Bcr–Abl is a chimeric oncoprotein that is strongly implicated in acute lymphoblastic (ALL) and chronic myelogenous leukemias (CML). This deregulated tyrosine kinase selectively causes hematopoietic disorders resembling human leukemias in animal models and transforms fibroblasts and hematopoietic cells in culture. Bcr–Abl also protects cells from death induced on cytokine deprivation or exposure to DNA damaging agents. In addition, the antiapoptotic function of Bcr–Abl is thought to play a necessary role in hematopoietic transformation and potentially in leukemogenesis. The transcription factor NF-κB has been identified recently as an inhibitor of apoptosis and as a potential regulator of cellular transformation. This study shows that expression of Bcr–Abl leads to activation of NF-κB-dependent transcription by causing nuclear translocation of NF-κB as well as by increasing the transactivation function of the RelA/p65 subunit of NF-κB. Importantly, this activation is dependent on the tyrosine kinase activity of Bcr–Abl and partially requires Ras. The ability of Bcr–Abl to protect cytokine-dependent 32D myeloid cells from death induced by cytokine deprivation or DNA damage does not, however, require functional NF-κB. However, using a super-repressor form of IκBα, we show that NF-κB is required for Bcr–Abl-mediated tumorigenicity in nude mice and for transformation of primary bone marrow cells. This study implicates NF-κB as an important component of Bcr–Abl signaling. NF-κB-regulated genes, therefore, likely play a role in transformation by Bcr–Abl and thus in Bcr–Abl-associated human leukemias.