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 of T. 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. Relapse infections occurred in all the groups given the drug s/c whilst all the animals treated i/m remained parasitologically negative up to 80 days after treatment. Results from serological assays, however, suggested the possible persistence of trypanosome infection in the animals treated at a dose rate of 0.50 mg/kg i/m although trypanosomes could not be demonstrated parasitologically. A dose rate of 0.75 mg/kg administered i/m is recommended, therefore, for the treatment of T. evansi infection in Friesian Holstein cattle.