The activity of erythrocyte cytosolic superoxide dismutase from rat, bovine, man and duck was considerably increased when measured after preparation or incubation in media pretreated with negative air ions (mostly superoxide) from electroeffluvial ion generator. 0.5-1.0 microM H2O2 was found in incubation medium after treatment with air ions. The stimulatory effect of air ions on superoxide dismutase activity was mimicked by addition of 0.5-6 microM H2O2. The primary physicochemical mechanism of beneficial biological action of negative air ions is suggested to be related to the stimulation of superoxide dismutase activity by micromolar concentrations of H2O2.