JNK has been suggested to be proapoptotic, antiapoptotic, or have no role in apoptosis depending on the cell type and stimulus used. The precise mechanism of JNK action, under conditions when it promotes cell survival, is not entirely clear. Here, we report that JNK is required for IL-3-mediated cell survival through phosphorylation and inactivation of the proapoptotic Bcl-2 family protein BAD. IL-3 withdrawal-induced apoptosis is promoted by inhibition of JNK but suppressed by expression of a constitutively active JNK. JNK phosphorylates BAD at threonine 201, thereby inhibiting BAD association with the antiapoptotic molecule BCL-X(L). IL-3 induces BAD phosphorylation at threonine 201, and replacement of threonine 201 by alanine generates a BAD mutant, which promotes IL-3 withdrawal-induced apoptosis. Thus, our results provide a molecular mechanism by which JNK contributes to cell survival.