The effect of ribonucleosides on 8-oxoguanine formation in salmon sperm DNA dissolved in 1 mM phosphate buffer, pH 6.8, upon exposure to gamma rays was examined by ELISA using monoclonal antibodies against 8-oxoguanine. Nucleosides (1 mM) decreased the radiation-induced yield of 8-oxoguanine in the order Guo > Ino > Ado > Thd > Urd > Cyd. Guanosine and inosine considerably reduced deamination of cytosine in the DNA solutions upon heating for 24 h at 80 degrees C. The action of nucleosides on the heat-induced generation of reactive oxygen species in the phosphate buffer was studied. The concentration of hydrogen peroxide was measured by enhanced chemiluminescence in a peroxidase-luminol-p-iodophenol system; the hydroxyl radical formation was measured fluorometrically by the use of coumarin-3-carboxylic acid. Guanosine and inosine considerably decreased the heat-induced production of both hydrogen peroxide and OH radicals. Guanosine and inosine increased survival of mice after a lethal dose of radiation. They especially enhanced the survival of animals when were administered shortly after irradiation. The results indicate that guanosine and inosine, natural antioxidants, prevent oxidative damage to DNA, decrease the generation of ROS, and protect mice against gamma-radiation-induced death.