Growth factors and cytokines signal survival through their cognate receptors, activating signaling pathways often composed of protein kinase cascades. On the other hand, inadequate growth factor initiates a signaling cascade that leads to cell death. Both pro-and anti-apoptotic BCL-2 family members lie downstream of these proximal signals. The “multi-domain” BCL-2 members either suppress (e.g., BCL-2, BCL-XL) or promote apoptosis (e.g., BAX, BAK), whereas the “BH3-only” subfamily members identified to date (e.g., BAD, BID, BIM) function to promote cell death. It has become clear that BH3-only proteins are upstream initiators of apoptosis. BIM was identified as a BH3-only protein that induces apoptosis and can be antagonized by anti-apoptotic BCL-2 family members. Studies in BIM-deficient mice/cells indicate important roles for BIM in hematopoietic cell homeostasis. In the present study, an siRNA knockdown of BIM confirms this BH3-only member is important for the cytokine-mediated homeostasis of hematopoietic cells. We show that the phosphorylation status of BIM controls its pro-apoptotic activity. Interleukin-3 (IL-3), a hematopoietic survival factor, induces ERK/MAPK-mediated phosphorylation of BIM on three serine sites (Ser55, Ser65, Ser100). IL-3 withdrawal results in the dephosphorylation of BIM and only non-phosphorylated BIM interacts with the multi-domain pro-apoptotic effector BAX. Phosphorylation of BIM upon exposure of cells to IL-3 dramatically reduces the BIM/BAX interaction. A non-phosphorylatable BIM molecule (S55A, S65A, S100A) demonstrates enhanced interaction with BAX and enhanced pro-apoptotic activity. Thus, ERK/MAPK-dependent phosphorylation of BIM in response to survival factor regulates BIM/BAX interaction and the pro-death activity of BIM.