The immunomodulating effects of Anapsos, an aqueous hydrosoluble extract obtained from the rhizomes of the fern Polypodium leucotomos, on both pathogenicity and cytokine levels in serum (IFN-gamma/IL-4) were assayed in a Trichomonas vaginalis experimental model (BALB/c mice infected with 10(7) trichomonads and examined at day 15 after infection). Doses of 20 mg/kg/day administered for 10 days before the infection with the parasite induced a decrease of the experimental pathogenicity approximately 10-20% compared to controls. Gross histopathologic changes at abdominal organs and mortality rate, as a consequence of pathogenicity of the protozoa and the immune response of the host, were evaluated. IFN-gamma and IL-4 cytokines were determined on days -5, 0, 5, 10, and 15 postinfection by indirect ELISA. Treatment with PAL before infection modulates and downregulates the IFN-gamma concentration, while anticipates and upregulates the IL-4 level. The assays performed have showed the utility of the murine model of experimental trichomoniasis for the evaluation of immunomodulatory activity of synthetic or natural products.