The T cell growth factor IL-2 induces T cell progression through the cell cycle and ultimately controls T cell mitosis. Here we show that the guanine nucleotide-binding proteins p21ras may be involved in IL-2 signal transduction pathways. IL-2 causes a rapid and prolonged activation of p21ras in both murine and human T cells. The concentration-dependence of IL-2-mediated stimulation of p21ras correlated with IL-2 stimulation of T cell proliferation, which indicates that p21ras activity can be controlled by signals generated via the interaction between IL-2 and its high affinity cellular receptor. These results suggest that p21ras may play a role in the regulation of T cell growth by IL-2.