Development of effectors from naive CD4 cells occurs in two stages. The early stage involves activation and limited proliferation in response to T cell receptor (TCR) stimulation by antigen and costimulatory antigen presenting cells, whereas the later stage involves proliferation and differentiation in response to growth factors. Using a TCR-transgenic (Tg⁺) model, we have examined the effect of aging on effector generation and studied the ability of γc signaling cytokines to reverse this effect. Our results indicate that responding naive CD4 cells from aged mice, compared with cells from young mice, make less interleukin (IL)-2, expand poorly between days 3 to 5, and give rise to fewer effectors with a less activated phenotype and reduced ability to produce cytokines. When exogenous IL-2 or other γc signaling cytokines are added during effector generation, the Tg⁺ cells from both young and aged mice proliferate vigorously. However, IL-4, IL-7, and IL-15 all fail to restore efficient effector production. Only effectors from aged mice generated in the presence of IL-2 are able to produce IL-2 in amounts equivalent to those produced by effectors generated from young mice, suggesting that the effect of aging on IL-2 production is reversible only in the presence of exogenous IL-2.