Cell-free extracts of Campylobacter sputorum subspecies bubulus contained superoxide dismutase. The enzyme was located in the cytoplasmic fraction and insensitive to cyanide. After centrifuging a cell-free extract at 144000 x g for 1.5 h the total activity in the supernatant fraction was threefold higher than in the crude cell-free extract. The pellet fraction thus obtained was shown to have a lowering effect on superoxide dismutase activities from different sources in the assay method used here. C. sputorum responded to a raised oxygen tension in the culture by an increase in the superoxide dismutase activity. The ability to produce superoxide anion radicals (O2-.) during oxidation of formate and lactate was demonstrated. Furthermore C. sputorum was found to produce H2O2 while oxidizing formate. In experiments in which the reduction of cytochrome c by formate was followed, step-wise kinetics were observed. One of the steady states then obtained was attributed to the oxidizing action of H2O2, because it was abolished by the addition of catalase and lengthened by H2O2 added in addition to H2O2 formed as a product of formate oxidation. An overall reaction for formate oxidation by C. sputorum is discussed.