Bleomycin, in the presence of ferric salts, oxygen and a suitable reductant, degrades DNA with the release of base propenals, detected as thiobarbituric acid (TBA) reactivity, and the formation of 8-hydroxydeoxyguanosine (8OHdG) detected by HPLC. When xanthine oxidase is added to the incubated mixture of DNA degradation products, TBA-reactivity is destroyed but 8OHdG formation is increased. EPR Spin trapping experiments show that hydroxyl radicals (OH) are formed in the reaction mixture and can be inhibited by the inclusion of either superoxide dismutase or catalase. These findings suggest that the base propenals and possibly malondialdehyde, formed from them, are aldehydic substrates for xanthine oxidase and, the product of this reaction is superoxide (O2-) and hydrogen peroxide (H2O2). Thus, TBA reactivity is destroyed in the formation of O2- and H2O2 which stimulate further oxidative damage to DNA resulting in increased 8OHdG formation.