We isolated and characterized an immunogenic protein of an oral spirochete, Treponema denticola Johnson. A genomic DNA library constructed with bacteriophage lambda EMBL3 as a vector was immunologically screened with a rabbit antiserum against the whole cells. Using Western immunoblot analysis, we found a particular clone encoding an antigen with a molecular weight of 53,000; we designated the antigen as T. denticola protein A (TdpA). Complete sequence determination revealed an open reading frame of 1,419 bp and a signal peptide sequence that was homologous to that of bacterial lipoprotein. Southern hybridization analysis revealed that the tdpA gene is highly conserved in six tested strains of T. denticola species. Furthermore, we found that sera from some periodontitis patients contained antibody against the TdpA protein, although the reactivities of the antibodies varied among individuals.