The epididymis of the dog has been studied with regard to its ability to bind androgens with a high affinity and low capacity. A proteinaceous molecule was demonstrated in the high-speed supernatant fraction of disrupted cells that bound DHT and sedimented in a sucrose density gradient with a sedimentation coefficient of 8S with respect to bovine serum albumin. This binding protein was able to transport DHT to the nucleus at elevated temperatures where it sedimented as a 4S in a 5-20% sucrose density gradient. Even though this protein appears to be similar to other androgen receptors in other accessory sex organs in a variety of mammalian tissue, it is different from androgen-binding protein (ABP). The apparent specificity of the receptor protein for androgens was demonstrated using competition studies. The molecular configuration of the steroids was found to be instrumented in this selectivity. A reduced affinity of the androgen receptor was found when the steroid did not possess a hydroxyl group at the 17 beta-position and a 3-oxo-group irrespective of the degree of saturation of the A-ring. Studies concerning the physical properties of the androgen binder revealed a protein with a molecular weight of 220,000, a Stokes' radius of 54A, and a frictional ratio of 1.35.