Granulosa cells were aspirated from human pre-ovulatory follicles following a combined clomiphene-gonadotrophin stimulation in an in-vitro fertilization (IVF) programme. The cells were cultured for 8 days in medium M199 containing 10% bovine fetal calf serum under 5% CO2 in air. Pure human FSH and human LH were added alone or in combination to the culture in various concentrations and the progesterone (P) and oestradiol-17 beta (E2) levels in the medium were measured every second day by a conventional RIA technique. In the presence of FSH or LH the formation of P increased 2- to 3-fold with the pronounced effect after 4 to 6 days in culture. Addition of testosterone (T) (3 X 10(-7) M) to the culture medium affected neither basal nor gonadotrophin stimulated P formation. In this system, only minute amounts of E2 were formed and neither FSH nor LH stimulated its formation. When the medium was fortified with T, basal E2 formation increased 50- to 100-fold. FSH further stimulated this conversion significantly after 6 and 8 days of culture, while LH had no significant influence.