Granulocytes were obtained from samples of peripheral blood of five patients who had untreated chronic granulocytic leukemia, and polymorphonuclear leukocytes were isolated from peripheral blood of three normal persons. Specific and nonspecific esterases were extracted from leukocyte preparations with cetyltrimethylammonium bromide and with lysolecithin, and subjected to polyacrylamide disk electrophoresis. In samples from both patients and normal persons, electrophoretic patterns of nonspecific esterase activity using alpha-naphthyl acetate and alpha-naphthyl butyrate were similar, and the esterase bands were weakly inhibited by fluoride. Lysolecithin extracts of specific esterase showed similar electrophoretic patterns for patients and normal subjects. However in cetyltrimethylammonium bromide extracts of specific esterase, 11 bands were seen in preparations from all of the patients with chronic granulocytic leukemia. In preparations of normal polymorphonuclear leukocytes, only eight bands were visualized. The results are consistent with an interpretation that these fast-moving components of specific esterase in chronic granulocytic leukemia granulocytes are present in normal polymorphonuclear leukocytes, but in quantities too small to be visualized with the technics used. Alternatively, the apparent "additional" bands of specific esterase may reflect abnormal metabolism of malignant granulocytes in chronic granulocytic leukemia.