[en] The aim of study is to investigate the induction of DNA damage and chromosomal aberrations (CA) by irradiation with mixed beam from Brookhaven Medical Research Reactor (BMRR) as well as alterations in biological effect due to boron pretreatment. Two different methods were applied to estimate the biological effect in normal and boron enriched cells: the single cell gel electrophoresis (SCGE) technique and the cytogenetic assay. In normal human lymphocytes and in the GS-L9 rat gliosarcoma cells, significant radiation dose-dependent increase of DNA and cytogenetic damage were observed. In boron-enriched cells, significantly higher level of damage was observed compared with the non-enriched cells for both cell types. Levels of the DNA damage detected in cells with and without boron enrichment were in a good agreement with a biological effectiveness of the beam evaluated on the base of the BNCT treatment planning procedure. However, the relatively high level of the DNA damage observed in the boron-enriched and non-irradiated gliosarcoma cells suggests that boric acid per se may also induce the damage in cells. Therefore, boron enrichment in cancer cells may be genotoxic and enhances the therapeutic effects of neutrons at low doses. Our data also indicates that biological efficiency of the boron neutrons capture therapeutic beam in spite of having a high impact from gamma rays (∼73% of the dose) is 9.3 times more effective than X-rays alone. (author)