SummaryOne volume of plasma with 6–8 units heparin per ml. incubated with two volumes of 0·3M d‐glucose formed clots after 30 to 100 minutes at 37°. Incubation with thiourea or sodium chloride produced smaller quantities of plasma clots. Heparinised plasma from a haemophilia A patient (factor VIII deficiency) and plasma from patients, under treatment with oral anticoagulants, deficient in factor II (prothrombin), factor VII (proconvertin), and factor X (Prower‐Stuart factor) did not clot after incubation with d‐glucose. Platelets are essential in this process but can be replaced by erythrocyte ghosts or by brain phospholipids but lecithins are inactive. Calcium ions must also be present. No appreciable loss of heparin occurred, but clotting was associated with reduction of heparin co‐factor (antithrombin II). The thrombin clotting time of heparinised plasma which contained d‐glucosc, mannitol, saccharose, glycine and also thiourea was shorter than that in the presence of sodium chloride. In contrast, thrombin clotting time of citrated plasma is not shortened by any of the above compounds except glycine. Incubation of plasma containing between 1 and 1·6 heparin units/ml. without further additions produced high yield of plasma clots. The addition of thromhin to heparinised plasma gave shorter clotting times in platelet‐containing plasma than in platelet poor plasma. The yield of plasma clot was dependent upon the thrombin concentration, but a low concentration of thrombin added to heparinised platelet‐containing plasma caused more fibrin formation than in platelet‐poor plasma. A low concentration of thrombin added to platelet‐containing plasma deficient in factor II (prothrombin), factor VII (proconvertin), factor X (Prower‐Stuart factor) gave a low yield of fibrin. The difference of fibrin yield between platelet‐containing and platelet‐poor plasma was seen with both commercial and purified bovine thrombin preparations. Very dilute human brain extracts added to heparinised platelet‐poor plasma gave, despite a very long clotting time, high yields of fibrin. Heparin co‐factor activity (antithrombin II) is greater in plasma than in serum.