OBJECTIVE: In the present study, the antiviral effects of polyphylla saponin I isolated from Parispolyphylla on influenza A virus are investigated both in vitro and in vivo. METHODS: Column chromatography and reversed phase liquid chromatography separation technology were used to extract and purify polyphylla saponin I. The purity of polyphylla saponin I was assayed by high performance liquid chromatography. Methyl thiazolyl tetrazolium assay and analyses of cytopathic effects were performed to examine the antiviral activity of polyphylla saponin I upon MDCK cells infected with influenza A virus. Model mice were made by intranasal inoculation of influenza a virus. Mice infected with influenza A virus were orally administered polyphylla saponin I and oseltamivir twice a day for 5 days to study their antiviral efficacy in vivo. RESULTS: Polyphylla saponin I had no cytotoxicity on MDCK cells at the concentration of 50 μg/mL. Polyphylla saponin I (6.25, 12.5, 25 and 50 μg/mL) and oseltamivir (40 μg/mL) had remarkable inactivation effects on influenza A virus, prevention effects on influenza A virus adsorption on MDCK cells, and inhibitory effects on the reproduction of influenza A virus in MDCK cells. In addition, polyphylla saponin I (5 and 10 mg/kg), and oseltamivir (3 mg/kg) significantly reduced viral hemagglutination titer, improved the pathologic histology of lung tissues, and decreased the mortality of mice infected with influenza A virus. Polyphylla saponin I (5 and 10 mg/kg) prolonged the survival time of mice from 8.5±0.3 days to 13.2±0.5 days, with the prolonged life rates being 49.4% and 55.3%, respectively. CONCLUSION: Polyphylla saponin I has antiviral activity on influenza A virus both in vitro and in vivo.