Water-soluble ingredients of the herbal medicine sho-saiko-to dose-dependently inhibited the proliferation of a human hepatocellular carcinoma cell line (KIM-1) and a cholangiocarcinoma cell line (KMC-1). Fifty % effective doses on day 3 of exposure to sho-saiko-to were 353.5 +/- 32.4 micrograms/ml for KIM-1 and 236.3 +/- 26.5 micrograms/ml for KMC-1. However, almost no suppressive effects were detected in normal human peripheral blood lymphocytes or normal rat hepatocytes. Sho-saiko-to suppressed the proliferation of the carcinoma cell lines significantly more strongly than did each of its major ingredients, i.e., saikosaponin a, c, and d, ginsenoside Rb1 and Rg1, glycyrrhizin, baicalin, baicalein, and wogonin, or another herbal medicine, juzen-taiho-to (P < 0.05 or 0.005). Because such ingredients are barely soluble in water, there could be synergistic or additive effects of the ingredients in sho-saiko-to. Morphological, DNA, and cell cycle analyses revealed two possible modes of action of sho-saiko-to to suppress the proliferation of carcinoma cells; (a) it induces apoptosis in the early period of exposure and (b) it induces arrest at the G0/G1 phase in the late period of exposure.