The initiation of DNA synthesis and secretion of Interleukin 2 (IL-2) was measured in isolated rat splenic lymphocytes following activation with Concanavalin A (ConA). The extent of 3H-thymidine incorporation into activated cells was tested when cultured with various concentrations of Adrenocorticotropic hormone (ACTH). A paradoxical dose-response curve resulted when ACTH caused a biphasic response of augmenting and inhibiting 3H-thymidine uptake in lymphocytes depending on the hormone concentration. Low levels of ACTH (0.001-1-nM) augmented 3H-thymidine uptake and high levels (10-1000 nM) reversed the effect. The optimal ACTH concentration was 10 pM ACTH in the presence of 5 ug/ml ConA and there was no ACTH effect on quiescent cells (no ConA). Conditioned media from splenic lymphocytes treated with various concentrations of ConA or ACTH was tested for increased uptake of 3H-thymidine by the IL-2 growth dependent Cytotoxic T Lymphocyte Leukemia (CTLL-2) cells. ConA conditioned medium could sustain the CTLL-2 cells indicating the presence of IL-2. Conditioned medium from splenic lymphocytes treated with both ConA and 100 pM ACTH further increased CTLL-2 cell proliferation indicating an additional increase of IL-2 secretion. The identity of IL-2 was confirmed by using an anti-rat IL-2 antibody to neutralize the growth potential of the conditioned medium. ACTH alone had no effect on the CTLL-2 cell proliferation indicating the effect is due solely to induced IL-2 found in the conditioned medium. IL-2 levels in the conditioned media were quantitated by ELISA assay; splenic lymphocytes produced 4.2 ng/ml to ConA only, 19.2 ng/ml in ConA plus 10 nM ACTH, and no detectable IL-2 at ConA plus 10 uM ACTH. These results demonstrated that ACTH modulates IL-2 secretion from activated lymphocytes, which is both biphasic and concentration dependent.