Summary: Anthrax is a deadly disease caused by the Gram-positive bacterium Bacillus anthracis. The organism infects humans and many other animals. The primary virulence factors are thought to be anthrax toxin and a glutamic acid capsule, both of which are encoded on large plasmids, although other important virulence factors may yet be discovered. Inhalation anthrax, the most severe form of the disease, results in a systemic infection in which the organism spreads to the lymph nodes and then into the blood where it is able to replicate at very high levels. The dissemination of anthrax spores into the air, which results in inhalation anthrax, is considered to be the most likely method by which this organism would be used as a biowarfare agent. The current vaccine for anthrax is thought to confer protection due to antibodies elicited by protective antigen. Future vaccines are likely to be based on this protein. Protective antigen by itself is non-toxic, however when it associates with either EF or LF, the complex is toxic to cells. We are initiating investigations to examine the antigenicity and immunogenicity of PA, an important vaccine candidate for anthrax vaccines. PA can be divided into four domains. We have expressed each of these domains individually and have purified the domains. These proteins were injected into mice to induce antibodies. We are now examining the ability of the antibodies to neutralize the action of anthrax toxin. In this manner, we will determine which portions of PA elicit neutralizing antibodies to anthrax toxin.