Аннотация:The present study of the radiation-induced DSBs level in Chinese hamster V79 cells exposed to IR at various dose rates showed a significant decrease in the DSBs level per IR dose in cells irradiated at a lower dose rate compared to a higher dose rate. Evidently, current findings support the threshold hypothesis on the biological effects of IR. The dose rate appeared to be an essential factor in cellular response to IR determining biological effects induction and realization, probably more important than the absorbed dose. It is therefore advisable to consider the dose rate factor in the radiological protection standards.