Department of Biochemistry (Division of Chemical Endocrinology), Medical Faculty, Erasmus University Rotterdam, Rotterdam, The Netherhnds (Received 31 March 1976) Studies on steroid hormone receptors in testicular tissue have previously revealed the presence of specific oestradiol receptors in interstitial tissue (Brinkmann, Mulder, Lamers-Stahlhofen, Mechielsen & van der Molen, 1972) and androgen receptors in tubular tissue (Hansson, McLean, Smith, Tindall, Weddington, Nayfeh, French & Ritzen, 1974; Mulder, Peters, van Beurden & van der Molen, 1974). Recently methods for the preparation of isolated Leydig cells (Janszen, Cooke, van Driel & van der Molen, 1976) and Sertoli cells (Fritz, Rommerts, Louis & Dorrington, 1976) have become available. In the present study, binding of testosterone and oestradiol was investigated in such Leydig cell and Sertoli cell preparations. Leydig cells were prepared, as described by Janszen et al. (1976), from adult rats 8 days after hypophysectomy. Sertoli cells were prepared, according to Fritz et al. (1976),