if the indemnification were of Losses with respect to an Assumed Liability.

4. Nabi CFO. Mr. Siegel shall remain as an employee of Nabi in the position of Senior Vice President, Finance and Administration, Chief Financial Officer and Treasurer from the Effective Time through March 28, 2008 unless Mr. Siegel resigns or is terminated by Nabi. During such time, Mr. Siegel may accept simultaneous appointment as Chief Financial Officer of Biotest and may perform the duties of that position to the extent that such duties do not unreasonably interfere with Mr. Siegel’s duties to Nabi. Biotest may announce the appointment of Mr. Siegel as its Chief Financial Officer from and after the Effective Date. Nabi shall pay Mr. Siegel’s compensation during such period in accordance with his existing employment agreement. In the event that Mr. Siegel determines that there is an actual or potential conflict between his duties to Nabi and his duties to Biotest with respect to any matter or issue, Mr. Siegel shall promptly inform the Chief Executive Officers of Nabi and Parent and their mutual determination with respect

to such conflict shall control. If the conflict cannot be resolved in the foregoing manner, either Party may refer the conflict to arbitration in accordance with Section 12.8 of the Purchase Agreement. Pending resolution of the conflict, Mr. Siegel shall not have a duty to either Nabi or Biotest with respect to the matter or issue. In any such circumstance, Nabi shall as soon as reasonably possible appoint another Nabi officer who shall have principal on-site authority for exercising on behalf of Nabi the oversight and control authority described herein during the Interim Period. Biotest hereby confirms for the benefit of Mr. Siegel that his continued employment by Nabi pursuant to this Section 4 will not constitute grounds for termination for cause under his employment agreement with Biotest. Nabi hereby confirms for the benefit of Mr. Siegel that his employment by Biotest pursuant to this Section 4 will not constitute grounds for termination for cause under his Employment Agreement with Nabi dated April 29, 2006 and that his continued employment by Nabi will not effect his status as an “affected employee” for purposes of certain benefits to which he is eligible as a result of the closing of the transactions contemplated by the Purchase Agreement.

5. Mutual Waiver of Certain Purchase Agreement Provisions. Each Party waives any breach by the other Party of a representation, warranty or covenant of such other Party under the Purchase Agreement or any Other Agreement that occurs as a direct result of such other Party’s performance of its obligations under this Agreement.

6. Incorporation by Reference of Certain Provisions of Purchase Agreement. Except as specifically provided herein or unless the context requires otherwise, Article XII of the Purchase Agreement is hereby incorporated by reference herein as if set out at length herein, and shall be read to apply to this letter agreement standing on its own.

7. Controlling Document. In the event of any inconsistency between the terms of this letter agreement and any term of the Purchase Agreement or any Other Agreement, the terms of this letter agreement shall govern.

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If the foregoing reflects your understanding with respect to the subject matter of this letter agreement, please acknowledge your agreement with and acceptance of the same by signing the enclosed counterpart of this letter in the space provided below and returning it to the undersigned.

Acknowledged and agreed as of the date first written above:

Attention: Russell Pallesen