Several tyrosine kinase oncogenes have been associated with myeloproliferative diseases, including Bcr/Abl, Tel/Abl, Tel/Jak2, and Tel/PDGFR. One target molecule shared by these oncogenes is known to be STAT5. We generated sublines of Ba/F3 cells in which either wild-type STAT5 or a constitutively active mutant of STAT5 (STAT5-1*6) were expressed under the control of a tetracycline-inducible promoter. These cell lines were compared with a Ba/F3 cell line in which the expression of p210^Bcr/Abl was made inducible by a similar promoter. Before induction, all cells were dependent on interleukin 3 (IL-3) for growth and survival. Both STAT5-1*6 and Bcr/Abl enhanced viability and induced proliferation in the absence of IL-3. We found that the proviability protein Bcl-X_L, but not Bcl-2, was induced by both p210^Bcr/Abl and STAT5-1*6. Using a Bcl-X gene promoter construct fused to a luciferase complementary DNA (cDNA), both p210^Bcr/Abl and STAT5-1*6 were shown to induce transcription of Bcl-X. The increase in transcription of the Bcl-X promoter and the increase in Bcl-X protein, due to p210^Bcr/Abl, were blocked by expression of a dominant negative STAT5 mutant. Interestingly, however, STAT5-1*6 required the continued presence of IL-3 to cause a significant increase in Bcl-X_L protein, whereas p210^Bcr/Abl did not need IL-3. Studies with enzyme inhibitors suggest that the extra signal supplied by IL-3 may be supplied by the PI3K pathway. Overall, these data suggest that constitutively activated STAT5 can increase viability and proliferation of Ba/F3 cells. This may contribute to, but is not likely sufficient for, the enhanced viability associated with Bcr/Abl transformation.