During the last years, much attention was focused on the measurement of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) as a marker of oxidative DNA damage. Among various analytical techniques, the (32)P-postlabeling assay has been applied in determination of 8-oxo-dG. However, artefactual DNA oxidation could take place during the work-up procedures leading to over-estimate the level of 8-oxo-dG. In the present study, we optimized the (32)P-postlabelling assay with thin layer chromatography to measure 8-oxo-dG in standard samples of 8-oxo-dG, calf thymus DNA and primary cultured rat hepatocytes. The background levels of 8-oxo-dG in calf thymus DNA and in primary cultured rat hepatocytes were lesser than those determined by the previously described (32)P-postlabeling procedures and were in the range of those determined by chromatography methods (GC-MS, HPLC-MS).