Specific receptor binding of estradiol (E2) and dihydrotestosterone (DHT) was studied in human myometrial tissue and in human mammary cancer tissue. The inhibition of binding for E2 and DHT by E2, testosterone (T), DHT, dehydroepiandosterone (DHEA), dehydroepiandrosteronesulfate (DHEA-S), androstenedione (A) and 5-androstene-3β,17β-diol (Adiol) was tested with the use of dextran-coated charcoal separation of bound and free E2, respectively, and DHT. The percentage of binding inhibition was calculated with reference to the inhibition obtained with nafoxidine in a molar concentration ratio of 1,000 for E2 binding, respectively, with cyproterone acetate in a molar concentration ratio of 10,000 for DHT binding. In 15 samples of myometrium tested, receptors were found for both E2 and DHT. From 19 samples of mammary carcinoma tissue one showed no binding activity, three samples did bind E2 only, five samples DHT only, and ten samples showed binding of both steroids. A 50% inhibition of E2 binding, in myometrial as well as in tumor tissue, required a molar concentration ratio of 40 for Adiol, of more than 2,000 for T and for DHT, and of about 20,000 for DHEA. No significant inhibiting activity could be found for A up to a molar concentration ratio of 10,000 and for DHEA-S up to 40,000. With regard to DHT binding, Adiol is more active than E2 and less active than T. Of the substances tested Adiol is therefore the only one which exerts a significant inhibiting influence at a molar ratio not far beyond the physiological range. This signifies that Adiol might interfere at the receptor level in the estrogenic stimulation of mammary cancer cells.