Alkaline dimethylsulfoxide as a superoxide anion-generating system in association with cytochrome c as a superoxide anion-indicating scavenger has been used to develop a new assay for superoxide dismutase. The assay is sensitive (one unit of enzymatic activity is provided by 110 ng of purified copper-containing superoxide dismutase) and highly specific. The nature of this system prevents the usual interferences and its simplicity allows for multiple, rapid measurements of superoxide dismutase activity in biological preparations using either normal or automated procedures.