In the present study, the effects of extracts and polyphenol-rich fractions as well as monomer polyphenols identified in them, from both red and white grapes, on mitomycin C (MMC) induced sister chromatid exchanges (SCEs) in human peripheral blood lymphocytes were investigated. The grape extracts and two of the three polyphenol-rich fractions promoted MMC-induced SCEs at concentrations from 75 to 300 microg/mL. However, none of the extracts or fractions alone induced SCEs. Thus, these results suggest caution especially with regard to the use of grape extracts as dietary supplements. On the other hand, the fact that these extracts were not genotoxic alone may indicate a selective activity against genetically damaged cells. This is the first study regarding the clastogenic effects of grape extracts in human cells. Moreover, from the tested polyphenols, caffeic acid, gallic acid, and rutin hydrate enhanced MMC-induced clastogenicity, whereas ferulic acid, protocatechuic acid, (+)-catechin, (-)-epicatechin, and trans-resveratrol had no effect at concentrations between 5 and 100 microM. The differences in the chemical structures of the tested polyphenols may account for their differential effects on MMC clastogenicity.