Anticoagulant cofactor activity of HDL in clotting assays in the presence of APC/protein S. APC and protein S, APC alone, or control buffer without APC/protein S (as indicated) were mixed with plasma, fibrinogen, and purified lipoproteins, and clotting was initiated by addition of tissue factor/Ca++ (see Methods). Final LDL (circles) or HDL (squares) concentration was based on choline-containing PL content.