Insulin, as well as other ligands which increase intracellular guanosine 3',5'-cyclic monophosphate (cGMP), augments thymic-derived (T)- lymphocyte effector activity as revealed by alloimmune lymphocyte-mediated cytotoxicity. The observation that insulin binds only to monocytes among circulating nonimmune human mononuclear cells fosterd reexamination of the mechanism by which insulin augments T-lymphocyte function. This report concerns a test of the hypothesis that the T cell is directly affected by insulin and that an insulin receptor emerges upon T lymphocytes consequent to immune activation. Spleens were removed from rats skin grafted across a major histocompatibility barrier. Lymphocytes were harvested from Ficoll-Hypaque density gradients and subsequently enriched for T cells by passage over one or two nylon wool columns. This population was composed of more than 98% T cells as assessed by surface marker techniques (Ig staining, erythrocyte antibody, and erythrocyte antibody complement rosetting, anti-T staining). There was no loss of augmentation of lymphocyte-mediated cytotoxicity induced by insulin, carbamycholine, and 8-bromo-cGMP in the purified cells when compared to unfractionated cells 7 days after transplantation. 125I-insulin bound saturably to the allostimulated T-enriched lymphocytes with maximum binding at 12.8 +/- 0.2 pg and a dissociation constant at equilibrium of 1.3 nM. In contrast, insulin receptors were not present on nonimmune T-enriched cells or on T cells from animals that received syngeneic grafts. The affinity of the lymphocyte insulin receptor was similar to that of more conventional insulin-sensitive tissues e.g., liver, adipocyte. After 89% of T cells from spleens on day 7 were lysed with anti-thy 1.1 antibody and complement, the ability to measure specific insulin binding was lost. These data confirm a physiologic role for insulin in T-lymphocyte effector function and describe the emergence of insulin receptors concomitant with cell sensitivity to ligand. Such receptors may play a role in hormonal modulation of the immune response.