The aim of this study was to investigate the chemoprotective effects of water and garden cress juices towards benzo(a)pyrene (B(a)P)-induced DNA damage using the single cell gel electrophoresis (SCGE)/Hep G2 test system. This experimental model combines the advantages of the SCGE assay with that of human derived cells possessing inducible phase I and phase II enzymes. Treatment of Hep G2 cells with small amounts of water cress or garden cress juice (0.1-1.25 mul/ml) and B(a)P reduced the genotoxic effect of the latter in a dose-dependent manner. Contrary to the results with the juices, unexpected synergistic effects were observed with benzyl isothiocyanate (BITC, 0.6 muM), a breakdown product of glucotropaeolin contained abundantly in garden cress. Although these concentrations of BITC did not cause DNA damage per se, at higher concentrations ( greater than or equal to 2.5 muM), the compound caused a pronounced dose-dependent DNA damage by itself. With phenethyl isothiocyanate (PEITC), the breakdown product of gluconasturtin contained in water cress, no synergistic effects with B(a)P were seen; however, significant induction of DNA damage was observed when the cells were exposed to the pure compound at concentrations greater than or equal to 5 muM. In experiments with (+/-)-anti-benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide (BPDE, 5.0 muM), the ultimate genotoxic metabolite of B(a)P, and the juices, only moderate protective effects were seen indicating that detoxification of BPDE is not the main mechanism behind the protective effect of the juices against B(a)P-induced DNA damage. In conclusion, our findings show that garden and water cress juices are highly protective against B(a)P-induced DNA damage in human derived cells and that their effects can not be explained by their isothiocyanate contents. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.