The capacity of blood to form a fibrin clot by means of the intrinsic hemostatic pathway requires coagulation factors I, II, V, VIII, IX, X, XI and XII, platelet lipids and calcium. The assay is performed by the addition in the sample of a suspension of rabbit brain cephalin with a surface activator. The APTT has proven to be a simple and highly reliable measurement of the intrinsic coagulation mechanism.