Ten congenic strains of mice were immunized with irradiated sporozoites of Plasmodium yoelii. When challenged with viable sporozoites, only two strains had a high proportion of animals which did not develop blood-stage infections. Genes both within and outside the H-2 region affected the degree of protection. Immunity did not correlate with anti-sporozoite antibody levels. In vivo depletion of CD8+ T cells did not alter immunity in two of three congenic strains, implying the existence of a novel mechanism of cell-mediated immunity.