The reaction of hydrogen peroxide with the copper-zinc bovine-liver superoxide dismutase at low molar ratios (0.2-20.0) of HzOz/active site between pH 7.3-10.0 leads to the loss of native enzyme as a distinct form monitored by electrophoresis. The pH dependence of the loss of native enzyme between 7.3 and 9.0 indicates the involvement of a conjugate base on the enzyme of pKa of 8.7 0.1. The rate of loss of the native enzyme is first order with respect to the concentration of both enzyme and hydrogen peroxide between pH 7.3 and 9.0 with no evidence for binding of peroxide. A second-order rate constant of 3.0 i 1.0 M-l s-' is obtained from these data.