To characterize fullerenes (C-80 and C-70) as photosensitizers in biological systems, the generation of active oxygen species, through energy transfer (singlet oxygen O-1(2)) and electron transfer (reduced active oxygen radicals such as superoxide anion radical O2-. and hydroxyl radical (OH)-O-.), was studied by a combination of methods, including biochemical (DNA-cleavage assay in the presence of various scavengers of active oxygen species), physicochemical (EPR radical trapping and near-infrared spectrometry), and chemical methods (nitro blue tetrazolium (NBT) method). Whereas O-1(2) was generated effectively by photoexcited C-60 in nonpolar solvents such as benzene and benzonitrile, we found that O2-. and (OH)-O-. were produced instead of O-1(2) in polar solvents such as water, especially in the presence of a physiological concentration of reductants including NADH. The above results, together with those of a DNA cleavage assay in the presence of various scavengers of specific active oxygen species, indicate that the active oxygen species primarily responsible for photoinduced DNA cleavage by C-60 under physiological conditions are reduced species such as O-2(-.) and (OH)-O-..