Oxidative DNA damage is generated in every tissue in the body, and is mainly excised by glycosylases. However, endonucleases and exonucleases are suggested as being present in tissues, since oxidized nucleosides such as thymidine glycol and 8-Oxo-2'-deoxyguanosine are detected in urine. For this reason, we studied repair enzymes induced in regenerating rat liver, and detected an enzyme cleaves phosphodiester bonds on the 5'side of 8-oxo-2'-deoxyguanosine nucleotides in DNA. The coexistence of this enzyme and phosphodiesterase II results in the release of 8-oxo-2'-deoxyguanosine monophosphate from calf thymus DNA enriched with 8-oxoguanine by γ-ray irradiation. This enzyme is found in regenerating rat liver, but not in normal rat liver. The enzyme may have a specific connection to DNA replication.