The hypothesis that a role for insulin in the metabolism of T cells would be evident after cell activation when receptors appear was tested to validate the T cell model and to analyze the mechanism by which insulin may function in immunoregulation. Measuring the flux rates of 3-O-[methyl-3H]-D-glucose and aminoisobutyric acid, alpha-[1-14C], lactate production and oxidation, and glucose oxidation from carbon 1- and carbon 6-labeled substrates, it was determined that (a) mitogens such as phytohemagglutinin enhance basal T lymphocyte intermediary metabolism, (b) physiologic concentrations of insulin have no impact on the metabolism of unstimulated, cultured, receptor-negative lymphocytes, and (c) insulin provided to receptor bearing lymphocytes augments intermediary metabolism above mitogen stimulated levels. The importance of the pentose phosphate shunt pathway for energy metabolism in the stimulated lymphocyte was confirmed. These studies demonstrate that insulin has a classical physiologic role to play in the activated lymphocyte further validating the use of this cell to examine potential receptor defects in disorders of carbohydrate metabolism. By enhancing energy metabolism of stimulated lymphocytes, insulin serves biologic economy and thus may perform its immunoregulatory role.