To assess the antimutagenic potentials of ginsenoside Rh2 (Rh2), its effects on the baseline and mitomycin C-induced sister chromatid exchange (SCE) were examined in human peripheral blood lymphocytes (PBLs). The SCE frequency in PBLs treated with various concentrations of Rh2 for 72 h was decreased in a dose-dependent manner and was significantly lower than the baseline levels at 1.0 x 10(-10) M and 1.0 x 10(-7) M. The SCE frequency in PBLs treated with both Rh2 and mitomycin C was significantly (P < 0.001) less than that in PBLs treated with only mitomycin C. Cell cycle kinetics, as indicated by the proliferation and mitotic indices, was not significantly affected by Rh2 of various concentrations in the PBLs throughout the present experiments. This is the first report which showed convincingly a reduction of SCE in normal human cells. The mechanism remains to be elucidated in future studies.