The life-cycle of a precocious and attenuated line (WisF96) of Eimeria tenella, derived from the Wisconsin (Wis) strain, contained only the first of the three generations of schizogony undergone by the parent strain. The reproductive capacity of WisF96 was less than that of the parent strain by a factor of about 2000, but inoculation of chickens on two occasions with a large number of its oocysts induced resistance against challenge with oocysts, or with second-generation merozoites of Wis. The immunizing abilities of the attenuated line and its parent were compared by priming groups of chickens with numbers of oocysts of WisF96 or Wis, designed to produce infections of equal magnitude in terms of oocysts production (standard inocula), and then challenging with oocysts of Wis. The results suggested that an equivalent parasite mass of WisF96 was more immunogenic than Wis. This was confirmed by the finding that, even when the priming dose of Wis was increased 10-fold, the standard inoculum of WisF96 still produced the greater protective effect. These results indicate that the first generation of schizogony of E. tenella is highly immunogenic.