[en] In the report DNA strand breaks in V79 cells caused by γ-rays and the protective effect of total flavonoids of astragalus mongholicus Bunge (TFA) have been studied by fluorometric assay of DNA unwinding. The results indicated that the rate of DNA strand break increased with the increase of γ-ray doses. Even when the concentration of TFA was as low as 0.6 mg/ml, it still showed some protective effect on DNA strand breaks. When its concentration was up to 1.2 mg/ml, TFA could effectively protect DNA from radiation damage