A simple method to screen trichomonacides, based on the quantification of acid phosphatase (AP) activity, has been designed. Using p-nitrophenyl phosphate as chromogenic substrate, we first determined the optimal conditions for enzyme reaction. After seeding, a linear correlation between number of trichomonads and optical densities at 405 nm was obtained at 24 hr but not at 48 hr. Then, the inhibitory effect of metronidazole was assessed both by microscope counts and by AP determination. Similar values for 50% inhibitory concentrations (2.6 μM), with 95% confidence limits of 1.91–3.33 for microscopic and 2.21–3.05 for colorimetric method, were obtained. We concluded that the colorimetric method described in this investigation is suitable for pharmacological studies and for the screening of new, potential antitrichomonal agents.