Addition of recombinant interleukin-2 (rIL-2) to normal adult murine thymocytes in vitro as the only exogenous stimulus leads to a dose-dependent mitogenic response characterized by two distinct dosage kinetic components. The high-affinity IL-2 thymocyte response is mounted by in vivo-activated (IL-2 receptor light chain positive) thymocytes, while the low-affinity IL-2 response, of larger amplitude, is carried out by resting thymocytes. Addition of IL-2 to thymocytes also triggers intense IL-3 secretory responses with both high and low IL-2 affinity components. Addition of high IL-2 dosages to thymocyte bulk cultures results in a dramatic increase in IL-2 responsiveness for both proliferation and IL-3 secretion on a per viable cell basis and with tightly coupled temporal kinetics. The low-affinity component of IL-2-proliferative and IL-3-secreting responses is carried out by resting mature CD4+ thymocytes, as assessed by negative selection with monoclonal antibodies (mAb) plus complement. The mechanism of resting thymocyte activation by high doses of IL-2 is partially characterized. Depletion of endogenous thymus-adherent cells abolished both proliferation and IL-3 secretion, and addition of splenic accessory cells or peritoneal macrophages to depleted thymocytes restored IL-2 responsiveness. Mature CD4+ thymocytes spontaneously form rosettes with adherent accessory cells, while CD8+ thymocytes do so with much less efficiency. Rosette formation of CD4+, but not of CD8+ thymocytes, can be blocked by anti-CD4 mAb GK1.5. At the same dosage as it prevents rosette formation, mAb GK1.5 also blocks the low-affinity thymocyte response to IL-2. The high-affinity IL-2 response is completely resistant to the action of cyclosporin A (CsA), but the low-affinity IL-2 response, although of much larger amplitude, can be almost completely suppressed by CsA. Together, these results demonstrate that resting CD4+ thymocytes can be induced to proliferation and lymphokine secretion by IL-2 alone in a process that is dependent on interaction with accessory cells, involves CD4 adhesion molecules and triggers activation through a CsA-sensitive pathway. In addition, the results demonstrate that IL-2 alone is able to enhance thymocyte IL-2 responsiveness and IL-3 secretory responses in vitro. The ability of IL-2 to induce and maintain thymocyte function is discussed in the light of these results.