Cytoplasmic and nuclear oestradiol-17β binding sites have been measured in human endometrium obtained at different stages of the menstrual cycle and from patients with cystic hyperplasia and endometrial carcinoma. In normal endometrium both sets of sites are maximal at about the time of ovulation. The number of nuclear sites obtained by incubating endometrium with oestradiol-17β in vitro paralleled the number of available cytoplasmic sites. Cytoplasmic and nuclear binding sites were present in cystic hyperplasia and endometrial carcinomata. There was no correlation between the degree of differentiation of the tumour and its oestradiol-17β binding capacity. All types of endometrium contained variable amounts of 8–10 S and 4–5 S cytoplasmic receptors. The proportions of these receptors were not affected by the presence of the protease inhibitor, p-phenylmethylsulphonyl fluoride.