SummaryThe anticlotting activity of a physiological heparin-like substance can be demonstrated by the study of dilution curves of fresh blood in non wettable material.The usual clotting tests appear unequally sensitive to the presence of an anticoagulant as far as the site and mechanism of action of the inhibitor and the interference of compensatory reactions are concerned. The procedures should be improved by using non wettable surfaces, reduced activators and concentrated inhibitor.The use of several methods is advisable for detecting abnormal inhibitors.