In a controlled single blind study to determine the minimal effective dose of estrogen for protection against bone loss, conjugated equine estrogens in doses of 0.625 and 1.25 mg per day were equally effective in reducing bone loss in postmenopausal and oophorectomized women when bone mass was estimated by single-photon absorptiometry or radiogrammetry . Daily dose levels of less than 0.625 mg were essentially ineffective. Fifty percent response level was calculated to be 0.45 mg per day. Concomitant biochemical effects, reduction in urine calcium and hydroxyproline, were compatible with the observed effects on bone mineral.