Seven acid phosphatase isoenzymes, namely 0, 1, 2, 3, 3b, 4 and 5, in human leukocytes have been separated both by acrylamide gel electrophoresis and by ion exchange chromatography. The study of these isoenzymes in preparations of various cell types indicates that at least some of the isoenzymes are cell type-specific, i.e., isoenzyme o in Gaucher cells, 1, 2 and 4 in neutrophils, 1 and 4 in monocytes, 3 in lymphocytes and platelets, 3b in primitive blasts and 5 in reticulum cells of leukemic reticuloendotheliosis. Isoenzyme 5 is different from others in its resistance to l(+)-tartaric acid treatment. The persistent presence of abnormally high isoenzyme 5 in the reticulum cells of leukemic reticuloendotheliosis may be important for the differential diagnosis of this disease. Possible relationships between these isoenzymes and subcellular organelles are discussed.