SUMMARY Fibrinolytic proteins in menstrual fluid collected from women with normal menstrual blood loss (<80 ml) were found to be completely exhausted. High levels of t-issue plasminogen activator, fibrin(ogen) degradation products, and plasmin-inhibitor complexes were present, but little fibrinolytically active plasmin remained. No difference was observed between days 1 and 2 of menstruation.The fluidity of menstrual discharge has intrigued investigators for many years,' 2 and the absence of clottable fibrinogen has been attributed to high fibrinolytic activity. The presence of plasminogen activator in endometrial tissue3 and the possibility that an increased level of this enzyme is an aetiological factor in menorrhagia have led to the use of synthetic antifibrinolytic agents to reduce excessive menstrual blood loss.4Since these reports the molecular control of the fibrinolytic enzyme system has been explored in detail.5 6 Plasmin, the major fibrinolytic protease, is generated from the inactive proenzyme plasminogen by the action of the enzyme plasminogen activator, present only in trace amounts in peripheral blood.'8 The action of plasmin on fibrin(ogen) is regulated by the inhibitors fast acting a2-antiplasmin and slow acting a2-macroglobulin.' '0 Fibrin, the primary substrate of plasmin, also plays an important regulatory role: both plasminogen activator and plasminogen bind to fibrin, bringing the proteins into catalytic contact and causing the generation of plasmin activity. In addition, fibrin bound plasmin is protected from a2-antiplasmin inhibition, localising the proteolytic action of plasmin-to the immediate vicinity of the fibrin clot.5 6The recent exploration of the fibrinolytic enzyme system has been facilitated by methodological advances, notably the preparation of antibodies to fibrinolytic proteins and the development of tripeptide chromogenic substrates for functional assays. We have used these techniques to re-examine fibrinolytic proteins in menstrual fluid collected from women with normal menstrual blood loss (less