A new NADH oxidase, useful for the regeneration of NAD+, was isolated and characterized from Lactobacillus brevis. In crude extracts the activity was from 10-15 U mg(-1). After purification by four chromatographic steps, an activity of 116 U mg(-1) was obtained with 14% yield. Highest activity was from pH 5.5-7 and at 40degreesC. The enzyme requires dithiothreitol to prevent oxidative deactivation. The K-m value for NADH was 24 muM.