Intact platelets do not show procoagulant phospholipids on their exterior. These phospholipids are located at the inside leaf of the bilayer membrane. They become available by (a) disrupture of the platelets (mechanical, osmotical etc.), (b) by a mechanism specific for the platelets, that we call the membrane flip-flop. Membrane flip-flop translocates procoagulant phospholipids (mainly phosphatidylserine) in the intact platelet from the inside to the outside. Thus the intact platelet becomes procoagulant. The trigger for the flip-flop mechanism is the simultaneous presence of small amounts of collagen and thrombin. The clotting factors IXa and VIIIa bind to the procoagulant lipids to form the factor X converting enzyme and factor Xa and factor Va to form prothrombinase.