There is an increased interest in development of therapeutic agents to treat disease states that involve reactive oxygen species in their pathophysiology. The metalloporphyrins, MnTBAP and ZnTBAP, were found to be active in a superoxide dismutase (SOD) assay. The efficacy of these compounds were tested against a well established model of intracellular superoxide-mediated cell injury. Paraquat generates superoxide by redox cycling with intracellular diaphorases and molecular oxygen. SOD mimetics were employed in a cell culture model with calf pulmonary artery endothelial cells (CPA-47) grown to near confluence in 24-well plates. Cell injury was assessed by measuring the release of cytosolic lactate dehydrogenase into the cell medium and by the number of adherent cells remaining after treatment. Exposure to various concentrations of paraquat for 4 h produced a dose-dependent injury response that was attenuated by 50 microM SOD mimetic, MnTBAP. MnTBAP also protected endothelial cells in a dose-dependent manner (EC50 approximately 40 microM) against paraquat (2 mM), whereas its less active analog (ZnTBAP) did not protect them. The protective effect of MnTBAP appears to be due to its intracellular superoxide-scavenging activity because neither the zinc form or exogenous CuZnSOD protected the cells against paraquat-induced injury. These studies suggest that metalloporphyrin-based SOD mimetics may be useful agents in preventing injury and disease states associated with the generation of intracellular reactive oxygen species.