The effectiveness and safety of CY 216 as anticoagulant for extracorporeal circulation were evaluated in 403 haemodialysis of haemofiltration sessions performed in 33 patients with chronic [24] or acute [9] renal failure; 149 of the sessions were carried at risk of haemorrhage. Initially CY 216 was administered as a bolus intravenous injection in doses of 7,500 anti-Xa Institut Choay units (AXa.IC.U) to patients under 50 kg, 15,000 AXa.IC.U to patients weighting between 50 and 80 kg and 22,500 AXa.IC.U to patients over 80 kg. Subsequently dosage was adjusted according to clinical results. With a median dose of 250 AXa.IC.U per kg, no haemorrhage was observed. Blood restitution was satisfactory in 84.6% of the cases, extracorporeal circulation was without clotting of fibrin deposit in 90% of the cases and the incidence of total coagulation was only 0.5%. Using CY 216 seems to be effective in preventing coagulation in the extracorporeal circuit and was well tolerated by all patients whether or not they were carried at risk of haemorrhage.