Cell extracts possessing considerable proteolytic activity have been obtained from human leukocytes by ultrasonic breakdown or by treatment with surface active agents. Three proteolytic enzymes are present; one with an optimum at approximately pH 8 is inhibited by organophosphorus compounds and resembles chymotrypsin. This enzyme is derived from polymorphonuclear leukocytes. Lymphocytes contain two proteases with pH optima of 3.0 and 5.5, respectively.