Stat5 (Signal Transducers and Activators of Transcription 5) is important in regulating T cell functions involving the receptors for Interleukin-2 (IL-2). IL-2 stimulates the rapid phosphorylation of both serine and tyrosine residues of Stat5a and Stat5b in human T lymphocytes and in several IL-2-responsive lymphocytic cell lines. IL-2 differentially induces serine phosphorylation of Stat5a and Stat5b on Ser726 and Ser731, respectively. Stat5b is preferentially phosphorylated and displays more protracted serine phosphorylation kinetics than Stat5a. Both the acid-rich region and the COOH terminus of IL-2Rb can independently mediate IL-2-induced Stat 5a/b serine phosphorylation, suggesting that Stat5a/b serine phosphorylation occurs at a postreceptor level. Stat5a is phosphorylated on Tyr694 in a prolactin-sensitive manner, whereas serine phosphorylation is constitutive. Activation of Stat5 by IL-2 may help govern the biological effects of IL-2 during the immune response.