Superoxide dismutase (SOD) was purified from Aerobacter aerogenes IFO 3317 to an electrophoretically homogeneous state and partially characterized. SOD was purified by ammonium sulfate fractionation, column chromatography on DEAE-Sephadex A-50, gel filtration on Sephadex G-100, phenyl-Toyopearl 650 M hydrophobic chromatography, and hydroxyapatite adsorption chromatography. The molecular weight and subunit molecular weight of the purified enzyme were estimated to be 45,000 and 22,000, respectively. The isoelectric point of the enzyme was about pH 4.0. The purified enzyme remained stable at pH 7.0-11.0, 25 degrees C for 36 hrs, but was rapidly inactivated below pH 7.0. SOD was stable up to 35 degrees C at pH 7.0, but was inactivated at temperatures above that. The absorption maximum in the visible range was found at 360 nm, and the enzyme was insensitive to cyanide and fluoride, and sensitive to hydrogen peroxide and azide. These results suggest that the enzyme is an iron-containing SOD. The amino acid composition and the N-terminal sequence of the first 15 amino acids of the enzyme exhibited close homology with the other iron-containing SODs.