An increased production of plasminogen activators, able to convert plasminogen into plasmin, has been found in experiments in vivo on rat ovarian granulosa cells at the time of ovulation, indicating an involvement in follicular rupture. The granulosa cells of 49 follicles from 20 patients undergoing in-vitro fertilization were obtained by laparoscopy and tested for the content of urokinase-type plasminogen activator (u-PA), tissue-type plasminogen activator (t-PA) and inhibitor of plasminogen activator (PAI). In the respective follicular fluids the concentrations of oestradiol (E2), progesterone (P) and testosterone (T) were determined and the levels of these enzymes and of the follicular steroid content were related to the fertilizing behaviour of the respective oocytes. Follicles containing oocytes which could be fertilized, revealed significantly higher follicular fluid E2 and P levels and significantly lower T levels than follicles with unfertilized oocytes. The respective granulosa cells of fertilized oocytes exhibited higher levels of t-PA compared to their unfertilized counterparts, whereas no significant difference occurred in the levels of u-PA and PAI. These data suggest that successful fertilization of human oocytes is associated with a high content of t-PA in granulosa cells and high E2 and P levels in the follicular fluid.