Antioxidant effects of isoflavonoids have recently been described. To learn whether the isoflavonoids genistein and equol have actions on the intracellular free radicals, human neutrophils and J774 monocyte-macrophage cell line were used to measure the intracellular production of O(2) (superoxide anion) and H(2)O(2) (hydrogen peroxide) by flow cytometry. The results shown significatives decrease in O(2) and H(2)O(2) production after 1 hour of incubation with equol and genistein. The phagcytic oxidant production decreased owing to the effects of both isoflavonoids in a concentration-dependent manner.