CASEASE, the proteolytic, casein-hydrolysing enzyme, has been reported in many bacteria. It has not hitherto been detected in the protozoan, Entamoeba histolytica. In earlier studies, Nakamura1 reported the absence of casease in certain strains of the amœbae. It was thought that the failure to demonstrate the enzyme might have been due to the comparatively large amounts of casein in the assay plates. Therefore, the experiments were repeated using a modification of the same method but incorporating smaller concentrations of casein in the test plates.