The protective effect of Baicalin, a flavonoid isolated from the root of Scutellaria baicalensis G., on H₂O₂-induced DNA single strand break (SSB) was examined in NIH3T3 mouse fibroblasts by Comet assay (single cell gel electrophoresis technique). When the cells were pulse-chased with H₂O₂ (0.1—0.5 mM) for 15 min in fetal bovine serum (FBS)-free Dulbecco's Modified Eagle's Medium (DMEM), SSB occurred in the DNA as reported elsewhere in dose-dependent manner. Baicalin (50, 100 μM) which was incubated with the cells for 24 h before the H₂O₂ chase did not give rise to significant protection against the SSB formation. However, when the time required to cause a change in the DNA damage histogram obtained by the Comet assay was precisely examined after the H₂O₂ chase, it was found that the H₂O₂ induced SSB was more promptly repaired in the cells pretreated with Baicalin prior to the H₂O₂ chase, compared to untreated control cells. At the same time, the cell viability examined by 3-(4,5-dimethylthiazol-2-yl) 2,5-diphenyltetrazolium bromide) (MTT) assay after the H₂O₂ abuse was moderately recovered in the Baicalin increased by the Baicalin treatment. It was thus concluded that Baicalin that was known as an antioxidant flavonoid in vitro also functions as a biological response modifier, improving the cellular repair potential of oxidatively damaged DNA.