To ascertain whether abnormalities in testicular binding of follicle stimulating hormone (FSH) are related with spermatogenic impairment found in idiopathic male infertility, we measured FSH receptors in testicular tissues obtained by biopsy from 37 infertile men. Since there had been few reports on the characterization of human testicular FSH receptor, we first established conditions for FSH receptor assay in small amounts of human testicular tissues taken by biopsy. Thirty-seven infertile men were divided into 3 groups by histological grading using Johnsen's score count, and testicular FSH receptors among these groups were compared. Although 6 of 12 infertile men with low Johnsen's score count and 3 of 9 men with middle score count had no obvious FSH binding sites with high affinity, all 16 men with high score count had high affinity FSH binding sites. The present findings demonstrate that the decrease in testicular high affinity binding sites for FSH is related with the degree of spermatogenic impairment found in idiopathic male infertility.