Abstract Previous studies established that low in vitro temperatures (27°C, termed nonpermissive) suppressed murine primary thymus-dependent (TD), but not primary thymus-independent (TI) or secondary TD, antibody responses. Suppression was rescued by the addition of oleic acid (18:1), recombinant interleukin (rIL)-2 and/or rIL-4, but not rIL-1, These observations suggested that low temperatures suppress the functions of virgin T cells (Tv) but not those of memory T cells (Tm), B cells, or accessory cells and that hypothetically 18:1 may rescue suppressed responses by altering the membranes of Tv cells, allowing them to proliferate and subsequently secrete interleukins. In this study negatively selected Tm and Tv cells were stimulated with various mitogens at 37 or 27°C. The results indicated that proliferation was suppressed at 27°C to all mitogens tested. The subsequent addition of 18:1 induced proliferative responses at 27°C to both concanavalin A (Con A) and a combination of 12- O-tetradecanoylphorbol 13-acetate (TPA) and calcium ionophore (A23187), but not to phytohemagglutinin-P (PHA). Both Tm and Tv cells showed significant secretion of IL-2 and expression of IL-2 receptor (IL-2R) at 27°C in response to all mitogens, irrespective of proliferation, and the subsequent addition of 18:1 caused little or no change in the levels of IL-2 secretion or IL-2R expression. These findings indicate that suppression of Tm and Tv cell proliferative responses occurs irrespective of IL-2R expression and, unlike antibody production, of IL-2 secretion. Furthermore, it appears that 18:1 can synergistically act with Con A or TPA/A23187, but not PHA, in activating Tm and Tv cells to induce proliferation at 27°C. These findings suggest differences in signal transduction mechanisms between these various mitogens.