Since acetyl-CoA produced through pyruvate dehydrogenase reaction is poorly oxidized by the Krebs cycle in rat lymphocytes, the fate of acetyl units was investigated in these cells. The results presented here show that 24-h cultured lymphocytes actively synthesize lipids from [3-14C]pyruvate. Furthermore, a considerable amount of these lipids have shown to be exported into the culture medium. Experiments with [1-14C] acetate as a lipid precursor showed a close similarity with the rates of incorporation of [3-14C] pyruvate into the same lipid fractions. Treatment of lymphocytes with the mitogen concanavalin A (Con A) markedly enhanced [1-14C] acetate incorporation into a variety of lipids, but the lectin did not affect [3-14C] pyruvate incorporation. The results suggest that lymphocytes convert pyruvate into lipids via the acetyl-CoA pathway and that Con A interferes in lymphocyte lipogenesis but does not seem to affect the pyruvate dehydrogenase reaction. The ability to incorporate pyruvate into certain lipids may have an important role for the rapidly dividing capacity of lymphocytes since the human cancer strain HeLa 155 (a quickly proliferating cell line) also exhibits this feature by converting much more [3-14C] pyruvate into lipids than do lymphocytes. In addition, comparative experiments with lymphocytes, peritoneal macrophages and HeLa cells indicate that pyruvate may provide precursors for cells with active lipid producing and exporting capacities.