The effect of high molecular weight split (HMWS) products and D and E products on the determination of one‐stage prothrombin time, prothrombin and proconvertin, factors V, VIII, and IX, platelet adhesiveness and the thrombin time was investigated. When the concentration of the split products (D, E or HMWS) was 0.5–1.0 mg per ml plasma or less, the thrombin time was only slightly prolonged, and the results of other tests studied, platelet adhesiveness included, were not affected significantly. Prolonged thrombin time and one‐stage prothrombin time and a decrease to half of the normal factor V value were noted when the concentration of the HMWS products was increased to 5–10 mg per ml plasma, while the P & P value was influenced only slightly. When the concentrations of the split products exceeded the maximal level achievable in vivo was a large effect noted on factor V and some effect on P & P. The HMWS products probably act partly as ‘antithrombin’, partly as inhibitor of the polymerization of the fibrin monomer, while the D and E products act only as ‘antithrombins’. The esterolytic effect of thrombin on TAME was not inhibited by D and E products in a molar concentration of 1/7 and 1/5, respectively, of that of TAME.