Bacillus anthracis is a facultative intracellular bacterial pathogen that can cause cutaneous, gastrointestinal or respiratory disease in many vertebrates, including humans. Commercially available anthrax vaccines for immunization of humans are of limited duration and do not protect against the respiratory form of the disease. Brucella abortus is a facultative intracellular bacterium that causes chronic infection in animals and humans. As with other intracellular pathogens, cell mediated immune responses (CMI) are crucial in affording protection against brucellosis. B. abortus strain RB51 has been shown to be useful in eliciting protective cell mediated immunity and humoral responses against Brucella in cattle and other animal species. Since the protective antigen (PA) of B. anthracis is known to induce protective antibodies, it was decided that the objective of this research was to test whether the gene encoding PA could be expressed in Brucella producing a bivalent vaccine to protect against both brucellosis and anthrax. The pag gene was transcriptionally fused to promoters of genes encoding superoxide dismutase or heat shock protein groE, subcloned into a broad host range plasmid (pBBR1MCS) and shown to express in E. coli by immunoblotting using antiserum specific for PA. The immunoblot results revealed that E. coli produced a PA protein of the expected size. In addition, the culture medium was shown to contain the same PA protein using immunoblotting. These results show that E. coli is capable of expressing B. anthracis PA in both the cellular and extracellular forms. The pBB/PA plasmid was used to transform B. abortus RB51 and CmR clones screened for the expression of PA by immunoblotting. Twenty clones of strain RB51/pBBSOD were show to express a 30kDa PA protein. Three clones of strain RB51/pBBGroE-PA were shown to express a 63-83kDa protein as detected by antiserum specific for PA. Using the A/J mouse, an immunocompromised vertebrate model, immunization and challenge studies were performed. Preliminary results demonstrate that the bivalent vaccine is capable of producing protection against a live challenge with B. abortus and some protection against live non-disease producing spores of B. anthracis.