The influence of an IUD on the fibrinolytic activity of uterine fluid was measured by radial enzyme diffusion on fibrin plates. In samples obtained from 48 women using Cu-IUDs, the fibrinolytic activity in mid-cycle phase was higher than that in the proliferative or the luteal phase. It increased in the premenstrual phase. Individual values before and after insertion of a Cu-T were determined in the proliferative-mid-cycle phase and the luteal phase in a group of 11 women. In contrast to earlier reports on the endometrium, the fibrinolytic activity of uterine fluid was lower after, than before, insertion of an IUD. This may be due to a lower concentration of activators and/or to an increased concentration of inhibitors of plasminogen activation.