Two studies on the efficacy of mel Cy (Cymelarsan, Rhone Merieux, France) for the treatment of cattle infected with Trypanosoma evansi were carried out with groups of 5 Friesian Holstein calves infected with an Indonesian stock ofT. evansi and treated 14 days after infection. In the first study 3 groups were injected subcutaneously with Cymelarsan at dose rates of 0·125, 0·25 and 0·50 mg/kg and in the second study 2 i/m at 0·50 and 0·75 mg/kg. The response to treatment was monitored parasitologically by daily microhaematocrit centrifugation technique and weekly mouse inoculation. Enzyme-linked immunosorbent assays were used to monitor trypanosomal antibodies and trypanosomal antigens in serum samples collected weekly.