The yeast Candida mycoderma and its mutant lacking cytochrome oxidase and cytochromes b were grown in the glucose-mineral Rieder medium and liberated hydrogen peroxide. The evolution of hydrogen peroxide was found also in the resting cells of the parent strain and its mutant when they oxidized glucose, ethanol and endogenous substrates. The evolution of hydrogen peroxide was registered also during the growth of other yeast cultures, in particular, those belonging to Saccharomyces and Torulopsis.