Tobacco-specific nitrosamines (TSNAs), nitrosonornicotine (NNN) and 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK), are metabolites of nicotine and the major carcinogens in cigarette smoke. To evaluate the effect of oxygen radicals on TSNA-induced genetic damage, MRC-5 fetal human lung cells were exposed to NNN and NNK (5 mM) and DNA single-strand breaks measured. Both NNN and NNK produced a dose-dependent increase in strand breaks up to 10 mM which was cytotoxic. In combination with enzymatically-generated oxygen radicals, strand breakage increased by approximately 50% for both NNN and NNK. Oxygen radical scavengers (superoxide dismutase, catalase, mannitol) significantly reduced the DNA damage caused by both the TSNAs and TSNAs plus oxygen radicals, suggesting that the genotoxicity is radical-mediated. Because both superoxide dismutase and catalase were protective, the hydroxyl radical may be playing an important role in the mediation of the DNA damage observed.