The immunogenic Plasmodium knowlesi (H strain) Mr 74,000 protein in membranes of schizont-infected rhesus erythrocytes was purified on a large scale, free of other polypeptides as monitored by dodecyl sulfate polyacrylamide gel electrophoresis and isoelectric focusing. In a limited vaccination trial, four rhesus monkeys were immunized four consecutive times with the Mr 74,000 protein and Freund's complete and incomplete adjuvants. Two monkeys were injected with adjuvant only. Upon challenge with 10(4) viable P. knowlesi schizonts of the heterologous W strain, the control monkeys developed fatal parasitemias after 7 d. In contrast, the vaccinated monkeys exhibited a delayed onset of patent parasitemias and underwent self-cure on days 14 to 16 after peak parasitemias of between 7 and 11%. The protective immunity that was induced crossed different strains of P. knowlesi. Blood smears at the time of cure demonstrated limited reinfection, as indicated by the presence of normally appearing ring and trophozoite stages. The absence of schizont stages in the peripheral blood suggested a specific interruption of the erythrocytic schizogony at that stage. Immunochemical analyses of the rhesus sera revealed antibody only against the Mr 74,000 protein after the first two immunizations. Upon repeated antigen injection, antibodies reacted with components of Mr of approximately 102,000, 140,000, and 230,000 in addition to the Mr 74,000 protein. Besides immunological cross-reactivity, relatedness between all four immune-precipitated proteins was indicated by a greater than 50% tryptic peptide homology, suggesting that the Mr 230,000 component represents a precursor protein that is cleaved within the infected erythrocyte into proteins with Mr of approximately 140,000, 102,000, and 74,000.