An increasing interest in the association of the presence of antibodies to Toxoplasma gondii and the development of schizophrenia in patients has been generated over the last several years. Some antischizophrenia agents have been shown to have activity against T gondii in cell culture assays and to ameliorate behavioral changes associated with chronic T gondii infection in rats. In the present study, we examined the effects of commonly used antipsychotic and mood stabilizing agents (haloperidol, clozapine, fluphenazine, trifluoperazine, and thioridazine) for activity against developing tachyzoites of the RH strain of T. gondii in human fibroblast cell cultures. Neither haloperidol nor clozapine had a measurable effect. Fluphenazine had an IC(50) of 1.7 mu M, thioridazine had an IC(50) of 1.2 mu M, and trifluoperazine had an IC(50) of 3.8 mu M. Our study demonstrates that some agents used to treat schizophrenia have the ability to inhibit T. gondii proliferation in cell culture.