Chickens given 200 oocysts of Eimeria tenella at day-old followed by a dose of between 300 to 500 oocysts at 8 days of age were afforded substantial protection against challenge at 15 and 22 days of age. Chickens given 2,000 oocysts of E. acervulina at day-old were partially protected against challenge infection given at 15 or 22 days of age. When chickens were given doses of 2,000 and 10,000 oocysts at 1 and 8 days, respectively, significant protection against challenge at 15 and 22 days was obtained. Chickens given five oocysts of E. maxima at day-old were partially protected against challenge at 15 or 21 days of age. A dose of 50 oocysts at day-old gave substantial protection judged by body weight changes and lesion scores. The protection was slightly greater when the immunising dose was given at 8 days of age. The results indicate that with chickens kept on wire floors where the conditions for reinfection were minimal, substantial immunity to challenge infection could be achieved by giving small numbers of oocysts to chickens 1 to 8 days of age.
