It has been proposed that reactive oxygen species, and in particular H2O2, may be involved in the activation of NF-kB by diverse stimuli in different cell types. Here we have investigated the effect of a range of putative antioxidants on NF-kB activation by interleukin-1 and tumor necrosis factor as well as the ability of H2O2 to activate NF-kB in primary human umbilical vein endothelial cells and the transformed human endothelial cell line ECV304. Activation of NF-kB and stimulation of IkBa degradation by H2O2 was only evident in the transformed cells and required much longer contact times than that observed with interleukin-1 or tumor necrosis factor. Furthermore, only H2O2 was sensitive to Nacetyl- L-cysteine, and no increase in H2O2 was detected in response to either cytokine. Pyrrolidine dithiocarbamate has been purported to be a specific antioxidant inhibitor of NF-kB that acts independently of activating agent or cell type. However, we found that tumor necrosis factor- but not interleukin-1-driven NF-kB activation and IkBa degradation were sensitive to pyrrolidine dithiocarbamate in transformed cells, while neither pathway was inhibited in primary cells. Phorbol ester-mediated activation was sensitive in both transformed and primary cells. Other antioxidants failed to inhibit either cytokine, while the iron chelators desferrioxamine and 2,2,6,6-tetramethylpiperidine-1-oxyl mimicked the pattern of inhibition seen for the dithiocarbamate. This suggested that pyrrolidine dithiocarbamate was inhibiting NF-kB activation in endothelial cells primarily through its iron-chelating properties. Tumor necrosis factor, but not interleukin-1, was found to induce lipid peroxidation in ECV304 cells. This was inhibited by pyrrolidine dithiocarbamate and desferrioxamine. t-Butyl hydroperoxide, which induces lipid peroxidation, activated NF-kB. Finally, butylated hydroxyanisole, which inhibits lipid peroxidation but has no iron-chelating properties, inhibited NF-kB activation by tumor necrosis factor but not interleukin-1. Taken together, the results argue against a role for H2O2 in NF-kB activation by cytokines in endothelial cells. Furthermore, tumor necrosis factor and interleukin- 1 activate NF-kB through different mechanisms in ECV304 cells, with the tumor necrosis factor pathway involving iron-catalyzed lipid peroxidation.