Twenty vaginal washes (VWs) and ten vaginal mucus (VM) samples from patients with trichomoniasis were examined for the presence of antibody to surface protein immunogens of Trichomonas vaginalis. Fourteen of 20 VWs (70%) and 8 of 10 VM (80%) had immunoglobulin G (IgG) antibody (Ab) that reacted in an immunoprecipitation (IP) assay with one iodinated Trichomonas vaginalis surface protein immunogen with a relative molecular mass of 230,000 daltons (230-kDa) (P230). No similar IP of any iodinated protein was observed when detergent extract was first depleted of P230 with monoclonal antibody (MAb), indicating a highly specific VW IgG response of patients to P230. VWs were also obtained from 10 patients from one to four weeks after treatment. These VWs had the same, or in one case a greater, level of IgG to P230. Under no circumstances was Ab to P230 or any other trichomonad protein detected in VWs or VM from normal, uninfected women. Flow cytofluorometry with VW Ab yielded heterogeneous fluorescent and non-fluorescent populations of trichomonads, reaffirming the restricted Ab response to one or a few epitopes on P230 in the vagina of patients. Under identical conditions, the MAb gave totally fluorescent parasite populations of some isolates, and the MAb again demonstrated variable epitope accessibility to Ab binding (Infect Immun 1987;55:1037). Finally, the MAb or VW Ab was never cytolytic for immunoreactive (fluorescent) parasites, even in the presence of complement. This study identifies the most important trichomonad surface immunogen on the basis of the vaginal Ab response, and data underscore the significance of immune evasion strategies of this sexually transmitted disease agent.