Despite differences in T cell responses induced by interleukin (IL)-2 and IL-7, both cytokines modulate T cell functions by activation of signal transducers and activators of transcription (STAT) proteins. We examined the contribution of the two isoforms of STAT5, STAT5A and STAT5B, to IL-2- and IL-7-induced activation of human peripheral blood T lymphoblasts. Both cytokines induced assembly of STAT5A and STAT5B containing complexes capable of binding to the interferon-gamma activation sequence (GAS), and these complexes rapidly translocated (within 1 min) into the nucleus of IL-2- or IL-7-treated cells. The kinetics of this translocation were delayed in IL-7-treated as compared to IL-2-treated cells. IL-2 and IL-7 were equivalent in their ability to induce tyrosine phosphorylation of STAT5A and STAT5B and to facilitate binding of these STATs to an immobilized GAS element. Both IL-2 and IL-7 induced substantial amounts of STAT5A/STAT5B heterodimerization. Moreover, we observed constitutive association of STAT3 with each STAT5 isomer. These data suggest that IL-2 and IL-7 induce assembly of STAT heterodimers in a similar manner and that subsequent cellular responses may be driven by induction of similar sets of genes.