Collagen enhances coagulation in the absence of tissue factor. Human citrate-anticoagulated plasma with phospholipid vesicles (PPP + PV) or platelets (PRP) was pretreated with 4 μM CTI or 100 nM FVIIai, as indicated. Preparations were then incubated with collagen type I Horm (type I/H) or vehicle solvent (control) for 10 minutes. Activation was with 16.6 mM CaCl₂; tissue factor (1 pM) was present only if indicated. (A,B) Collagen shortens the clotting time when using normal plasma (circles), but not FXII-deficient plasma (triangles). Bar graphs represent clotting times obtained with normal plasma and 40 μg/mL collagen. Note the potent collagen effect without tissue factor (gray), but not with tissue factor present (white). (C,D) Collagen (5 μg/mL) potentiates thrombin generation in the presence of phospholipids and platelets. Shown are representative thrombin generation curves, and bar graphs of thrombin peak height (relative to the control condition of vehicle solvent). Mean ± SEM; n = 3-5 donors; *P < .05 vs control; #P < .05 vs condition with collagen.