Control of coccidiosis by vaccination or by medication with anti-coccidial drugs were compared on a commercial broiler farm. An attenuated, embryo-adapted line of Eimeria tenella, a virulent laboratory strain of E. acervulina and a precocious (attenuated) line of E. maxima, used for vaccination, were given in three combinations: E. tenella alone, E. tenella plus E. acervulina or E. tenella together with E. acervulina and E. maxima. Oocysts were given via feed when chickens were 2 to 8 days old. In four trials the performance of 8,416 chickens vaccinated with E. tenella alone or in combination with E. acervulina was as that of chickens given anticoccidial drugs. The performance of 3,601 chickens vaccinated with E. tenella, E. acervulina and E. maxima was not better than that of chickens vaccinated with E. tenella and E. acervulina only. The performance of 61,772 chickens vaccinated with E. tenella and E. acervulina in four large scale crops was similar to that of the control chickens and that of the previous six crops, which had been medicated with anticoccidial drugs.