We have studied the co-mutagenic effect of verapamil and ascorbic acid on formation of radiation-induced chromosomal aberrations in T-lymphocytes of peripheral blood (PBL) of healthy donors. Test system of PBL treated in vitro with subsequent metaphase analysis of chromosomal aberrations has been used. Cells were exposed to γ-ray radiation in G 0 -and G 2 -periods of cell cycle and treated with verapamil (1.5; 2 and 4.0 µg/ml of blood) and ascorbic acid (20; 40 and 80 µg/ml of blood). The post-irradiation treatment of PBL with ascorbic acid in concentrations of 40 and 80 μg/ml, which exceeded therapeutic concentration value 2 and 4 times, increased overall chromosome aberrations frequency in 1.4 times compared to low dose (0.3 Gy) radiation effect. Verapamil at a concentration of 4.0µg/ml also increased the damaging effect of low dose of radiation in 1.5 times. Modification of radiation-induced cytogenetic effects in human PBL treated with co-mutagens was dependent on the concentration of the drugs, the absorbed dose of radiation, as well as cell radiosensitivity. It could be concluded that high concentrations of comutagens potentiate the damaging effect of low doses of ionizing radiation.