Nocardia asteroides resists killing by neutrophils despite the occurrence of the oxidative metabolic burst when the organism is phagocytosed. In a study of the apparent resistance of N asteroides to oxygen-dependent killing by neutrophils, this organism and (for comparison) Staphylococcus aureus were exposed to metabolites of the oxidative metabolic burst. N asteroides was more resistant than S aureus to H2O2, hydroxyl radical, and singlet oxygen and to the combination of H2O2, lactoperoxidase, and iodide. The rate of iodination of N asteroides by neutrophils or by the combination of lactoperoxidase and H2O2 was significantly lower than that of S aureus. Lysates of N asteroides had 2.8 times more catalase than lysates of S aureus, but levels of superoxide dismutase were similar in the two lysates. A reduction in the level of catalase activity of N asteroides with aminotriazole or azide resulted in a modest decrease in resistance to oxidative metabolites. Thus, the relative resistance of N asteroides to killing appeared to be due partially but not completely to its relatively high level of catalase activity.