Studies of anti-Trichomonas vaginalis antibodies in patients with vaginal trichomoniasis were undertaken in attempts to identify the predominant antibody isotype produced and to delineate clinically significant antigens. The total antibody content of serum samples from 23 patients was determined by an enzyme-linked immunosorbent assay (ELISA) that employed anti-human immunoglobulin and isotype-specific antisera. The immunochemical reactivity of these antibodies was examined by Western blot analysis. The anti-T. vaginalis titer of all but two of these serum samples was greater than 200 (range, greater than 200 to 12,800). By using an ELISA titer of at least 200 as a criterion, 21 of the serum samples contained antibodies of the immunoglobulin G (IgG) isotype, 17 contained IgM antibodies, and 6 contained IgA antibodies directed to the protozoan. Western blot analyses of these serum samples revealed approximately 29 antigenic trichomonad polypeptides, with apparent molecular sizes ranging from 14 to greater than 100 kilodaltons and with individual serum samples possessing different patterns of reactivity. These results add to the current understanding of the serological and secretory immune responses to T. vaginalis, as well as define potential antigens for use in immunodiagnostics.