The effect of xanthine oxidase, lactoperoxidase, and transition metals [Fe(III), Cu(II)] on the oxidation of ascorbate in raw milk was investigated. Data clearly showed that iron(III) (200 microM) does not accelerate ascorbate oxidation in raw milk in concentrations relevant for raw milk. In contrast, addition of copper(II) (10 microM) to the raw milk accelerated oxidation of ascorbate. Furthermore, both xanthine oxidase and peroxidase activity were found to accelerate ascorbate oxidation dramatically in raw milk, indicating that xanthine oxidase and lactoperoxidase might be some of the most obvious candidates for mediation of ascorbate oxidation in raw milk. The present data are discussed in relation to using the fate of ascorbate in raw milk as an indicator of the oxidative stability of the milk.