The maximum circulation time, as well as the rate of clearance, of viable, infective, Plasmodium berghei sporozoites was determined after intravenous injection into immunized and normal mice. In animals actively immunized by the previous injection of X-irradiated sporozoites, the circulation time of infective sporozoites was considerably reduced as a consequence of an increased clearance rate of the parasites. The transfer of large volumes of immune serum into normal mice also resulted in a considerably increased disappearance rate of infective sporozoites from their peripheral blood. Exo-erythrocytic forms (EEF) of the parasite were either not detectable or greatly reduced in number in the livers of both groups of actively and passively immunized animals, as compared to controls. However, in spite of the apparent absence of EEF in some of these animals and the reduced number of EEF in all the others, protection was never complete in the passively immunized animals, all of them developing parasitemia after a prolonged prepatent period. This differed markedly from the results obtained in the actively immunized animals, the majority of which was totally protected. Together with other experimental evidence, this seems to indicate that in addition to the important role of immune serum, another, as yet undetermined mechanism might be necessary to bring about complete protection to malaria.