Substantially purified Eimeria maxima gametocytes have been prepared and methods for their preparation are disclosed. A proteinaceous extract derived from the gametocytes comprises at least nine proteins of vary­ing molecular weights. Vaccines for conferring upon a chicken active immunity against infection comprise an effective immunizing amount of Eimeria maxima gameto­cytes or gametocyte extract and a carrier. Also pro­vided is a method if in vitro development of Eimeria spp. gametocytes or oocysts and a method of assaying drug or anti-gametocyte antibody effectivity on Eimeria spp. development in vitro .