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Although the germicide role of H 2 O 2 released during inflammation is well established, a hypothetical regulatory function, either promoting or inhibiting inflammation, is still controversial. In particular, after 15 years of highly contradictory results it remains uncertain whether H 2 O 2 by itself activates NF-κB or if it stimulates or inhibits the activation of NF-κB by proinflammatory mediators. We investigated the role of H 2 O 2 in NF-κB activation using, for the first time, a calibrated and controlled method of H 2 O 2 delivery—the steady-state titration—in which cells are exposed to constant, low, and known concentrations of H 2 O 2. This technique contrasts with previously applied techniques, which disrupt cellular redox homeostasis and/or introduce uncertainties in the actual H 2 O 2 concentration to which cells are exposed. In both MCF-7 and HeLa cells, H 2 O 2 at extracellular concentrations up to 25 μM did not induce …