Baicalein, an extract from Scutellaria baicalensis, was evaluated for its ability to inhibit the influenza virus in vivo. Oral administration of baicalein to BALB/c mice infected with the influenza A/FM1/1/47(H1N1) virus showed significant effects in preventing death, increasing the mean time to death, inhibiting lung consolidation, and reducing the lung virus titer in a dose-dependent manner. These effects are believed to be due to baicalin, the metabolite of baicalein in the serum. At a concentration of baicalin 2 microg/ml in overlay medium, it showed significant inhibition in the plaque assay, and the mean IC(50) value of baicalin was calculated as 1.2 microg/ml in the cytopathic effect assay. Our results showed that baicalein warrants further research as a potential antiinfluenza viral agent.