The clinical pattern of acquired immunity to malaria varies widely in different parasite host species. In some instances there is no effective immune response and the disease is rapidly fatal, e.g., Plasmodium knowlesi in the rhesus monkey. More commonly, however, induced immunity controls but does not eliminate the infection, which persists at low density over long periods—a sequence referred to as ‘premunition,’ e.g., P. falciparum in man. Finally, malaria may result in clinical cure, complete elimination of the parasite (sterilizing immunity) and life-long resistance to challenge, e.g., P. berghei in the rat. A given species of Plasmodium does not induce comparable immunity in different hosts, e.g., P. knowlesi produces a rapidly fatal infection in the rhesus monkey, Macaca mulatta, but mild intermittent parasitemia in the kra monkey, M. fascicularis. In addition, immunity may be age-dependent, as in the rat where the diminishing intensity of P. berghei infections is associated with altered immune responsiveness.