Although Stat5 is activated by various cytokines, only erythropoietin (Epo) and a small number of cytokines induce Stat5-dependent erythroid differentiation. Here, by using a reporter gene system to monitor transcriptional activity of Stat5, we showed that Epo but not interleukin (IL)-3 supports sustained activation of Stat5, which induces globin gene expression. IL-3 or IL-2 stimulation inhibits Epo-induced globin gene expression. The acidic region of the IL-2 receptor P-chain was essential for this inhibition. These results underscore the importance of temporal regulation of Stat activity for regulation of cytokine-specific cell differentiation. (C) 2007 Elsevier Inc. All rights reserved.