8-Oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) is one of the mutagenic base modifications produced in DNA by the reaction of reactive oxygen species. The biological significance of 8-oxo-dG is shown by the existence of repair pathways that are able to recognize and remove this lesion from both DNA and the nucleotide pool. The final outcome of these evolutionarily conserved repair mechanisms in man is excretion of 8-oxo-dG/8-oxo-Gua from the intracellular to extracellular milieu including the blood plasma and urine. The aim of this investigation was to establish dose response relations for radiation-induced appearance of extracellular 8-oxo-dG in cellular model systems. Here we report on excretion of 8-oxo-dG after in vitro irradiation of whole blood and isolated lymphocytes with clinically relevant doses. We find that this excretion is dependent on dose and individual repair capacity, and that it saturates above doses of 0.5-1 Gy of gamma radiation. Our data also suggest that the nucleotide pool is a significant target that contributes to the levels of extracellular 8-oxo-dG; hence the mutagenic target for oxidative stress is not limited to the DNA molecule only. We conclude that extracellular 8-oxo-dG levels after in vitro irradiation have a potential to be used as a sensitive marker for oxidative stress.