The recent availability of specific inhibin assays has demonstrated that inhibin B is the relevant circulating inhibin form in the human male. Inhibin B is a dimer of an alpha and a betaB subunit. It is produced exclusively by the testis, predominantly by the Sertoli cells in the prepubertal testis, while the site of production in the adult is still controversial. Inhibin B controls FSH secretion via a negative feedback mechanism. In the adult, inhibin B production depends both on FSH and on spermatogenic status, but it is not known in which way germ cells contribute to inhibin B production. The regulation of inhibin B production changes during life. There is an inhibin B peak in serum shortly after birth only partly correlated with an increase in serum FSH, probably reflecting the proliferating activity of the Sertoli cells during this phase of life. Afterwards, inhibin B levels decrease and remain low until puberty, when they rise again, first as a consequence of FSH stimulation and then as a result of the combined regulation by FSH and the ongoing spermatogenesis. In the adult, serum inhibin B shows a clear diurnal variation closely related to that of testosterone. The administration of FSH increases the secretion of inhibin B in normal men, but is much more pronounced in males with secondary hypogonadism. The treatment of infertile men with FSH, however, does not result in an unequivocal inhibin B increase. There is a clear inverse relationship between serum inhibin B and FSH in the adult. Serum inhibin B levels are strongly positively correlated with testicular volume and sperm counts. In infertile patients, inhibin B decreases and FSH increases. In general, there is very good correlation with the degree of spermatogenetic damage, with the arrest at the earlier stages having the lowest inhibin B levels. However, for unknown reasons, there are cases of Sertoli-cell-only syndrome with normal inhibin B levels. Inhibin B and FSH together are a more sensitive and specific marker for spermatogenesis than either one alone. However, the inhibin B concentrations are not a reliable predictor of the presence of sperm in biopsy samples for testicular sperm extraction. Suppression of spermatogenesis with testosterone and gestagens leads to a partial reduction of inhibin B in serum but it is never completely suppressed. In contrast, testicular irradiation in monkeys or humans leads to a rapid and dramatic decrease of inhibin B, which becomes undetectable when germ cells are completely absent. In summary, although inhibin B is a valuable index of spermatogenesis, the measurement of serum inhibin B levels is still of limited clinical relevance for individual patients.