The content of 8-oxoguanine, a biomarker of DNA damage by the action of reactive oxygen species, in native and denatured DNA upon heating at 37 degrees C was studied by the enzyme-linked immunosorbent assay using monoclonal antibodies against 8-oxoguanine. It was found that the content of 8-oxoguanine changes with time in a complicated multiphase manner, the maximum changes being as great as twofold. The production of hydrogen peroxide in water and 1 mM PBS, pH 6.8, at 37 degrees C over a period of 50 h was determined by the method of enhanced chemiluminescence in a peroxide-luminol-p-iodophenol system. The generation of hydrogen peroxide also changed in a complicated multiphase manner. After heating the DNA at 80 degrees C for 24 h, guanine oxidation products were excised by 8-oxoguanine-DNA-glycosylase. The products were separated and analyzed by liquid column chromatography on Sephadex LH-20 and Toyopearl HW-40 gel. The products were identified from UV adsorption spectra. The results indicated the generation of reactive oxygen species at 37 degrees C, which leads both to the generation of 8-oxoguanine in DNA and its elimination as a result of its further oxidation. The oxidation of 8-oxoguanine was accompanied by the formation of a number of unstable products of further oxidation of 8-oxoguanine. Among these products, aminoimidazolone, spiroiminodigidantoin, and diiminoimidazole were identified from UV spectra. The appearance of the products of further oxidation of 8-oxoguanine explains the origin of G : C --> C : G transversions by the action of reactive oxygen species.