Plasmin and elastase degrade fibrin and inhibit the blood coagulation system by degrading key proteins. Elastase can facilitate plasmin expression via an alternative pathway of plasminogen activation. Elastase modifies plasminogen to yield a zymogen that is a better substrate for activators than native plasminogen. Furthermore, elastase inactivates the inhibitor system of plasmin and plasminogen activators without affecting plasmin and plasminogen activators. While plasmin activity develops from a blood zymogen as a consequence of activators synthesized and secreted by endothelium and possibly other cells, elastase is secreted in an active form primarily by polymorphonuclear leukocytes. Plasmin and elastase may play mutual roles in thrombolysis, inflammation, and tumour invasion and metastasis.