A cyanide-insensitive superoxide dismutase was purified to electrophoretic homogeneity from leaves of the eucaryote, Ginkgo biloba L. A molecular mass of 47,000 was determined for the enzyme, which consisted of two subunits of equal size. The enzyme preparation contained two isoenzymes with isoelectric points of 5.25 and 5.15. Metal analysis after dialysis against EDTA revealed the presence of 1.4 gram atoms of iron per molecule. Approximately 2 gram atoms each of copper and zinc per enzyme molecule were also detected, although removal of copper by other chelators had no effect on enzymatic activity. The purified Ginkgo enzyme exhibited a sensitivity to hydrogen peroxide and insensitivity to cyanide, which is typical of iron-containing superoxide dismutases. Ginkgo iron superoxide dismutase was localized in the stroma of chloroplasts, but was absent from mitochondria. The enzyme from Ginkgo was most similar to iron superoxide dismutases of Nuphar, Brassica, and Escherichia coli when compared on the basis of S delta Q analysis of amino acid composition. Peptide fragments formed by proteolytic digestion of these four enzymes were compared qualitatively; similar-sized fragments which denote possible areas of homology were observed.