Leucocytes of normal persons and patients with acute and chronic granulocytic leukemia, chronic lymphocytic leukemia, and non-Hodgkin lymphoma were separated into subfractions by centrifugation in discontinuous Ficoll density gradient. Osmotic resistance was examined in hypotonic NaCl solutions with decreasing concentration and by determining LDH activity in the supernatant. Suspensions of myelocytes, polymorphnuclear granulocytes, and lymphocytes of normal persons and patients with chronic lymphocytic leukemia demonstrated the same osmotic resistance. Only myeloblasts were osmotically less fragile, and tumor cells of non-Hodgkin lymphoma more fragile.