In T cell-deficient conditions, naive T cells undergo spontaneous "homeostatic" proliferation in response to contact with self-MHC/peptide ligands. With the aid of an in vitro system, we show here that homeostatic proliferation is also cytokine-dependent. The cytokines IL-4, IL-7, and IL-15 enhanced homeostatic proliferation of naive T cells in vitro. Of these cytokines, only IL-7 was found to be critical; thus, naive T cells underwent homeostatic proliferation in IL-4(-) and IL-15(-) hosts but proliferated minimally in IL-7(-) hosts. In addition to homeostatic proliferation, the prolonged survival of naive T cells requires IL-7. Thus, naïve T cells disappeared gradually over a 1-month period upon adoptive transfer into IL-7(-) hosts. These findings indicate that naive T cells depend on IL-7 for survival and homeostatic proliferation.