Human plasma inhibin was measured by radioimmunoassay in 26 women undergoing ovarian hyperstimulation for in-vitro fertilisation. Plasma inhibin rose in parallel with oestradiol during gonadotropin therapy (r = 0.79, p less than 0.001). Peak plasma inhibin levels correlated with follicle number as determined by ultrasonography (p less than 0.001) and with the number of oocytes recovered at laparoscopy (p less than 0.05). The increase in production of human ovarian inhibin by exogenous follicle stimulating hormone (FSH) is thus consistent with its proposed role in FSH regulation. Human plasma inhibin may be useful as a circulating protein marker of granulosa-cell function and may have therapeutic implications in human ovulation-induction programmes.