Outer membrane (OM) antigens of several oral treponemes were studied. SDS-PAGE revealed a 56-58-kDa protein as a major component of isolated OM vesicles. Immunoblot analysis with eluted antibodies prepared from Treponema denticola ATCC 33520 recognised the 56-58-kDa protein, which was highly conserved in the 17 T. denticola strains tested. The protein was also a component of a T. denticola desoxycholate-extractable, ethanol-soluble antigen (DES-Ag) but was not present in T. pectinovorum or T. vincentii ATCC 35580 and strain N9 whole-cell lysates. Electronmicroscopy of OM vesicles showed typical treponemal ultrastructure. Immunogold labelling of T. denticola ATCC 33520 with T. denticola ATCC 33520-specific eluted antibodies recognised only the OM surface of the cell. These results suggest that the 56-58-kDa antigen comprises surface-orientated epitopes and that this antigen may be specific for T. denticola.