During phagocytosis, neutrophils generate reactive oxygen metabolites and release lysosomal enzymes into the extracellular medium. We have investigated the possibility that these enzyme are inactivated by the oxygen compounds. Phagocytosing neutrophils from 12 patients with chronic granulomatous disease, which do not generate these oxygen metabolites, released two to three times more activity of lysozyme and beta-glucuronidase than did normal neutrophils. This difference proved to be due to a decrease of approximately 20% of the total activity of these enzymes in normal neutrophils, but not in neutrophils of patients with chronic granulomatous disease. This inactivation of enzymes took place during phagocytosis of opsonized zymosan particles as well as during stimulation of normal cells with phorbol myristate acetate. The inactivation was not due to formation of inhibitors. The lysosomal enzymes were not activated when the neutrophils were stimulated under anaerobic conditions. Addition of catalase, superoxide dismutase, or albumin gave no protection against the oxidative damage; reduced glutathione gave partial protection. The oxidative inactivation was more pronounced in the presence of azide. Measurement of the activity and the amount of protein of acid alpha-glucosidase in the cells showed that the specific activity of this enzyme decreased by approximately 50% during 30 min of phagocytosis. This indicates that the inactivation of the lysosomal enzymes takes place in the phagolysosomes, before the enzymes have leaked into the extracellular medium.