A superoxide dismutase (superoxide:superoxide oxidoreductase, EC 1.15.1.1), distinct from previously known superoxide dismutases, has been isolated from human lung tissue. It is probably of the same nature as a previously demonstrated high molecular weight superoxide dismutating factor in human extracellular fluids. The enzyme has a molecular weight around 135,000 and is composed of four equal noncovalently bound subunits. Each molecule appears to have four copper atoms. No iron or manganese was found in the enzyme. Cyanide inhibits the enzyme efficiently. The enzyme brings about a first-order dismutation of the superoxide radical, the rate constant for the catalyzed reaction being about 1 X 10(9) M-1 s-1 per copper atom. The enzyme has hydrophobic properties. Affinity for various lectins indicates the presence of carbohydrate. Upon chromatography on heparin-Sepharose it is divided into three fractions, one with no, one with weak, and one with strong affinity for heparin.