[en] For years mercury has been considered to be dangerous for human health. It was shown in vitro studies that mercury ions produce various types of DNA damage: single strand breaks as well as alkali labile lesions. The aim of this study was to compare levels of the DNA damage and cytogenetic damage induced in vivo, DNA's susceptibility to radiation, as well as repair capabilities of DNA damage induced in vitro by UV-exposure or X-rays, in lymphocytes from unexposed donors and from persons occupationally exposed to mercury vapours. In order to estimate cytogenetic damage, the analysis of sister chromatid exchange frequency (SCE) was used, while to detect DNA damage alkaline version of the single cell gel electrophoresis (SCGE) was applied. To analyse in vitro susceptibility of cells to genotoxic factors such as UV-C or X-rays, lymphocytes were exposed to 6 J/m² of UV or irradiated with 2 Gy of X-rays. After exposure the cells were incubated for 2 hours with or without the presence of phytohemagglutinin (agent stimulating cell divisions). In the present study, results do not show any statistically significant differences between the examined groups, either in the levels of DNA damage of untreated lymphocytes or in the sister chromatid exchanges. Neither the levels of DNA damage detected in lymphocytes after UV exposure and 2 h incubation, without or in the presence of a cell- division stimulating agent, nor the repair efficiencies of the DNA damage induced by UV exposure, differed significantly between the unexposed group and that occupationally exposed to mercury vapours. Statistically significant higher levels of DNA damage measured in X-ray-irradiated lymphocytes, without incubation and after 2 hours of incubation, with or without the presence of phytohemagglutinin, were observed in the group exposed to mercury vapours. So, donors exposed to mercury vapours have shown a statistically lower repair efficiency of X-ray-induced DNA damage both in non- stimulated lymphocytes (70.8% for the exposed, 84.5% for the unexposed) and stimulated lymphocytes (85.7% for the exposed and 90.4% for the unexposed). (author)