Oxygen uptake by washed cell suspensions of Bifidobacterium bifidum DSM 20082 was studied by using spectrophotometric measurements of the degree of oxygenation of added myoglobin as a measure of the concentration of dissolved O2. The absorbance changes during consumption of O2 in a closed reaction vessel were analysed by computer to obtain estimates of the changes in dissolved O2 concentration with time. These were then used to calculate the rate of O2 uptake as a function of the dissolved O2 concentration. The cell suspensions showed Michaelis-Menten kinetics with an apparent Km value of 0.06 microM O2. Cell-free extracts contained a soluble NADH oxidase activity with a stoichiometry corresponding to the reduction of O2 to H2O and with a high affinity for O2.