Lambs fed monensin mixed in a complete fattening ration at concentrations of 5, 10, or 20 ppm were protected against death, impaired body weight gain, and diarrhea due to infections of Eimeria ninakohlyakimovae and E ahsata. Monensin given at the concentration of 10 ppm reduced oocyst passage and, at the concentration of 20 ppm, almost completely controlled oocyst passage. Protection was afforded by monensin given at the concentration of 5 ppm, although oocyst passage was excessive and body weight gains were suboptimal. Infected control lambs were severely affected, with mortality rate of 30% to 50% on days 14 to 18 and oocyst passage averaging 286,000,000 oocyst/lamb, Oocyst production peaks on days 12 to 15 and 21 to 27 correlated with the appearance of oocysts of E ninakohlyakimovae and E ahsata, respectively. Body weight gains and feed consumption of lambs fed monensin at the concentration of 10 or 20 ppm indicated that the drug was well tolerated at these dose levels and this route of administration. The compound was efficacious in Dorset and Suffolk X Hampshire lambs, and caused no problems when fed to pregnant ewes.