The ability of human leukocyte enzymes to hydrolyze phosphorus is compared in terms of the conventional substrate sodium β-glycerophosphate and the metabolically important phosphate esters, adenosine 5'-phosphate and glucose 1-phosphate. At pH 9.9, there is marked and comparable variation in phosphatase activity toward all three substrates, this being low in chronic myelocytic leukemia and high in the presence of infection and certain "stressful" states. Moreover, substrate mixture experiments show no increased hydrolysis of phosphorus when two substrates are present in the incubation mixture. Increased phosphatase activity toward both glucose 1-phosphate and sodium β-glycerophosphate resulted when corticosteroids were administered in large doses for 72 hours. The data, while not providing absolute proof, are compatible with the hydrolysis of phosphorus at pH 9.9, being due in the case of all three substrates to the activity of the same phosphomonoesterase or group of phosphomonesterases. At pH 5.5, phosphatase activity toward both sodium β-glycerophosphate and adenosine 5'-phosphate was likewise demonstrated, but, in leukocytes, the pH of maximal activity varies from subject to subject and is dependent to a large extent on the amount of the highly variable "alkaline phosphatase" activity present in any given cell population at the time of analysis.