The v-Rel oncoprotein belongs to the Rel/NF-kappaB family of transcription factors. It transforms chicken lymphoid cells in vitro and induces fatal lymphomas in vivo. In this study, we used a tetracycline-regulated system to characterize the role of v-Rel in cell transformation. We show that the continued expression of v-Rel is necessary to maintain the viability of transformed lymphoid cells and enables primary spleen cells to escape apoptosis in vitro culture. In agreement with a possible role for v-Rel in the inhibition of programmed cell death, its inducible expression in HeLa cells prevented TNFalpha-induced apoptosis. While the repression of v-Rel was accompanied by the rapid degradation of IkappaBalpha, changes in the steady-state levels of the apoptosis inhibitors Bcl-2 and Bcl-X(L) were only observed following the onset of cell death in transformed lymphoid cells. This suggests that the anti-apoptotic activity of v-Rel may affect other apoptosis inhibitors or other factors in the death pathway. Together, these findings demonstrate that v-Rel blocks apoptosis and suggest that this activity may be an important component of its transforming function.