We screened ethanol extracts from a total of 134 species of edible Thai plants for their suppressive effects on superoxide (O2 -) generation using a xanthine (XA)-xanthine oxidase (XOD) assay system. When the extracts were tested at a concentration of 500 ìg/ml, 28.4% significantly suppressed O2 - generation. Of these active extracts, it was found that in 17.9% of cases the action was due to XOD inhibition, in 1.5% due to O2 - scavenging activity, and in 9% due to both XOD inhibition and O2 - scavenging. In addition, some plant extracts (25 species) which had been known to possibly possess anti-tumor promoting activity were tested for O2 - and NO generation in cellular systems. In this test, 13 species exhibited strong inhibitory activity toward both O2 - and NO generation. From the fruit pods of Oroxylum indicum (Bignoniaceae), a traditional vegetable in Thailand, two flavones, oroxylin A and chrysin, and a triterpene carboxylic acid, ursolic acid (UA), were identified as inhibitors of O2 - generation in XA/XOD system. These compounds also showed marked inhibitory effects on the tumor promoter 12-O-tetradecanoylphorbol-13- acetate (TPA)-induced O2 - generation in dimethylsulfoxide (DMSO)-differentiated HL-60 cells. Our results suggest that, as we have reported earlier, edible Thai plants are promising sources of antioxidants with chemopreventive potential.