The activity of the copper- and zinc-containing superoxide dismutase decreased with increasing ionic strength. Modification of lysine residues by acetylation or succinylation inverted the effect of increasing ionic strength, whereas modification of arginine with phenylglyoxal did not. These results were noted in both photochemical and pulse-radiolysis assays. It appears that interaction of O2- with the anionic enzyme is assisted by the positive charge on lysine residues, presumably those close to the active site. By the criterion of responsiveness to ionic strength, the arginine residue close to the active site does not appear to provide electrostatic facilitation to the catalytic process. Elimination of the charge on epsilon-amino groups by raising the pH suppressed activity to the same extent as did elimination of these charges by acetylation. Activity was similarly suppressed to the same extent by covalent modification or by ionization of arginine residues, indicating that the positive charge on arginine is important for the catalytic process even though its effect is not responsive to changes in the ionic strength of the solution.