Although the beneficial effects of estrogen in the treatment of postmenopausal osteoporosis are well documented, such effects were difficult to demonstrate in in vitro models. However, recent improvements in bone cell culture models (better defined osteoblastic cell populations, omission of Phenol Red from culture media) enabled several investigators to show albeit small, but reproducible, direct effects of estradiol in various osteoblastic cell types. Such findings were supported by the identification of low numbers of high-affinity estrogen receptors in bone cells derived from different mammalian species. The likely physiological relevance of the in vitro results is indicated by the specificity for 17 beta-estradiol, and the requirement for nanomolar concentrations of the hormone, consistent with a Kd of 0.6 nM for estradiol binding to its receptor . In bone in vitro, estradiol may have anticatabolic effects by decreasing parathyroid hormone responsiveness, and anabolic effects by stimulating matrix synthesis and cell proliferation. Insulin-like growth factor-I is likely to be an autocrine/paracrine mediator for the anabolic effects and may, when associated with its binding proteins, effectively act in the bone compartment.