Hybridomas producing monoclonal antibodies (mAbs) directed against Trichomonas vaginalis (Tv) have been produced in three fusions using mice immunized with live or killed Tv. The ELISA technique was used to test the binding activity of six out of the 48 mAbs produced. It was found that acetone fixation enhanced the binding activity of the antibodies and revealed hidden antigenic determinants. Thirty percent of the mAbs obtained from splenocytes of mice immunized with live Tv were of the IgG3 subtype. Two mAbs of the IgM and IgG3 subtypes demonstrated complement-fixing capacity. Incubation of these mAbs with live Tv and complement at 37 degrees for 30 min lysed the parasites. The lytic process was complement-dependent since in its absence the antibodies only agglutinated the parasites. The mAbs, when partially purified from ascitic fluids, had the same lytic activity as the native preparation. MAbs of the IgG1 subtype which did not fix complement, bound to Tv but did not lyse it. The lytic activity of the mAbs was not inhibited by cervico-vaginal secretions obtained from 14 women. It is suggested that mAbs could be used for diagnostic as well as for therapeutic purposes.