The development of an assay to measure the sensitivity of drugs against Blastocystis hominis using the incorporation of 3H-hypoxanthine is described. The activity of 42 compounds have been measured. Four of the 5-nitroimidazoles tested (satranida-zole, S75 0400 A, flunidazole and ronidazole) were found to be more active than metronidazole, the drug commonly used to treat infections caused by B. hominis in humans. Other potentially useful compounds include emetine, furazolidone and quinacrine. Ketoconazole and iodoquinol reported to have therapeutic activity in infections caused by this parasite were found to be significantly less active than metronidazole.