alpha-Difluoromethylornithine, an inhibitor of polyamine biosynthesis, affected the pathogenicity of Trichomonas vaginalis in two model systems. Firstly, it blocked the cytotoxic effect of the parasite towards mammalian cells in culture: at a concentration of 1 mM it prevented the death of mouse myeloma cells in mixed culture with trichomonads. Secondly, when administered orally to mice (multiple doses of 750 mg/kg body wt), the drug delayed the development of subcutaneous abscesses due to T. vaginalis infection. The same drug dose did not cure intravaginal infections by the parasite. The implications of these findings with respect to the value of the model systems used and the potential of DFMO as an antitrichomonal drug are discussed.