We have studied the utilization of free fatty acid and glucose by human leukemic blast cells. Palmitate was both incorporated into complex cellular lipids, primarily phospholipids and triglycerides, and oxidized to CO2. The predominant phospholipid synthesized was phosphatidylcholine. Only a small proportion of the incoming fatty acid was modified structurally before incorporation into lipid esters. After incubation with [1-14 C]palmitate, 91% of the radioactivity recovered in cell lipids remained in fatty acids containing 16 carbon atoms. Studies with labeled glucose revealed little de novo synthesis of fatty acid, and the majority of the radioactivity from glucose was located in the water-soluble fraction after saponification of the esters. We conclude that the free fatty acids contained in the extracellular fluid provide much of the fatty acid for required cellular lipid synthesis in human leukemic blast cells. Since there is little elongation of incoming palmitate before incorporation into cellular lipids, it may be possible to alter the fatty acid composition of membrane phospholipids by changing the proportion of the various free fatty acids available to the leukemic cells.