Pulsed field gel electrophoresis (PFGE) was used to quantitatively measure the double-strand breaks (dsbs) of DNA damage in human lymphocytes induced by hydroxyl radicals generated from H₂O₂－Fe³⁺ system.It was shown that there was a significant dose-dependent relationship between concentration of H₂O₂－Fe³⁺ and the content.of DNAdsbs occuring in cells. Moreover, the DNA damage in terms of dsbs became heavier as the time of incubating cells with H₂O₂－Fe³⁺ was extended but could be inhibited by catalase considerably.The sensitivity of PFGE was estimated by the concentration of H₂O₂－Fe³⁺ which were 0.3 mmol/L and 6 pmol/L, respectively.