Sporulated and unsporulated oocysts of Eimeria bovis were irradiated with 10,000, 50,000, 75,000, 100,000, or 200,000 rads of gamma rays from Cobalt-60 to determine how the oocysts were affected by radiation. In two experiments the oocysts were irradiated after sporulation and in two experiments they were irradiated before sporulation. Nonirradiated oocysts were used as controls. The ability of sporulated oocysts to cause coccidiosis in young Holstein-Friesian calves was used to measure the effect of irradiation. Sporulated oocysts irradiated at 10,000 rads caused infection similar to the controls while those irradiated with 50,000 rads caused only mild infections in a few calves. Spor- ulated oocysts irradiated with 75,000 rads or more did not cause coccidiosis. When previously inoc- ulated calves were challenged with nonirradiated oocysts only those that had infections caused by oocysts irradiated with 10,000 rads were resistant to reinfection. This indicates that it was necessary for infections to occur before immunity could be developed. Calves given sporulated oocysts that had been irradiated in the unsporulated condition at the levels stated above all became infected although calves receiving oocysts irradiated at 200,000 rads were not affected as severely as were those receiving oocysts irradiated at lower levels. Calves recovering from these infections were resistant to challenge inoculations. Unsporulated oocysts withstood much greater amounts of ionizing radiation with retention of viability, than did the sporulated oocysts. In vitro studies of the rates of excystation indicated that irradiation did not adversely affect, but may have enhanced excystation. Irradiation did not appear to be advantageous in attenuating Eimeria bovis for use in immunizing calves.