Salinomycin, administered prophylactically, was effective in controlling clinical sarcocystosis resulting from infection with Sarcocystis ovicanis in two experiments involving a total of 50 lambs. In each experiment, five lambs were used in each of five test groups--uninoculated and unmedicated, inoculated and unmedicated, uninoculated and medicated (2 mg/kg), inoculated and medicated (2 mg/kg), and inoculated and medicated (1 mg/kg). Salinomycin was included in the feed of each medicated group for 30 days beginning on the day of oral inoculation with 10(6) S. ovicanis sporocysts. Lambs were challenged with 10(6) sporocysts 9 wk PI and the experiment was terminated 7 wk later. Data from deaths, weight gains, body temperatures, packed cell volumes, hemoglobin values, serum protein levels, and postmortem and histological examinations indicated that salinomycin at both doses tested reduced the number of deaths and severity of signs of sarcocystosis in infected lambs as compared with unmedicated controls. Infected, medicated lambs developed a protective immunity as determined by challenge inoculation. An additional group of five lambs, each inoculated orally with 10(6) sporocysts, were treated therapeutically with 2 mg/kg per day beginning 21 days after inoculation. All five died from acute sarcocystosis.