Abstract: Asuperoxide dismutase from leek mitochondria has been purified to homogeneith by ammonium sulfate fraction, DEAE-Sephacel chromotography and Sephadex G-200 gel filtration.2.5 mg purified enzyme was obtained from 6000 g leaves.The specific activity of the purified enzyme is 1200 U/mg protein.The enzyme is not sensitive to inhibitors:KCN nad H₂O₂.It has good heat .stability.The largest amount of ultraviolet absorption is at 280 urn.The molecular weight of the enzyme is about 82000 Das assayed by Sephadex G-200 gel filtration,that of the enzyme subunit is about 22000 Das tested by SDS-polyacrylamide gel electrophoresis.The N-terminal amine acid of the enzyme is valine as tested by DNS-CI.These results show the enzyme is a manganese superoxide.