Consistent with the constitutive activation of Rel/NF-kappaB in human hematopoietic tumors, the v-Rel oncoprotein induces aggressive leukemia/lymphomas in animal models. v-Rel is thus a valuable tool to characterize the role of Rel/NF-kappaB in cancer and the mechanisms involved. Prior studies by our group identified a serine-rich domain in v-Rel that was required for biological activity. Here, we investigated the molecular basis for the transformation defect of specific serine mutants. We show that the transforming efficiency of these mutants in primary lymphoid cells is correlated with their ability to mediate kappaB site-dependent transactivation and with specific changes in phosphorylation profiles. Interestingly, coexpression of the death antagonists Bcl-xL and Bcl-2 significantly increased their oncogenicity, whereas other NF-kappaB-regulated death inhibitors showed little or no effect. The fact that a subset of apoptosis inhibitors could rescue v-Rel transactivation mutants suggests that their reduced transcriptional activity may critically affect expression of defined death antagonists essential for oncogenesis. Consistent with this hypothesis, we observed selection for high endogenous expression of Bcl-2-related death antagonists in cells transformed by weakly transforming v-Rel mutants. These results emphasize the need for Rel/NF-kappaB to efficiently activate expression of a subset of antiapoptotic genes from the Bcl-2 family to manifest its oncogenic phenotype.