Bcr-Abl constitutes a deregulated tyrosine kinase involved in the pathogenesis of chronic myeloid leukemia (CML) and a subset of acute lymphoblastic leukemia (ALL). Although activation of the transcription factor NF-kappaB/Rel has been demonstrated, mechanisms of NF-kappaB/Rel activation by Bcr-Abl remain obscure. In this paper we demonstrate activation of NF-kappaB/Rel by Bcr-Abl and for the first time by v-Abl. Furthermore, we investigated mechanisms of NF-kappaB/Rel induction by Bcr-Abl and v-Abl. Both Bcr-Abl and v-Abl induced NF-kappaB/Rel DNA binding in Ba/F3 cells. DNA binding was a result of nuclear translocation of p65/RelA, whereas p65/RelA expression was unaffected. Nuclear translocation of p65/RelA is at least partially due to increased IkappaBalpha degradation, which is independent of IkappaB kinase (IKK) activity. IKK activity is not deregulated by Bcr-Abl and v-Abl. NF-kappaB/Rel transactivation was dependent on abl kinase activity but independent of Grb2 and Grb10 binding tobcr sequences. In addition, NF-kappaB/Rel activation was dependent on Ras activity. Primary CML blasts showed constitutive p65/RelA NF-kappaB/Rel DNA binding activity. Thus NF-kappaB/Rel represents a potential target for molecular therapies in CML.