Oxygen free radicals generated by the interaction of Fe2+ and H2O2 (Fenton reaction) are capable of reacting with DNA bases, which may induce premutagenic and precarcinogenic lesions. Products formed in DNA by such reactions have been characterized as hydroxylated derivatives of cytosine, thymine, adenine, and guanine and imidazole ring-opened derivatives of adenine and guanine. As shown here by 32P-postlabeling, incubation of DNA under Fenton reaction conditions gave rise to additional oxidation products in DNA that were characterized as putative ribonucleosides by enzymatic hydrolysis of the oxidized DNA, 32P-postlabeling, and co-chromatography in multiple systems with authentic markers. Formation of these products in DNA was enhanced by the presence of L-ascorbic acid in the reaction mixtures and their total amounts were similar to those of the major DNA oxidation product, 8-hydroxy-2'-deoxyguanosine. The ribonucleoside guanosine was also formed in kidney DNA of male rats treated with ferric nitrilotriacetate, a renal carcinogen. It is postulated that ribonucleotides alter conformation and function of DNA and thus their presence in DNA may lead to adverse health effects.