The transcription factor nuclear factor-κB (NF-κB) is constitutively activated in malignancies from enhanced activity of inhibitor of NF-κB (IκB) kinase, with accelerated IκBα degradation. We studied whether redox signaling might stimulate these events. Cultured melanoma cells generated superoxide anions (O) without serum stimulation. Ogeneration was reduced by the NAD(P)H:quinone oxidoreductase (NQO) inhibitor dicumarol and the quinone analog capsaicin, suggesting that electron transfer from NQO through a quinone-mediated pathway may be an important source of endogenous reactive oxygen species (ROS) in tumor cells. Treatment of malignant melanoma cells with the H₂O₂ scavenger catalase, the sulfhydryl donorN-acetylcysteine, the glutathione peroxidase mimetic ebselen, or dicumarol decreased NF-κB activation. Catalase,N-acetylcysteine, ebselen, dicumarol, and capsaicin also inhibited growth of melanoma and other malignant cell lines. These results raise the possibility that ROS produced endogenously by mechanisms involving NQO can constitutively activate NF-κB in an autocrine fashion and suggest the potential for new antioxidant strategies for interruption of oxidant signaling of melanoma cell growth.