Rel/nuclear factor kappaB transcription factors were shown to have either pro- or antiapoptotic as well as pro- or antiproliferative functions, and it is often assumed that the outcome of their activation depends on the cell type or cellular context. Inconsistent with this assumption, we show here that cRel is able in one cell type to inhibit proliferation, protect against apoptosis induced by tumor necrosis factor alpha (TNF-alpha) + cycloheximide (CHX), and increase the basal rate of apoptosis. Both the effects of proliferation inhibition and protection against TNF-alpha + CHX-induced apoptosis are massive and occur in the same cells. Using reverse transcription-PCR, Western blot and immunofluorescence, and transactivation assays, we found that the manganese superoxide dismutase (MnSOD), an enzyme that converts O2*- in H2O2, is up-regulated by cRel through a kappaB site in intron 2. Inhibition of MnSOD induction by antisense oligonucleotides and overexpression of MnSOD respectively reverts and mimics both the antiproliferative and antiapoptotic effects of cRel, suggesting that they both occur via the induction of this gene. On one hand, MnSOD could improve the efficiency of cRel-overexpressing cells in eliminating toxic O2*- produced on TNF-alpha treatment, explaining why they escape TNF-alpha-induced apoptosis. On the other hand, cRel-overexpressing cells should accumulate H2O2. We present evidence linking this H2O2 accumulation to the proliferation arrest induced by cRel. Therefore, different effects on proliferation and apoptosis could arise from the induction of MnSOD and thus coexist in cRel-overexpressing cells.