Human follicular fluid (hFFl) was harvested from follicles (5-15 mm) of human ovaries obtained at laparotomy. Addition of native hFFl, or a low molecular weight fraction of it, to the culture medium was found to inhibit the spontaneous maturation of cumulus-enclosed porcine oocytes. It was also found that hFFl inhibited progesterone secretion by the cumulus cells. The results extend earlier observations in other mammalian species, indicating that in the human, also, a specific oocyte maturation inhibitor is responsible for keeping the oocyte in meiotic arrest.