The European Standards Committee on Oxidative DNA Damage (ESCODD) was set up to resolve the problems associated with the measurement of background levels of oxidative DNA damage (in particular 8-oxo-7,8-dihydroguanine, or 8-oxoGua) in human cells. A tendency for DNA oxidation to occur during sample preparation prior to chromatography has been recognized as the source of a very substantial artefact. To assess the success of attempts to eliminate the artefact, ESCODD has distributed to its members standard samples of pig liver and HeLa cells for analysis. Estimates of 8-oxoGua in pig liver, using chromatographic techniques, ranged from 2.23 to 441 per 106 guanines, with a median of 10.47 per 106 guanines. Chromatographic analysis of HeLa cell DNA gave a range of 1.84 to 214 per 106 guanines with a median of 5.23 per 106 guanines. HeLa cell DNA was also analysed by an enzymic approach, in which whole cell DNA was treated with formamidopyrimidine DNA glycosylase, which nicks DNA at sites of 8-oxoGua, and the breaks measured with the comet assay, alkaline unwinding or alkaline elution. Values with these methods ranged from 0.06 to 4.98 8-oxoGua per 106 guanines, with a median of 0.79 per 106 guanines. Although there are clearly still serious discrepancies between methods and laboratories, the lowest estimates by chromatography (arguably those in which the artefact was best controlled) are only 2.5 times higher than the median value obtained with the enzymic approach.