Hydrogen peroxide accumulated in aerated cultures of Streptococcus lactis C10, the amount depending on the sugar being utilized. When the energy source was galactose or (to a lesser extent) lactose or maltose, H2O2 reached autoinhibitory levels. With glucose, H2O2 reached levels which did not cause obvious inhibition, and then declined. Washed cell suspensions utilized several media ingredients including glucose, galactose and yeast extract as substrates for O2 uptake, but cell-free extracts utilized none of these. The only substrate which supported O2 uptake by cell-free extracts was NADH. The activity of NADH oxidase was not affected by several intermediates of carbohydrate metabolism. Evidence was obtained to show that its synthesis was regulated via induction by O2 with an overriding catabolite repression type control, when cells were grown at the expense of glucose.