Puerarin, a daidzein-8-C glucoside, is the major isoflavonoid in Kudzu (Pueraria lobata), and is unique in that it contains C-C conjugated glucose at position 8 of the isoflavonoid structure. A puerarin diet at a dose of 5 mg/kg b.w./d to fed ovariectomized mice for 2 mo diminished the urinary deoxypyridinoline, a typical bone-degradation product. Since the bone absorption marker, serum tartarate-resistant acid phosphatase (TRAP) activity of puerarin-fed mice decreased but the bone formation marker, osteocalcin level, did not alter, the puerarin diet was proved to specifically depress the bone absorption, but not the overall bone metabolism. In accordance with that results, the femur structure of puerarin-fed mice was restored compared with that of puerarin-free diet mice. The atrophied uterine due to low estrogen (E2) level after ovariectomy was not restored by the puerarin diet, suggesting that puerarin exerted the anti-osteoporotic action through a non estrogen receptor (ER) mediated-pathway, in vivo. The growth of an ER-positive human breast cancer cell, MCF-70, was not enhanced by puerarin, suggesting that puerarin did not show estrogen-like action on MCF-7 cells, even at a ten thousand times higher concentration than that of E2. Furthermore, ICI182,780 (ICI), an estrogen antagonist, suppressed the enhanced growth of MCF-7 cells by E2, but not that by puerarin. In an ER-binding assay, puerarin was proved not to bind to ERα or β, or if all, extremely weakly, although daidzein, an aglycon of puerarin, showed a little stronger binding compared with puerarin. All these results strongly indicate that puerarin exerts its anti-osteoprotic action independently of the ER-mediated pathway.