Mice genetically deficient in IL-7R(alpha) are highly lymphopenic in the peripheral lymphoid organs. The functional competence of T cells that have developed in the absence of an IL-7R signal was investigated. Three important observations were made using several in vitro activation regimens. First, stimulation of T cells from IL-7R -/- mice at limiting dilution with immobilized Abs to CD3, CD4 or CD8, and CD18 revealed a six- to sevenfold reduction in the frequency of clonogenic T cells compared with T cells from IL-7R +/+ mice. IL-7R -/- T cells were also significantly less responsive to alloantigen as well as to receptor-independent stimuli such as PMA and ionomycin. Furthermore, the average clone size of single IL-7R -/- T cells was 50% smaller than that of IL-7R +/+ T cells. These data suggest that the reduced clonogenicity was predominantly due to intrinsic deficiencies in the ability of IL-7R -/- T cells to proliferate upon stimulation. Second, analysis of the kinetics of cell growth of IL-7R -/- T cells revealed that a significant proportion of T cells failed to proliferate within the first 72 h of in vitro stimulation, with the majority undergoing programmed cell death. Third, both clonogenic IL-7 -/- T cells and IL-7R +/+ T cells showed a similar proliferative response in the presence of IL-2 and similar survival kinetics, indicating that a subpopulation of IL-7R -/- T cells is functionally mature. We propose that an absence of IL-7R signaling not only affects T cell development in the thymus, but also results in the accumulation of functionally inactive T cells in the periphery.