With the use of the DNA comet assay and immunocytochemical staining, regularities have been studied in the induction and repair of DNA double-strand breaks(DSBs) in human cells after exposure to Co-60 -rays and accelerated heavy ions with different linear energy transfer (LET) in the presence of the DNA repair inhibitors cytosine arabinoside and hydroxyurea. It is shown that for heavy ions the agents' modifying effect decreases with increasing particles' LET. The approach involving DNA synthesis inhibitors used in this study allows an estimation of the proportion of enzymatic DNA DSBs in total DSB yield after exposure to ionizing radiations of different quality.