Calves experimentally infected with Eimeria bovis and E zuernii were used in a controlled experiment to determine the anticoccidial activity of lasalocid sodium. Eleven-week-old Holstein calves were given an inoculum of 300,000 E bovis and 200,000 E zuernii oocysts; medication was initiated on the day of inoculation and continued for a 4-week period. The progress of parasitic infection was monitored with quantitative fecal oocyst examinations for the 6 weeks before calves were inoculated, and then in the 4-week treatment period and a 3-week observation period. The calves were given different doses of lasalocid sodium (0.5, 0.75, 1.0, and 3.0 mg/kg) and were compared with both nonmedicated inoculated calves and controls (nonmedicated, noninoculated calves). There were overall numerical reductions of oocysts produced in the medicated groups when compared with the nonmedicated inoculated controls.