In vitro inhibition of cytoplasmic SOD by cyanide ions, nitriles, dimethyldithiocarbamate, paraquat and Gramoxon was determined. In this experiment NBT reduction in aerobic xanthine oxidase system was used. Inhibition by polluting agents against crude extract of SOD of human erythrocytes and purified SOD from bovine blood. The following results were obtained. 1, Potassium cyanide inhibits SOD reaction, giving 50 percent inhibition at 0.036mM in crude extract of human blood and 0.043mM in purified bovine blood SOD. 2, No inhibition was observed by nitriles up to 20mM both in crude extract and purified SOD. 3, Sodium thiocyanate gives no inhibition up to 40mM both in crude extract and purified SOD. 4, Dimethyldithiocarbamate inhibits SOD reaction giving 50 percent inhibition at 0.66mM in crude extract and 0.96mM in purified SOD. 5, Paraquat inhibits SOD reaction giving 50 percent inhibition at 0.06mM in crude extract and 1.09mM in purified SOD. And Gramoxon containing 24 percent paraquat and certain surfactants gives 50 percent inhibition as low as 0.0054mM in crude extract and 0.0084 in purified SOD.