Identification of surface macromolecules of Porphyromonas gingivalis that act as virulence factors in periodontal disease has important implications for studying host-parasite interactions as well as for potential vaccine development. The objective of this study was to determine whether a cloned, P. gingivalis hemagglutinin gene could be expressed in an intact form in an avirulent Salmonella typhimurium vaccine construct and to characterize the recombinant protein. The recombinant protein was purified from the vaccine strain, characterized, and tested for biological activity as a competitive inhibitor of hemagglutination. Cells of S. typhimurium SL3261/pST7 grown in Luria broth were broken by sonic disruption and fractionated. The purified recombinant protein was found to inhibit hemagglutination of erythrocytes by whole P. gingivalis cells. The same purified protein was analyzed for its N-terminal amino acid sequence and amino acid composition and found to match that predicted from the nucleotide sequence of the cloned gene. These results indicate that a surface macromolecule of P. gingivalis can be expressed in an intact and biologically active form in a Salmonella carrier strain.