Thirty-six Trypanosoma brucei spp. stocks isolated from man and domestic animals in south-east Uganda were studied for susceptibility to diminazene, isometamidium and melarsoprol in vitro. All stocks were susceptible to melarsoprol. One T.b. rhodesiense stock isolated from a sleeping sickness patient showed reduced susceptibility to the veterinary drugs diminazene and isometamidium. More than 100 ng/ml diminazene or 0.78 ng/ml isometamidium were required to eliminate that stock during 10 days drug exposure. In contrast, the remaining stocks were eliminated by 0.8-6.3 ng/ml diminazene and 0.01-0.20 ng/ml isometamidium. Clones derived from the resistant T. b. rhodesiense stock showed reduced susceptibility to isometamidium and diminazene comparable to the parental population. Control of sleeping sickness by treatment of the animal reservoir could, therefore, face serious problems since drug-resistant stocks as reported here would most likely not be eliminated by recommended doses of diminazene and isometamidium.