The effect of hypothermia on heparin anticoagulation was studied using (siliceous earth) activated clotting time (ACT) measured by the modified Hattersley technique and anti-factor Xa assay measured by the modified Teien technique during cardiopulmonary bypass (CPB). Seventeen adult patients with normal blood coagulation function undergoing their first time coronary artery bypass grafting surgery or valve replacement surgery were randomly selected. Blood samples were collected 3-5 minutes after CPB initiation, before cooling at a mean temperature of 32.9°C, and then again after cooling to a mean temperature of 28.4°C. No additional heparin, citrate or packed red blood cells were added to the CPB circuit between sampling points.