The study of the plasminogen-plasmin system has, in the past, contributed much to the understanding of fibrinolysis and thrombolysis. Attention is now focused on the role of the components of this system in many biologic functions. Findings of uPA, its receptor and its inhibitor in many tumor tissues and tumor cell lines, strongly implicate their involvement in tumor invasion, tumor cell proliferation and metastasis. The characteristics of the plasminogen activators, the uPA receptor and the plasminogen activator inhibitors as well as their expression and regulation in tumors and tumor cell lines are reviewed.