In August 2000, the US Food and Drug Administration (FDA) approved ciprofloxacin hydrochloride (Cipro; Bayer) for management of postexposure inhalational anthrax. This was the first antimicrobial drug approved by the FDA for use in treating an infection due to a biological agent used intentionally. The terrorist attacks of 2001 involving anthrax underscore the imperative that safe and effective drugs to manage such infections be readily available in the United States. The approval of ciprofloxacin hydrochloride, which was made on the basis of a surrogate human marker of efficacy, made extensive use of data from an animal model of disease. This represents a new direction in the development of efficacy data in support of drug approval and facilitates the availability of those drugs for which there is an urgent need. This article presents the scientific data and regulatory mechanism that supported the approval of ciprofloxacin hydrochloride for management of postexposure of inhalational anthrax.