Leukotriene B4 directly enhanced progesterone release from superfused human granulosa cells. This secretory effect was observed in concentrations from 10(-12) to 10(-10) M. Lower and higher concentrations failed to affect progesterone release. When we analysed the high performance liquid chromatography profile of supernatant from human granulosa cell cultures, we detected a leukotriene B4 peak. In conclusion, these data support the hypothesis that leukotriene B4 may participate in the intracellular mechanism of progesterone release in human granulosa cells.