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 micro M) 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.