The efficacy of amprolium against clinical sarcocystosis resulting from Sarcocystis ovicanis was determined in two experiments involving 40 lambs. In each experiment, four lambs were used in each of five test groups--uninoculated unmedicated, inoculated unmedicated, uninoculated medicated (100 mg/kg), inoculated medicated (100 mg/kg), and inoculated medicated (50 mg/kg). Amprolium was provided as a feed additive to each medicated group for 31 days beginning 1 day before oral inoculation with 100,000 S. ovicanis sporocysts. Data from deaths, body temperatures, weight gains, serum protein levels, hematocrits, hemoglobin values, LDH and SGOT values, postmortem examinations, and histological examinations indicated that amprolium at both levels tested reduced the number of deaths and severity of clinical signs of sarcocystosis in experimentally infected lambs as compared with unmedicated controls.