The effect of protease inhibitors obtained from the culture filtrates of actinomycetes (pepstatin, chymostatin, leupeptin, phosphoramidon and elastatinal) on the in vitro invasion of erythrocytes from rhesus and assamese monkeys by Plasmodium knowlesi merozoites was studied. The presence of these inhibitors had no effect on release of merozoites from schizonts but inhibited entry of the parasite into the red cell. Thus, at 50 micrograms/ml, chymostatin and leupeptin completely blocked the invasion whereas pepstatin and elastatinal showed 50% inhibition. Phosphoramidon at this concentration gave only 30% inhibition. Pretreatment of the erythrocytes with these inhibitors did not block invasion. Also, the intracellular development of the parasite was totally unaffected in the presence of these agents. These results suggested that proteases of merozoites might play some crucial role in the invasion process.