Both murine cuprozinc and manganosuperoxide dismutases were purified to electrophoretic homogeneity from liver; the former (dimer) had a sp. act. of 2600 U/mg and a subunit mol. wt. of 17,000, the latter (tetramer) 2300 U/mg and mol. wt. 23,000. Heterologous radioimmunoassays had sensitivities of 3.1 ng/ml for the former enzyme and 2.5 ng/ml for the latter, and were appicable across murine genetic lines (Balb/c, ob/ob, db/db). Islets had the lowest concentrations of both enzymes among the tissues studied, and this could explain their vulnerability to free-radical damage.