To clarify the clinical significance of the redox-controlling effects of Kampo, a traditional Japanese herbal medicine, we determined the scavenging activities of various reactive oxygen species in clinically used Kampo formulae using an electron spin resonance-based technique. Formulae containing Rhei Rhizoma (i.e., mashiningan and daiobotanpito) showed high scavenging activity against the alkoxyl radical, and crude extract quantity was significantly correlated with scavenging activity. Hydroxyl radical scavenging activity was positively correlated with the quantity of Zingiberis Rhizoma . Strong hydroxyl radical scavenging activity was also found in formulae containing both Bupleuri Radix and Scutellariae Radix , a widely used anti-inflammatory combination. Formulae containing a clinically common combination of Scutellariae Radix , Coptidis Rhizoma , and Phellodendri Cortex induced high superoxide scavenging activity. Singlet oxygen scavenging activity was high in formulae containing Bupleuri Radix and Glycyrrhizae Radix . In contrast, formulae containing Rehmanniae Radix showed generally low reactive oxygen species scavenging activities, and the quantity of Rehmanniae Radix was negatively correlated with hydroxyl radical and singlet oxygen scavenging activities. These results indicate that the antioxidative effects of Kampo formulae are not uniform but complexly varied against multiple reactive oxygen species. Some formulae have almost no antioxidant effects but may act as pro-oxidants.