The killing by human polymorphonuclear leukocytes of several species of bacteria, some of which were catalase positive, was examined in vitro in aerobic and anaerobic conditions. When all conditions other than the oxygen tension were identical, killing after 30 min was slightly greater in aerobic than in anaerobic conditions. However, after 60 and 120 min the difference between aerobic and anaerobic killing was smaller, and killing was nearly complete for all strains tested. These results conflict with the common opinion that oxygen is essential for efficient killing. Minor differences in experimental conditions can greatly influence results, and may be responsible for the discrepancy between this study and some previous studies on this subject.