Irradiation of sporulated oocysts of intestinal Eimeria reduce their pathogenicity for two-weeks old chickens as determined by weight gains, lesion score and the number of oocysts discharged in droppings. During seven days of infection with 105 irradiated sporulated oocysts, the mean weight gains were increased proportionally with the dose of irradiation. The
number of discharged oocysts (6-14 days) post infection as well as lesion score (7th day post infection) was decreased by increasing the dose of irradiation. Repeated inoculation (immunization) of two weeks old chicks with graded doses of irradiated oocysts (18 and 22 K Rad) partially protected chickens against challenge with non-irradiated viable oocysts of intestinal Eimeria. After challenge, the mean weight gain of the immunized challenged chickens was significantly higher than unimmunized-challenged group. The lesion scores of the immunized-challenged groups were low compared with the control unimmunized-challenged group.