In 10 patients (8 men and 2 women) aged 28 to 58 years (mean 44.4 years) treated by repeated haemodialysis due to end-stage renal failure, the bleeding time, whole-blood coagulation time, one-stage prothrombin time, thrombin time of plasma, activated partial thromboplastin time (APTT), fibrinogen level and euglobulin lysis time have been determined (1) during a 4-hour haemodialysis using heparin as an antithrombotic agent, and (2) one week later in the course of another haemodialysis using prostacyclin-heparin. The values for any of the above parameters with both anticoagulant treatment types did not differ. Plasma fibrinogen level after haemodialysis was significantly lower after administration of heparin alone as compared with the group treated by prostacyclin-heparin infusion. During haemodialysis performed with prostacyclin-heparin infusion, activation of the blood fibrinolytic system was manifested by a significant shortening of euglobulin lysis time, observed after 1.5 hours and after the end of haemodialysis. The above phenomenon did not occur when haemodialysis was performed with heparin alone.