In human leukemic myeloblasts, the granule enzymes beta-glucuronidase, myeloperoxidase and acid phosphatase were associated with light particles of varying densities that were separable from each other by means of zonal density gradient centrifugation. In more mature granulocytic cells of chronic myelogenous leukemia the three enzymes merged within a single group of denser particles; such particles were absent in myeloblasts. Myeloblast particles had two to three times higher activity of beta-glucuronidase and acid phosphatase, but only one-tenth of the myeloperoxidase activity. Some of the cationic proteins and lysozyme were not found in leukemic myeloblasts but were present in particles of chronic myelogenous leukemia; alkaline phosphatase was absent from both types of leukemic cells.