Manganic porphyrins are redox active metal complexes that have been employed as superoxide dismutase mimics. We tested whether these metalloporphyrins could also dismute hydrogen peroxide (H₂O₂) and whether they could protect endothelial cells against H₂O₂. Both of the manganic metalloporphyrins tested were found to catalytically dismute H₂O₂. These manganic porphyrins also protected endothelial cells in dose-dependent manners against H₂O₂-mediated injury with MnTMPyP having an EC₅₀of 8 μmand MnTBAP having an EC₅₀of 15 μm. The zinc containing analogs of these porphyrins were inactive in dismuting H₂O₂and did not protect. These studies further define the antioxidant capacity of metalloporphyrins in converting superoxide to H₂O₂and H₂O₂to water. These data suggest that manganic porphyrins may be useful therapeutics against disease states associated with the overproduction of reactive oxygen species.