Summary. The procoagulant activity obtained from bovine thrombocytes has been compared to that of lipids isolated from platelets, with and without the addition of purified bovine factor V. A one‐stage assay, which consisted of delipidated bovine plasma containing RVV‐activated factor X, was used to assess the activity. At low lipid concentrations no difference in coagulant activity was found between sonicated vesicles of extracted platelet lipid and lysed platelets. At higher lipid concentrations, however, the extracted lipids were found to be less active than lysed platelets. Determination of factor V in suspensions of gel‐filtered platelets demonstrated that suspensions containing 2 × 109 platelets per ml possessed about 1% of the factor V activity present in a normal bovine plasma pool. Platelet lysis by sonication produced a five‐fold increase in factor V activity. Addition of factor V to sonicated vesicles of extracted platelet lipid, so as to produce an identical factor V activity per amount of lipid as found in lysed platelets, decreased the clotting time only in the higher lipid concentration range. A further three‐fold increase in the amount of factor V added to the lipid vesicles made the coagulant properties of the lipid vesicles indistinguishable from those of lysed platelets over the whole range of phospholipid concentrations tested. When the conditions of the test were changed by diminishing the concentration of factor Xa in the substrate plasma, the difference between lysed platelets and extracted platelet lipid disappeared completely. It is concluded that the higher coagulant activity of lysed platelets, as compared to that of extracted platelet lipid, can be ascribed to platelet factor V activity. Therefore there is no compelling necessity to postulate the existence of a specific procoagulant factor in the platelet other than factor V or phospholipids.