Acid phosphatase in human leukocytes was examined in a large number of patients with a variety of hematologic diseases. In chronic lymphocytic leukemia the total leukocyte acid phosphatase activity was markedly decreased. This was due to the drastic increase of enzyme-poor leukemic lymphocytes and the concomitant decrease of enzyme-rich monocytes and neutrophils. Further examination by disc gel electrophoresis revealed that the leukocytes in this disease contained only one of the five acid phosphatase isoenzymes present in a normal leukocyte preparation. Total acid phosphatase activity was not significantly altered in other hematologic disorders, yet different ratios of the isoenzymes shown by disc gel electrophoresis were observed in Hodgkin's disease, chronic granulocytic leukemia, acute granulocytic leukemia, infectious mononucleosis and leukemic reticuloendotheliosis.