To investigate the protective effect of Sophorae Radix (SR) on the damage by pulmonary vascular endothelial cells by xanthine oxidase (XO)/hypoxanthine (HX)-induced oxygen tree radical, Neutral Red (NR) and c-fos immunopositive cell assay were used. The results were obtained as follows ; The viability of vascular endothelial cells treated with XO/HX was decreased. And c-fos immunopositive cells represented a maximal increase in group treated with XO/HX for 2 hour in pulmonary vasvular endothelial cells. But pretreated groups with SR extracts were not inhibited the increase of c-fos immunopositive cells by XO/HX in a dose-dependent manner. These results show that XO/HX elicits toxic effects in cultured pulmonary vascular endothelial cells, and suggest that SR extract is very effective in the prevention of XO/HX-induced increase of c-fos immunopositive cells.