Interleukin (IL)-2 and IL-15 are redundant in stimulating T-cell proliferation in vitro. Their precise role in vivo in governing T-cell expansion and T-cell homeostasis is less clear. Each may have distinct functions and regulate distinct aspects of T-cell activation 1, 2, 3, 4, 5, 6. The functional receptors for IL-2 and IL-15 consist of a private α-chain, which defines the binding specificity for IL-2 or IL-15, and shared IL-2 receptor β-and γ-chains. The γ-chain is also a critical signaling component of IL-4, IL-7 and IL-9 receptors 7. Thus, the γ-chain is called the common γ or γ-c. As these receptor subunits can be expressed individually or in various combinations resulting in the formation of receptors with different affinities, distinct signaling capabilities or both 7, 8, we hypothesized that differential expression of IL-2 and IL-15 receptor subunits on cycling T cells in vivo may direct activated T cells to respond to IL-2 or IL-15, thereby regulating the homeostasis of T-cell response in vivo. By observing in vivo T-cell divisions and expression of IL-2 and IL-15 receptor subunits, we demonstrate that IL-15 is a critical growth factor in initiating T cell divisions in vivo, whereas IL-2 limits continued T-cell expansion via downregulation of the γ-c expression. Decreased γ-c expression on cycling T cells reduced sustained Bcl-2 expression and rendered cells susceptible to apoptotic cell death. Our study provides data that IL-2 and IL-15 regulate distinct aspects of primary T-cell expansion in vivo.