NADH oxidase was purified to homogeneity from aerobic cells of Sporolactobacillus inulinus. The N-terminal amino acid sequence showed high similarity to those of the NADH oxidase from Amphibacillus xylanus and the H2O2-forming NADH oxidase from Streptococcus mutans. S. inulinus NADH oxidase is a homodimer composed of subunits and with a molecular mass of 53 kDa, It catalyzes the reduction of oxygen to hydrogen peroxide, and, in the presence of a low molecular weight disulfide-containing component (AhpC) from A. xylanus, of hydrogen peroxide to water, The V-max values for hydrogen peroxide and cumene hydroperoxide reduction were 9,300 and 8,500 min(-1), respectively at 25 degrees C. These values are similar to the V-max values for hydrogen peroxide and cumene hydroperoxide reduction with the NADH oxidase from A. xylanus. Cell extracts from S. inulinus cultures grown under anaerobic and aerobic conditions reacted with antibodies against the AhpC protein from A. xylanus. An immunoreactive AhpC-like 21 kDa protein was induced in S. inulinus under aerobic, but not under anaerobic conditions, The results suggest that AhpC is involved in the aerobic metabolism of S. inulinus, which lacks a respiratory chain and catalase. The AhpC-NADH oxidase system is thought to function as a scavenging system for hydrogen peroxide formed aerobically.