Peroxidases catalyze many reactions, the most common being the utilization of H2O2 to oxidize numerous substrates (peroxidative mode). Peroxidases have also been proposed to produce H2O2 via utilization of NAD(P)H, thus providing oxidant either for the first step of lignification or for the "oxidative burst" associated with plant-pathogen interactions. The current study with horseradish peroxidase characterizes a third type of peroxidase activity that mimics the action of catalase; molecular oxygen is produced at the expense of H2O2 in the absence of other reactants. The oxygen production and H2O2-scavenging activities had temperature coefficients, Q10, of nearly 3 and 2, which is consistent with enzymatic reactions. Both activities were inhibited by autoclaving the enzyme and both activities had fairly broad pH optima in the neutral-to-alkaline region. The apparent Km values for the oxygen production and H2O2-scavenging reactions were near 1.0 mM H2O2. Irreversible inactivation of horseradish peroxidase by exposure to high concentrations of H2O2 coincided with the formation of an absorbance peak at 670 nm. Addition of superoxide dismutase (SOD) to reaction mixtures accelerated the reaction, suggesting that superoxide intermediates were involved. It appears that horseradish peroxidase is capable of using H2O2 both as an oxidant and as a reductant. A model is proposed and the relevance of the mechanism in plant-bacterial systems is discussed.