Abstract In the 105,000 g prostate cytosol fraction of rats a specific receptor for 5α-dihydrotestosterone and testosterone has been demonstrated under in vivo and in vitro conditions. Six criteria which allow conclusions regarding the specificity of an androgen receptor have been elaborated: (1) A binding has been obtained in the target organ only, not in skeletal muscle and in plasma. (2) The thermolability of the receptor has been proved. (3) The plasma contamination of the cytosol fraction in question could not account for the binding. (4) A complete displacement of the labelled androgen by a 100-fold excess of the unlabelled compound was obtained. In this respect, too, a higher affinity of the receptor to 5α-dihydrotestosterone has been observed. (5) The non-androgen cortisol was ineffective in displacing the androgen from its receptor. (6) An inhibition of binding of the androgen was demonstrated with the anti-androgen cyproterone acetate in vivo as well as in vitro. The relevance of these findings in regard to androgen receptors and to the problem of androgen binding in human prostatic hypertrophy has been discussed.