Sotiriades, in 1917, reported clinical improvement in the condition of a patient with acute malaria following the inoculation of 10 ml of serum obtained from a “chronic” case. Kauders confirmed this finding by observing that beneficial effects resulted in 9 of 12 patients who received small quantities of serum from a person difficult to infect with malaria. While these reports suggested that the passive transfer of malarial immunity was feasible the first convincing evidence was adduced by Coggeshall and Kumm. These authors showed that protection against Plasmodium knowlesi could be conferred upon non-immune rhesus monkeys through serum obtained from monkeys previously immunized against this parasite. Although the degree of protection achieved was variable, best results being obtained when parasitic inocula were small, this study clearly indicated the existence of protective antibodies in the blood of monkeys with established malarial infections.