 Anthrax is rarely encountered in the U.S. since the introduction of attenuated vaccines for cattle in the 1930s. A vaccine for humans, composed of a cell-free preparation absorbed onto alum, was licensed in 1970 after successful clinical trials. Vaccination against anthrax in the U.S. was confined to individuals at risk such as wool sorters and some veterinarians until the prospects were raised of B. anthracis being used as a weapon after the Iraq war. Now anthrax vaccine is administered routinely to the armed forces. Interest in improving this vaccine was stimulated by the use of B. anthracis spores for bioterrorism. 