Fifty-six fresh specimens removed during vascular operations were examined histochemically to determine tissue fibrinolytic activity in the endothelium and adventitia. Prosthetic grafts, saphenous vein grafts, saphenous and varicose veins, and atheromatous femoropopliteal arteries were studied with the following results: (1) Normal vein walls (adventitia) were rich in plasminogen activator with minimal fibrinolytic activity in the endothelium. Fibrotic varicose veins showed reduced lysis activity in the adventitia. (2) There was no fibrinolytic activity in the neointima of all prosthetic materials, except where there was capillary loop formation. (3) Lysis was not present in any of the occluded saphenous vein grafts. (4) Fibrinolytic activity was reduced in atheromatous arteries, except in the area adjacent to fibrin plaques. We conclude that plasminogen activator is of vascular origin; that its activity is dynamic; and that absence of fibrinolytic activity may be an important contributing factor in graft failures.