Trichomonas vaginalis, Tritrichomonas foetus and Trichomitus batrachorum grown in modified Diamond's medium all had high concentrations of putrescine and lower concentrations of spermidine and spermine. Ornithine decarboxylase (ODC; EC 4.1.1.17) was detectable in all three species although at significantly different levels. Trichomonas vaginalis had the highest activity (typically around 1.85 nmol min-1 (mg protein)-1), Trichomitus batrachorum the lowest (0.11 nmol min-1 (mg protein)-1). The Trichomonas vaginalis ODC had an apparent Mr of 230 000 and was severely inhibited by alpha-difluoromethylornithine (DFMO). S-Adenosyl-methionine decarboxylase (EC 4.1.1.50) could not be detected in T. batrachorum but was present in the other two species. Arginine decarboxylase was apparently absent from all three. All three trichomonad species were able to accumulate spermidine and putrescine from the medium. When T. vaginalis was grown in the presence of DFMO (4 mM), which had little effect on parasite growth, ODC activity was reduced by over 99% and the polyamine content was altered; putrescine concentrations were decreased, those of spermidine and spermine remained the same or were raised. DFMO-treated cells accumulated more exogenous putrescine than untreated control cells. The results suggest that the lack of effect of DFMO on T. vaginalis in culture was due to the parasite being able to accumulate polyamines from the growth medium. It appears, therefore, that testing DFMO and similar compounds in axenic trichomonad cultures may well not give a true indication of their effectiveness in vivo where sources of exogenous polyamines may not be available.