Eighteen Holstein-Friesian four week old calves were inoculated with 5 X 10(5) sporocysts of Eimeria bovis by stomach tube. The calves were divided into three equal groups. The first group was treated with monensin, 1 mg kg-1 body weight daily, from the 10th-20th day inclusive after infection; the second group was treated with amprolium, 10 mg kg-1 body weight daily, for the same period of time and the third group acted as infected controls. Both drugs were for the most part effective in preventing clinical signs, in suppressing reduced rates of weight and in reducing oocyst production. The calves of all three groups were resistant to the second infection given 35 days after the first infection with E. bovis as measured by rates of weight gain, fecal oocyst output and lack of clinical signs.