It has been suggested that the isoflavone, genistein,, may have some role as a chemopreventive agent against cancer in humans. Levels of genistein and its beta-glucoside conjugate, genistin, ingested in soybeans and related bean products by the Japanese were quantified by HPLC, to estimate daily intake of these compounds. Amounts of genistein and genistin in soybeans, soy nuts and soy powder were in the range of 4.6 to 18.2 and 200.6 to 968.1 micrograms/g food, respectively. The values for soy milk and tofu (bean curd) were 1.9 to 13.9 and 94.8 to 137.7 micrograms/g food, respectively. Levels of isoflavones in fermented soybean products, miso (bean paste) and natto (fermented soybeans), were 38.5 to 229.1 micrograms/g food for genistein and 71.7 to 492.8 micrograms/g food for genistin. Thus, the level of genistein in the fermented soybean products was higher than in soy beans and soybean products such as soy milk and tofu. From these observations, it is suggested that the beta-glycosyl bond of genistin is cleaved to produce genistein by microbes during fermentation to yield miso and natto. Soy sauce was also found to contain both isoflavones, but at levels lower than in miso and natto. On the basis of these data for average annual consumption of soybeans and related products, daily intake of genistein and genistin by the Japanese is calculated to be 1.5-4.1 and 6.3-8.3 mg/person, respectively. These levels are much higher than those for Americans or Western Europeans, whose mortality rates for breast, colon and prostate cancers are greater than the Japanese.