Fibrinolytic activity of colic venous and arterial blood has been studied at laparotomy in 18 patients with ulcerative colitis or Crohn's colitis and compared with that of 17 patients with colonic carcinoma or diverticulosis. Using the dilute blood clot lysis time technique, no difference was detected between the fibrinolytic activity in colic venous blood from the two groups. When the clinical complications of bleeding or thrombosis are compared with fibrinolytic activity the only important association is between a low peripheral venous fibrinolytic activity and the occurrence of postoperative thrombo-embolic complications.