The purpose of this guidance is to assist sponsors in the development of drugs for the indication of prophylaxis of inhalational anthrax in persons who have or may have inhaled aerosolized Bacillus anthracis spores but who have not yet manifested clinical evidence of disease. The indication also applies to persons with anticipated exposure to B. anthracis spores (e.g., first responders for anthrax incidents); in such cases, initiation of antibacterial therapy would begin immediately before entering the B. anthracis-contaminated environment. For more information regarding the indication, see section II.B., Indication for Prophylaxis of Inhalational Anthrax.