Test of the conversion of fibrinogen to fibrin by thrombin in which clotting time of plasma mixed with a thrombin solution is measured. Time is prolonged by afibrinogenemia, abnormal fibrinogen, or the presence of inhibitory substances, e.g., fibrin-fibrinogen degradation products, heparin. BATROXOBIN, a thrombin-like enzyme unaffected by the presence of heparin, may be used in place of thrombin.