Superoxide dismutases (EC 1.15.1.1) are metalloenzymes that catalytically scavenge the superoxide radical. They are essential for the aerobic survival of all forms of life. There are three types of superoxide dismutase, containing manganese, iron, or copper and zinc. The copper--zinc type has generally been isolated from eukaryotic cells except for the enzyme for the symbiotic marine bacterium Photobacterium leiognathi. The copper--zinc type, from different sources, has a molecular weight of about 32 000, and is composed of two identical subunits, each containing one atom of copper and one atom of zinc. The copper participates in the catalytic activity of the enzyme, while the zinc plays only a structural role. The enzyme has been resolved reversibly. Superoxide dismutases provide protection against oxygen toxicity, against compounds that cause exacerbation of oxygen toxicity, against ionizing radiation, and also against the damaging sequelae of prolonged inflammation.