A major product of the radiation-induced decomposition of 2'-deoxyguanosine in oxygen-saturated aqueous solution has been isolated by reverse phase high performance liquid chromatography and characterized by carbon and proton NMR spectroscopy, fast-atom bombardment mass spectrometry, and chemical analysis as (2S)-2,5'-anhydro-1-(2'-deoxy-beta-D-erythro-pentofuranosyl)-5-guanid inylidene- 2-hydroxy-4-oxoimidazolidine(d < G). This compound is stable in aqueous solution at room temperature but decomposes upon heating (45 degrees C). The lesion is also observed following type I (riboflavin, benzophenone, and acetophenone) photosensitized irradiation of 2'-deoxyguanosine at 350 nm in oxygen-saturated aqueous solution. A similar reaction mechanism, involving a neutral guanine radical intermediate, is proposed to explain the generation of d < G following both types of irradiation.