Clotting within the dialyser is one of the most significant clinical parameters of biocompatibility. A study was designed to evaluate the biocompatibility of two different dialysis membranes (cuprophane and polyacrylonitrile) during therapy with conventional heparin. Transient leukopenia during cuprophane but not during polyacrylonitrile haemodialysis was observed, and elastase release using polyacryonitrile membranes was reduced (P less than 0.001). An elevation in F VIII:C activity during cuprophane haemodialysis has to be taken as an indication of endothelial disturbances. There was a significant (P less than 0.001) platelet activation (beta-thromboglobulin) and combined thrombin/plasmin generation using cuprophane membranes. This new synthetic polyacrylonitrile membrane inactivates the clotting in an extracorporeal system to a sufficient degree and allows a reduction in dosages of heparin. Platelet activation, platelet turnover, disturbances of endothelium, fibrinolysis activation, and granulocyte activation are reproducible parameters of a described interaction model. They also permit a comparison of different haemodialysis membranes.