In the course of a search for natural estrogen-like materials from mushrooms, the proliferating effects of the methanol extracts of 14 edible mushrooms on the estrogen-dependent growth of MCF-7 cells were examined. Of the extracts, that from the fruiting body of Pleurotus eryngii showed the strongest estrogen-like activity, specifically on the proliferating effects of MCF-7 cells. This proliferation effect of the extract from the fruiting body of P. eryngii is related to its estrogenic activity, because this proliferation activity was inhibited by the addition of the antiestrogenic compound ICI 182 780. The ethanol extract of the fruiting body of P. eryngii prevented ovariectomy-induced bone loss. The present data provide evidence that the ethanol extract of P. eryngii helps to protect against the bone loss caused by estrogen deficiency, without having a substantial effect on the uterus.