To prove up production and effect of free radicals on virus-induced pathogenesis, the influenza A/FM1/1/47 (H₁N₁) was adapted to mice by nostril inoculation. The results demonstrated that the oxygen free radical levels and xanthine oxidase activity in the lung tissues from the mice after infection by virus raised remarkably. These were positively correlated with lung tissues injury and mice mortality. The results demostrated that oxygen free radicals may involve in influenza-induced pathogenesis in mice, and may be an important factor causing tissue injury.