PIP: Inhibin is defined as a gonadal hormone of the male and female that causes selective suppression of the secretion of FSH by the pituitary gland. Because of its ability selectively to suppress the secretion of FSH, which in turn would lead to a reduction in germ cell production, inhibin often has been promoted as an agent to regulate fertility, particularly in men. This review summarizes the recent reports on purification of inhibin and sequence analysis of cDNA coding for inhibin mRNA and then analyzes, in men and women, the likelihood of inhibin being used as a regulator of fertility and as a diagnostic in cases of infertility. Inhibin has been a difficult molecule to isolate and identify, and there has been considerable confusion about its physiochemical characteristics. Inhibin will have to consistently and reversibly induce azoospermia to be considered any further as a potential fertility-regulating agent in ment. If it is shown to do ths, then the problems of long-term administration of a polypeptide contraceptive will have to be addressed, including frequency of administration, the type of delivery system, and the possibility of antibody production, especially after long-term therapy. 2 strategies are possible during women's menstrual cycles: 1) treatment with inhibin every 5 to 7 days to prevent establishment of the dominant follicle and thereby continually suppress ovulation, r 2) continuous treatment with inhibin, at least for the 1st 7 days of the menstrual cycle, to suppress FSH to an extent that will not suppress ovulation but will result in an inadequate luteal phase. Potential problems include short and irregular cycles, breakthrough bleeding, and amenorrhea. Measurement of inhibin is a potentially useful adjunct for the diagnosis of infertility in men with disorders of the seminiferous tubules and is a potentially useful index of folliculogeresis and the time of ovulation in women.