Abstract: The prooxidant and apoptosis-inducing effects of Sho-saiko-to, a traditional Sino-Japanese herbal medicine and its active ingredients were analyzed. Among the components of Sho-saiko-to, wogon, the extract of Scutellaria and licorice root extract induced apoptosis of HL60 cells and increased the intracellular levels of reactive oxygen species. Lower concentrations (5 to 20 μM) of baicalein, the principal flavonoid in the Scutellaria root extract, showed induction of cell apoptosis and elevated the intracellular reactive oxygen species. However, the increase in the concentrations of baicalein rather inhibited the induction of apoptosis and the elevated levels of reactive oxygen species in cells. Induction of baicalein-mediated apoptosis was inhibited by addition of Tempol, the scavenger of reactive oxygen species. Glycyrrhetinic acid, an ingredient of licorice root extract, also induced apoptosis followed by increase in the intracellular reactive oxygen species. The effect of Sho-saiko-to on cell differentiation can be explained by the action of two ingredients, baicalein and glycyrrhetinic acid, which cause apoptosis and increase in reactive oxygen species in cells.