The protective antigen (PA) gene from Bacillus anthracis has been expressed in Salmonella typhimurium SL 3261 (aroA). Expression was achieved by cloning the gene after the plac promoter in a high copy number plasmid. The recombinant PA was exported into the periplasm. This construct was unstable in vivo and also reduced the colonization ability of the host S. typhimurium. Mouse-passaging of the recombinant Salmonella resulted in a strain with enhanced colonization ability and increased stability of the plasmid in vivo. This effect appeared to be due to a reduction in copy number of the PA-encoding plasmid. Mice were vaccinated with recombinant S. typhimurium and adjuvanted PA and challenged with virulent B. anthracis. Only mice vaccinated with adjuvanted PA or orally with the mouse-passaged recombinant showed partial protection. The degree of protection observed after oral vaccination with the recombinant S. typhimurium was similar to the degree of protection afforded by adjuvanted PA and suggested that the use of S. typhimurium to deliver PA is an effective approach for inducing protection against B. anthracis. The results presented also suggest that the degree of protection demonstrated in the mouse may not fully indicate the potential of the recombinant Salmonella as an effective vaccine in other species.