Osteoclasts were isolated from membranous bone from four children without metabolic bone disease who were undergoing craniotomy for either tumor or trauma. Both freshly isolated osteoclasts and those cultured for 4–7 days exhibited the following characteristics: production of tartrate‐resistant acid phosphatase (9.5–14.8 units), contraction in response to application of 100 mg/ml of human calcitonin, and formation of resorption lacunae on devitalized bone wafers. Nuclear estrogen and progesterone receptors were demonstrated by immunohistochemical techniques and quantitated in two of the patients by radioimmunoassay (estrogen receptor RIA, 23.6 and 23.8 cpm/μg protein; progesterone receptor RIA, 36.7 and 74.2 cpm/μg protein). The demonstration of sex steroid hormone receptors in the nucleus of osteoclasts derived from children with normal membranous bone has estabished a potential mechanism whereby direct modulation of bone resorption by the sex steroid estrogen and progesterone may occur.