set at the maximum speed (5 x 1O s at 0C). The receptor preparation was suspended at a concentration of 2 g in 10 ml of assay buffer (0.05 M, Tris-buffer, pH 7.5, containing 0.1 % BSA, 5 mM MgCI2 and 0.1 M sucrose).The testicular receptors were obtained from immature (21-day-old) Holtzman rats (Reichert & Abou-Issa, 1977). The receptor preparations were suspended at a concentration of I g in 10 ml in the assay buffer.Human FSH (hFSH-13) was iodinated by the chloramine-T method (Reichert & Bhalla, 1974) with a specific activity of about 15 tCi yg-'.Binding studies were performed using labelled hFSH (5 ng hFSH in 50 d l assay buffer) in the presence of serial dilutions of unlabelled FSH (I -250 ng) with 50 mg of testicular receptor in 500 gl of assay buffer. The total reaction volume was made up to I ml by addition of assay buffer. The tubes were then incubated in a metabolic shaker at 37°C for 2 h. Incubation was terminated by addition of 2 ml of chilled buffer. The tubes were then centrifuged at 1,500g for 15 min. The supernatant was discarded by decantation and the tubes were drained and counted in the well type gamma-counter. The non-specific binding was determined in presence of a 1000-fold excess of oFSH (NIH-FSH-S-16).