In this trial suramin, diethylcarbamazine, trichlorphon, levamisole, mebendazole, melarsonyl potassiu, Hoechst 33258 and tinidazole were administered to cattle infected with O. gibsoni and O. gutturosa to determine the usefulness of this screen in predicting the effect of drugs in man against. O. volvulus except for melarsonyl potassium which was macrofilarticidal against O. gutturosa but not O. gibsoni when cattle were slaughtered 6 weeks after treatment. It was concluded that cattle infected with O. gibsoni are a satisfactory substitute for chimpanzees infected with O. volvulus, as a tertiary screen for drugs against O. volvulus, but that their use would be restricted to centres in O. gibsoni endemic areas where the necessary facilities and specialised knowledge required to use cattle as experimental animals exist.