The fibrinolytic system, including the fibrinolytic activator activity of vein wall and fibrinolytic activity in blood after venous occlusion, was analysed in 43 patients before and on the third day after operation. Twenty-two of the patients underwent abdominoperineal amputation of the rectum and 21 had a cholecystectomy. The incidence of postoperative thrombosis was asssessed by means of the isotope method and phlebography. The results indicate that a decrease in fibrinolytic activity of the vein wall and/or in the release of fibrinolytic activator from the vein wall is a regular postoperative phenomenon, especially after major surgery. Malignant tumour of the rectum did not have any depressing effect on the fibrinolytic activator. All the patients with thrombosis after operation had a postoperative fibrinolytic defect of the vein wall compared with about 60 per cent of the other patients. It is concluded that postoperative changes in the parameters studied might be of significance for the development of thrombosis after operation.