We have examined the antioxidant properties of structurally related flavones/isoflavones in 12-O-tetradecanoylphorbol-13-acetate (TPA)-stimulated HL-60 cells. In the presence of 1.3% dimethyl sulfoxide in the medium for seven days, promyelocytic HL-60 cells were differentiated into neutrophil-like cells possessing phagocytic properties and the capacity to generate H2O2 on TPA stimulation. The effects of five selected flavones/isoflavones on the formation of H2O2 and 8-hydroxy-2'-deoxyguanosine (8-OHdG) were examined in TPA-stimulated HL-60 cells. The results indicated that genistein was the most potent inhibitor of H2O2 production by TPA-stimulated HL-60 cells, followed by apigenin and daidzein, whereas prunectin and biochanin A exhibited no effect. This inhibitory effect correlates well with the scavenging capacity of H2O2 by these flavones/isoflavones in an in vitro system. The formation of 8-OHdG in cellular DNA of HL-60 cells was induced by TPA and further enhanced by the addition of FeCl2 to the medium. Most flavones/isoflavones significantly inhibited TPA + FeCl2-induced 8-OHdG formation in HL-60 cells, with genistein being the most potent quencher. The inhibition of H2O2 production and 8-OHdG formation by these structurally related flavones/isoflavones may contribute to their chemopreventive potentials against human cancers.