A method is described for the separation and analysis of isoflavone β-glycoside conjugates and aglucones in various foods derived from soybeans. After initial extraction using 80% aqueous methanol and defatting of the extract with hexane, the isoflavones were analyzed by gradient elution reversed-phase high-pressure liquid chromatography. Their structures were confirmed by fast atom bombardment ionization mass spectrometry and by proton nuclear magnetic resonance spectroscopy. The results reveal that most Asian and American soy products, with the exception of soy sauce, alcohol-extracted soy protein concentrate, and soy protein isolate, have total isoflavone concentrations similar to those in the intact soybean. Asian fermented soy foods contain predominantly isoflavone aglucones, whereas in nonfermented soy foods of both American and Asian origin isoflavones are present mainly as β-glycoside conjugates. Since the much larger estimated daily intake of these isoflavones by Asians compared to Americans is similar on a body weight basis to the isoflavones in soybean-containing diets which inhibit mammary tumorigenesis in animal models of breast cancer, it is possible that dietary isoflavones are an important factor accounting for the lower incidence and mortality from breast cancer in Asian women. © 1993, American Chemical Society. All rights reserved.