Infantile pneumonia has been treated successfully with Huayu decoction (HYD) in our department for more than ten years. According to the principles of Huoxue Huayu of TCM, the composition of Huayu decoction is as follows: Angelica sinensis, Paeonia lactiflora, Ligusticum wallichii, Spathollobus suberectus, Hirudo nipponica, Tabanus bivittatus, Paeonia suffroticosa, Astragalus membranaceus . 49 cases of infantile pneumonia were treated with HYD. The activity of erythrocyte superoxide dismutases (ESOD) was measured in these patients. The results showed that the activity of ESOD reduced in the acute stage and returned to normal in convalescence. There was significant statistical difference in activity of ESOD between the patient's group. In experimental studies, the producing of free radicals was induced by inhaling ozone in mice. It was found that HYD had the effect of scavenging free radicals in these animal models. The action of anti-oxidate of HYD was also detected in vitro. The mechanism of HYD in treating infantile pneumonia might be elucidated in some respects by these clinical and experimental studies.