During blood clot formation, soluble fibrinogen is transformed to insoluble fibrin under the action of thrombin. Fibrin, the matrix of a blood clot, has a temporary function, and after it has fulfilled its role in haemostasis and tissue repair, it is degraded to soluble fragments by the action of plasmin. By this process, designated as fibrinolysis, the clot desintegrates. Data on fibrinogen binding of plasminogen and plasminogen fragments are conflicting. Some investigators were unable to show binding of glu-plasminogen or lys-plasminogen to fibrinogen, whereas others obtained direct or indirect evidence that binding occurs. Plasminogen has been shown to have binding sites for lysine-analogues such as tranexamic acid and 6-amino hexanoic acid. The binding of lysine analogues by plasminogen and the fibrin binding seem to be interrelated; that is, plasminogen fragments that bind to lysine-Sepharose also bind to fibrin. The binding of arginine to human plasminogen is weaker than to rabbit plasminogen.