Several superoxide dismutases (SODs), such as porcine and bovine erythrocyte Cu/Zn-SODs and N.tabacum Mn-SOD, were found to exhibit the activity to cleave supercoiled DNA in vitro. They converted supercoiled DNA into nicked and further into linear form. They did not act on linear double-stranded DNA. Activity assays after they were inactivated by H₂O₂ or guanidine and hydrolyzed by proteases suggested that these two activities of dismuting O^·₂ and cleaving supercoiled-DNA involved in different sites in SODs.