A specific agglutination of Plasmodium knowlesi detectable both by macroscopic and by microscopic methods is described. Agglutinins for Plasmodium knowlesi appear in the sera of monkeys between 15 and 45 days after the onset of the infection and become progressively stronger as the malarial infection gradually subsides. Agglutinins persist in the sera of chronically infected animals for a year or longer. The sera of animals which have been repeatedly superinfected agglutinate parasites at dilutions as high as 1:1,000. Sera from normal monkeys, from monkeys acutely ill with malaria, and from monkeys chronically infected with a different species of malarial parasite (Plasmodium inui) do not agglutinate Plasmodium knowlesi. Immune serum agglutinates mature intracellular or extracellular parasites but does not agglutinate unparasitized cells or cells containing immature parasites. The relation of these observations to the mechanism of active and passive immunity in monkey malaria is discussed.