Protein antigens of Treponema pallidum precipitated by immunoglobulin M (IgM) and IgG antibodies of sera from patients with untreated primary and secondary syphilis as well as treated secondary syphilis were characterized on a molecular basis. T. pallidum was labeled internally with [35S]methionine and solubilized in 0.1% sodium dodecyl sulfate-1% Triton X-100. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis on 12.5% gels followed by autoradiography revealed 32 distinct proteins with molecular weights between 13,500 and 200,000. Twenty-three proteins of T. pallidum with molecular weights between 15,500 and 115,000 were identified as antigens by double antibody radioimmunoprecipitation with IgM and IgG antibodies of sera from syphilitic patients. The molecular analysis of the IgM and IgG immune response to T. pallidum in human syphilis is in accord with earlier immunological observations. Finally, utilizing syphilitic human sera, we characterized 15 protein antigens of T. pallidum that are common to Treponema phagedenis by partial absorption of IgM and IgG antibodies with an ultrasonicate of T. phagedenis.