A substantial body of evidence indicates that high intracellular concentrations of inorganic manganous ions render some cells resistant to ionizing radiation and provide substantial antioxidant protection to aerobic cells lacking superoxide dismutase (SOD) enzymes. We found that manganous phosphate is unique among those manganous salts studied in its ability to remove superoxide rapidly and catalytically from aqueous solution via a disproportionation mechanism that is entirely different from those of the SOD enzymes.