In this study, lomefloxacin (LMX), a widely used quinolone antibiotic with a high frequency of clinical phototoxicity, was investigated by measuring the effects of several antioxidants on its ability to form of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) in cultured adult rat liver cells after exposure to UVA. In the current study the observed DNA damage, reflected by the formation of 8-oxo-dG, was almost completely inhibited by co-incubation of LMX and cultured cells with sodium azide (NaN3) that specifically quenches singlet oxygen. Vitamin E (alpha-tocopherol), known to quench both superoxide and singlet oxygen, inhibited 8-oxo-dG formation by approximately 54%. Mannitol, a hydroxyl radical scavenger, inhibited 8-oxo-dG formation by 64%. Butylated hydroxyanisole (BHA), a scavenger of hydroxyl, peroxy and alkoxy radicals, showed no inhibition of 8-oxo-dG formation but in fact enhanced levels of 8-oxo-dG by 169%. The results of this study suggest that the mechanism for the photodynamic generation of 8-oxo-dG by LMX is mediated, at least in part, by both singlet oxygen and hydroxyl radical and involves both type I and type II photosensitization.