Summary: Serum from rats actively immune to Plasmodium berghei can inhibit the development of parasitemia and reduce morbidity to a marked extent when administered to normal recipients at the time of challenge. Smaller doses of potent serum, equivalent to less than 0.05% of the recipient's plasma volume, can also reduce parasitemia to a statistically significant extent. The protective activity of hyperimmune serum, which can be measured by a simple passive transfer assay, is primarily associated with the 7 S immunoglobulins. It persists in the circulation for more than 6 months after infection and rapidly increases on restimulation with the infectious agent.