In this paper, we present evidence that Ginsenoside-Rs3 (G-Rs3), a new diol-type ginseng saponin isolated from the roots of Panax ginseng C.A. Meyer, efficiently arrests the cell cycle at the G1/S boundary at lower doses, 0.1-5 microM, but induces apoptosis at higher doses, 10-25 microM, the effects of which were associated with selectively elevating protein levels of p53 and p21WAF1 in SK-HEP-1 cells. The cell growth suppressive and apoptosis inducing effects were confirmed by MTT assays together with flow cytometric analyses, morphological changes and DNA fragmentation. Immunoblotting showed that G-Rs3 significantly elevated protein levels of p53 and p21WAF1 prior to inducing apoptosis, while it did not elevate those of cyclin E, cyclin A, p27Kip1, and PCNA. Immune complex kinase assays showed that G-Rs3 downregulated the activities of both cyclins E- and A-associated kinases. Collectively, we suggest that G-Rs3 selectively elevates protein levels of p53 and p21WAF1 and hence downregulates the activities of the cyclin-dependent kinases, resulting in cell cycle arrest at the G1/S boundary. We also propose that apoptosis induced by G-Rs3 is related to the elevations of p53 and p21WAF1 in the cells.