The iron-containing superoxide dismutase from Escherichia coli is inactivated by H2O2 to a limit of approximately 90%. When corrected for the H2O2-resistant portion, this inactivation was first order with respect to residual activity and exhibited a pseudo-first-order rate constant of 0.066 min-1 at 25 degrees C in 0.24 mM H2O2 at pH 7.8. The superoxide dismutase activity remaining after treatment with H2O2 differed from the activity of the native enzyme with respect to heat stability, inhibition by azide, and inactivation by light in the presence of rose bengal and by N-bromosuccinimide. The native and the H2O2-modified enzymes were indistinguishable by electrophoresis on polyacrylamide gels. Inactivation of the enzyme by H2O2 was accompanied by loss of tryptophan and some loss of iron, but there was no detectable loss of histidine or of other amino acids. H2O2 treatment caused changes in the optical spectrum of the enzyme. Inactivation of the enzyme by H2O2 depends upon the iron at the active site. Thus, the apoenzyme and the manganese-substituted enzyme were unaffected by H2O2. We conclude that reaction of H2O2 with the iron at the active site generates a potent oxidant capable of attacking tryptophan residues. A mechanism is proposed.