The cytogenotoxic effect of Na_2SeO_3 on lymphocytes in human whole blood culture was studied. This was done by means of analysis of sisterchromatid exchange (SCE) and determination of unscheduled DNA synthesis (UDS) with the use of scintillometric procedure and autoradiographic observation. The experiments showed that Na_2SeO_3induced a significant increase of SCE value and higher levels of UDS. These results indicate that Na_2SeO_3 has DNA-damaging potential and cytogenotocity, its threshold concentration was 6.48?10~(-7)mol/L.