The specific estrogen binding capacity of the cytosol has been investigated in 51 human breast cancers (49 women, 2 men). The estrogen binding sites of the cytosol has been analysed by Korenman's dextrancoated charcoal technique. Several methods have been used in order to distinguish the specific estrogen binding sites and the unspecific binding: temperature differentiation and saturation of the binding sites with nonlabelled estrogen or its antagonist Clomid. It has been concluded that the differentiation of the estradiol binding sites (specific or unspecific) with the elevated temperature effect or the additon of Clomid may be helpful only as a qualitative test. The elevated temperature of Clomid may influence the increase of the unspecific estradiol binding. The most suitable method to eliminate the unspecific binding of estradiol is the saturation of cytosol from control samples with non-radioactive estradiol-17-beta. The specific estrogen binding capacity of the cytosols of different tumours varied over a wide range (0--840 fM E2/mg protein). It has been found that among 51 persons there were 31 patients (61%) with measureable amounts of high affinity estradiol binding sites in cancerous tissues. In 20 cases (39%) the existence of estradiol specific binding sites could not be observed.