The effects of two polyphenols, chebulinic acid and tellimagrandin I, on DNA strand breaks mediated by H2O2/Cu(Ⅱ), hydroquinone (HQ)/Cu(Ⅱ) and H2O2/Fe(Ⅱ) in pBR322 plasmid DNA and genomic DNA of cultured MRC-5 human embryo lung fibroblasts were examined. The results demonstrated that chebulinic acid and tellimagrandin I obviously inhibited HQ/Cu(Ⅱ)-and H2O2/Cu(Ⅱ)-mediated pBR322 DNA strand breaks. When MRC-5 cells were treated with HQ/Cu(Ⅱ), the presence of chebulinic acid or tellimagrandin I inhibited HQ/Cu(Ⅱ)-mediated double strand breaks of genomic DNA. The presence of chebulinic acid or tellimagrandin I did not affect the H2O2-and HQ-mediated reduction of Cu(Ⅱ) to Cu(I). Both polyphenols could slightly inhibit H2O2/Fe(Ⅱ)-mediated plasmid DNA strand break at the lower concentration (1-10 lM), but potentiate the DNA strand break at the higher concentration (over 50 lM). These results demonstrated that chebulinic acid and tellimagrandin I possessed antioxidant action in certain conditions and exerted prooxidant action on DNA strand breaks in other conditions.