Whether the anabolic effect of genistein on bone metabolism in the femoral-metaphyseal tissues of elderly rats is inhibited by the anti-estrogen tamoxifen was investigated in vitro. Bone tissues were cultured for 24 h in the medium containing either vehicle or genistein. The presence of genistein (10−6 and 10−5 M) caused a significant increase in calcium content, alkaline phosphatase activity and DNA content in the bone tissues. The effect of genistein (10−5 M) in increasing bone alkaline phosphatase activity and DNA content was equal to the stimulatory effect of 17β-estradiol (10−10 and 10−9 M) on the bone components. The genistein effect was not significantly enhanced by the presence of 17β-estradiol (10−10 and 10−9 M). The anti-estrogen tamoxifen (10−7 M) completely blocked the effect of 17β-estrogen in increasing bone alkaline phosphatase activity and DNA content. The genistein (10−5 M)-induced increases in bone alkaline phosphatase activity and DNA content were completely abolished by the presence of tamoxifen (10−7 and 10−6 M), although tamoxifen itself had no effect on the bone components. The present study demonstrated that the anabolic effect of genistein on bone components is inhibited by anti-estrogen tamoxifen. The effect of genistein may be mediated through estrogen-like action.