Active oxygen species are possible initiators of oxidative reactions in foods, and their elimination could provide one mechanism of antioxidative effect. In this work we studied the antioxidative properties of superoxide dismutase (SOD) from yeast (Saccharomyces cerevisiae) in various model systems. The crude yeast extract was partially purified by ammonium sulfate precipitation. The purified fraction was found to inhibit the oxidation of emulsified linoleic acid, emulsified cholesterol, and ascorbic acid. Catalase had a prooxidative effect in the linoleic acid system, while at high concentrations it protected ascorbic acid against oxidation. No synergistic effect was observed between catalase and SOD. In order to evaluate possible applications of yeast SOD as an antioxidant in food, the influence of pH and heat treatment on the antioxidative effect was studied. Within the range pH 4.5-9.0 the antioxidative effect of the SOD increased with increasing pH. In heat treatment experiments the SOD showed unusually high thermal stability and an interesting inactivation-reactivation behavior. The enzyme activity was for instance retained at about 60% of its original value after heating at 70°C for 15 min. A few percent of the original enzyme activity remained even after 30 min at 100°C. The possible technological implications of these results are discussed. © 1989 American Chemical Society.