Kudzu-root and soy phytoestrogens have been associated with anti-cancer and anti-intoxication activities. Sales of capsules containing kudzu-root and soy extracts through health food stores and the Internet are unregulated. To compare efficacy, the amount of phytoestrogens present in commercial preparations and their fate in biological samples must be determined. In this study, the levels and composition of phytoestrogens in kudzu-root and soy extracts were studied using high-performance liquid chromatography with ultraviolet light detection. The bioavailability of phytoestrogens was studied by measuring red blood cell (RBC) uptake and serum protein binding ability. Phytoestrogen levels in acidified kudzu-root samples were 5- to 10-fold greater than those in nonacidified samples. Puerarin accounted for 80% of total phytoestrogens in kudzu-root. In soy extract, puerarin was absent while genistin, glycetein, and daidzin or daidzein were the major phytoestrogens. The RBC uptake depended on the phytoestrogen's polarity and molecular length. When serum was dialyzed with phytoestrogen standards in a buffer, the protein binding of phytoestrogens correlated negatively with their polarity. However, when serum was dialyzed with kudzu-root or soy extract, almost all of the phytoestrogens present in the extract bound to serum protein. Therefore, this study suggests differences in the bioavailability of phytoestrogens when they are ingested as purified compounds compared with crude plant extract. The differential composition of phytoestrogens in kudzu-root and soy may account for the differences in their therapeutic activities.