Human endometrial cytosol (90,000 g, 60 mins) contains a protein that binds non-steroidal anti-oestrogens with high specificity and high affinity (Kd = 10(-10)M). It binds oestrogens and other steroid hormones poorly, and can be physically distinguished from the oestrogen receptor protein by gel filtration and ion exchange chromatography. The endometrial anti-oestrogen binding protein is larger and less negatively charged than the oestrogen receptor. In this regard, it resembles the human myometrial anti-oestrogen binding protein. Unlike the myometrial protein, however, levels of the endometrial anti-oestrogen binding protein vary during the menstrual cycle, being higher in the proliferative phase (mean 29 fmol/mg protein, SD 13 fmol/mg protein) than in the secretory phase (mean 13 fmol/mg protein, SD 7 fmol/mg protein). These findings suggest that the concentration of the anti-oestrogen binding protein in human endometrium may be regulated, at least in part, by the cyclical pattern of hormonal secretion that characterizes the menstrual cycle.