The major superoxide dismutase of the unicellular red alga, Porphyridium cruentum, has been purified to homogeneity. This enzyme has a molecular weight of 40,000 and is composed of two subunits of equal size, which are joined by noncovalent interactions. Manganese constituted 0.13% of this superoxide dismutase. T,is is equivalent to 1 manganese atom/molecule of enzyme. Cyanide at 5 mM and H2O2 at 3 mM had no effect on the activity of this superoxide dismutase but 20 mM azide caused 50% inhibition. The isoelectric point, assessed by isoelectric focusing, is 4.2. The optical spectrum of this enzyme exhibited a maximum at 280 nm (Em = 49,000 M-1 cm-1) and a broad band centered at 450 nm (Em = 170 M-1 cm-1). Exposure to a pH of 3.8 in the presence of 8.0 M urea labilized the manganese and allowed the preparation of a colorless and inactive apoenzyme which could be reconstituted by subsequent treatment with MnCl2. The reconstituted enzyme was found to have regained both manganese and activity. The amino acid composition was also determined. The P. cruentum superoxide dismutase did not cross-react with antibody to the Escherichia coli manganese-containing superoxide dismutase.