Generation of factor XII, thrombin antithrombin complexes, prothrombin fragment 1+2 and thrombus precursor protein has been monitored in 16 subjects during haemodialysis. Immediately after starting treatment, contact of blood with the negatively charged surfaces of the polyacrylnitril membrane AN-69 resulted in a 9-45% decrease in factor XII activity. Peak concentrations for thrombin antithrombin complexes (50 to 120 microg/L) were observed 30 min after the start of haemodialysis. Establishment of thrombus precursor protein concentrations yielded steadily increasing results without any tendency to decrease during treatment. Determination of thrombin antithrombin complexes is considered to establish the most sensitive short-term reacting parameter indicating activation of coagulation. A steady generation of fibrin and fibrinogen-fibrin complexes during treatment with haemodialysis is indicated by increasing results for thrombus precursor protein. In order to prevent clotting during haemodialysis, an additional supplementation of anticoagulant is needed.