The Centers for Disease Control and Prevention (CDC), Coordinating Centers for Disease Control and Prevention, intends to negotiate a sole source purchase order with Sanofi Pasteur Biologics Company, Acambis Campus, under the authority of FAR 13.106-1(b) Soliciting from a single source, only one source reasonably available (e.g., urgency, exclusive licensing agreements, brand-name or industrial mobilization). The contractor shall perform potency testing using the Food and Drug Administration (FDA) qualified and validated vaccinia based potency assay on samples of Wetvax provided by the CDC. CDC has an agreement with the FDA to perform yearly testing of the Strategic National Stockpile (SNS) Aventis Pasteur Smallpox Vaccine.