 This study will examine the recombinant, that is, produced by genetic engineering, protective antigen rPA that brings about antibodies to neutralize the anthrax toxin and that could therefore be predicted to offer protection against anthrax. Today, anthrax is rarely encountered in the United States, since the introduction of vaccines for cattle in the 1930s. A human vaccine was licensed in 1970. Vaccination against anthrax has been confined to people at risk, such as wool sorters and some veterinarians. However, the rising prospects of B. anthracis being used as a weapon have led to routine administration of the anthrax vaccine to members of the armed forces. 