Neutral lipids, free and total cholesterol, glycolipids, and phospholipids were determined in 20 preparations of leukocytes distributed in four groups. Group I consisted of leukocytes from nonleukemic patients; group II, from patients with chronic myelogenous leukemia; group III, from patients with chronic lymphocytic leukemia; and group IV, from patients with acute leukemia. Two neutral glycolipids were found in nonleukemic mixed leukocyte populations. They were identified as glucosylceramide and lactosylceramide. The same glycolipids were also present in leukemic cells, but striking differences in glycolipid composition were found in various types of leukocytes. Glycolipids accounted for 8.9-12.6% of the total lipids in leukocytes from group I, 11.4-20.4% in group II, 1.2-1.6% in group III, and 0.5-4.9% in group IV. Glucosylceramide was the only glycolipid found in seven out of eight analyzed samples of lymphocytes, both normal and leukemic. Lactosylceramide was the major glycolipid in preparations consisting mainly of polymorphonuclear, myeloid, and blastic cells. Only lactosylceramide was found in platelets, where its concentration was about 100 times lower than in mixed leukocyte populations.