A protein hormone produced in women by developing follicles as well as by the corpus luteum, and in men by the testis in the presence of spermatogenesis, and acting on the pituitary gland to inhibit the production of follicle stimulating hormone. In women, falling levels occur as the number of developing follicles reduces to low numbers leading up to menopause, thus causing the elevation of serum FSH and shortening of the follicular phase that characterises the premenopause. In men, appreciable levels of serum inhibin B, a subclass of the inhibin family, predicts the presence of at least a small amount of sperm production.

In women, when measured during menstruation, inversely related to the serum FSH (and probably provides no more information than the day 3 FSH level in predicting depletion of eggs); in men with azoospermia, levels predict the likelihood of being able to obtain sperm for ICSI at testicular sperm extraction.