Bad, Bim, and PI3 kinase, but depends in part on Puma Cell death provoked by loss of interleukin-3 signaling is independent of Growth and survival of hematopoietic cells is regulated by growth factors and cyto-kines, such as interleukin 3 (IL-3). When cytokine is removed, cells dependent on IL-3 kill themselves by a mechanism that is inhibited by overexpression of Bcl-2 and is likely to be mediated by proapoptotic Bcl-2 family members. Bad and Bim are 2 such BH3-only Bcl-2 family members that have been implicated as key initiators in apopto-sis following growth factor withdrawal, par-ticularly in IL-3–dependent cells. To test the role of Bad, Bim, and other proapoptotic Bcl-2 family members in IL-3 withdrawal-induced apoptosis, we generated IL-3–de-pendent cell lines from mice lacking the genes for Bad, Bim, Puma, both Bad and Bim, and both Bax and Bak. Surprisingly, Bad was not required for cell death follow-ing IL-3 withdrawal, suggesting changes to phosphorylation of Bad play only a minor role in apoptosis in this system. Deletion of Bim also had no effect, but cells lacking Puma survived and formed colonies when IL-3 was restored. Inhibition of the PI3 ki-nase pathway promoted apoptosis in the presence or absence of IL-3 and did not require Bad, Bim, or Puma, suggesting IL-3 receptor survival signals and PI3 kinase survival signals are independent.