Isolation and characterization of a novel radiation-induced product, i.e., the 8-hydroxyguanine residue, produced in DNA, 2''-deoxyguanosine and 2''-deoxyguanosine 5''-monophosphate by .gamma.-irradiation in aqueous solution, are described. For this purpose, .gamma.-irradiated DNA was first hydrolyzed with a mixture of 4 enzymes, i.e, DNase I, spleen and snake venom exonuclease, and alkaline phosphatase. Analysis of the resulting mixture by capillary gas chromatography-mass spectrometry after trimethylsilylation revealed the presence of a product, which was identified as 8-hydroxy-2''-deoxyguanosine on the basis of the typical fragment ions of its trimethylsilyl (Me3Si) derivative. This product was then isolated by using reversed-phase high-peformance liquid chromatography. The UV and proton NMR spectra taken from the isolated product confirmed the structure suggested by the mass spectra of its Me3Si derivatives. The accurate molecular mass of the Me2Si derivatives of the isolated product was determined by MS. The obtained value agreed with the theoretical molecular mass within 1 millimass unit. The yield of 8-hydroxyguanine was also measured. Its mechanism of formation is believed to involve OH radical addition to the C-8 position of guanine followed by oxidation of the radical adduct.