The interaction of 125I-labeled human follitropin (hFSH), human lutropin (hLH), and human choriogonadotropin (hCG) with a particulate fraction prepared from the testes of adult men was investigated. The hormone specific binding was maximal at pH 7.5 and 34 °C in the presence of 10 mM MgCl2. The FSH receptor was relatively more stable than the LH receptor (t1/2 at 34 °C of 14 and 4 h, respectively). The dissociation constants (Kd) calculated for hLH or hCG were very similar (approximately 1.0 × 10−10 – 1.4 × 10−10 M). The affinity of FSH and LH to their receptors did not appear to change with age, as shown from analysis of testes from 17- to 80-year-old men. The number of FSH receptors was greater than the number of LH receptors in these tissues. The hFSH receptor did not show any preference to binding of the homologous hormone, because FSH from nonprimates could displace 125I-labeled hFSH, but the LH receptor showed a marked preference for hLH or hCG as the ligand.