In the present study the effect of unfractionated heparin (UFH) (Liquemin, 750-1000 IU/h), low molecular weight heparin (LMWH) (Fragmin, 3000-7250 IU bolus), and prostacyclin (Flolan, 5 ng/kg body weight/min) on the activation of blood coagulation and fibrinolysis, induced by polysulfone membrane dialyzers during hemodialysis, was compared. Plasma levels of thrombin-antithrombin III complex (TAT), fibrin split product D-dimer, and plasmin-plasmin inhibitor-complex (PPI) were measured in the arterial and venous line of the dialyzer at the beginning and at 10, 60, 120, and 180 minutes of hemodialysis. Five patients on chronic hemodialysis treatment were investigated in a cross over study. Clinically all three anticoagulation regimen were sufficient for hemodialysis treatment. Using UFH or LMWH TAT, PPI, and D-dimer levels were similar in the venous and the arterial line of the dialyzer. However, during prostacyclin treatment the levels of these activation markers were significantly higher in the venous line. Based on these data the dialyzer membrane can be considered as a site of activation of blood coagulation and of fibrinolysis during anticoagulation with prostacyclin in hemodialysis.