Summary Based on experimental evidence the following fundamental characteristics of sporozoite-induced immunity have, so far, been established. 1. The sporozoite stage of malaria parasites is strongly immunogenic. The intravenous injection of small numbers of attenuated, X-irradiated sporozoites induces a high degree of protective immunity and formation of anti-sporozoite antibodies. 2. Sporozoite-induced immunity is highly stage-specific. Its effects are mainly directed against the sporozoite stage. The erythrocytic stages of the parasite are not affected by this immune mechanism. 3. Considerable cross-protection and serologic cross-reactivity occur among sporozoites of different rodent malaria strains and species. 4. Sporozoites undergo antigenic changes during their process of maturation. These might possibly be correlated with the development of sporozoite infectivity. 5. The spleen does not play an essential role in sporozoite-induced protective immunity. The immunization of splenectomized animals renders them resistant to sporozoite challenge in the absence of antibodies detectable by the CSP reaction. 6. Sporozoite-induced immunity is, at least in part, serum-mediated. The incubation of sporozoites with immune serum considerably reduces their infectivity. The passive transfer of immune serum considerably increases the rate of sporozite clearance observed in normal animals. 7. The non-specific stimulation of the reticulo-endothelial system which occurs during sporozoite immunization is very shortlived. This reticulo-endothelial system stimulation does not play an important role in protection against sporozoite challenge.