Effects of platelets and factor XI in thrombin-activated coagulation assays. Coagulation assays were performed as described in the Materials and Methods. Either factor XI-deficient plasma (A and C) or normal plasma (B and D) was incubated with phospholipid vesicles (PS:PC 1:3 ratio, 10 μmol/L; A and B) or gel-filtered platelets (200,000 platelets/μL; C and D) activated with the thrombin receptor peptide, SFLLRN-amide (5 μmol/L), and CaCl₂ (5 mmol/L). In some assays, affinity-purified goat antihuman factor XI (14.6 μmol/L) was added, followed by thrombin at various concentrations (0.05, 0.1, 0.25, 0.5, and 1.0 U/mL) to initiate clot formation. Data shown are the means (±SEM) for four experiments, each performed in triplicate. In the absence of added thrombin, the clotting times of all samples were greater than 5 minutes. (A) Factor XI-deficient plasma and phospholipid vesicles in the absence (○) or presence (•) of anti-factor XI antibody. (B) Normal plasma and phospholipid vesicles in the absence (○) or presence (•) of anti-factor XI antibody. (C) Factor XI-deficient plasma and normal activated platelets (□) or platelets from factor XI-deficient patients (▵) or normal platelets in the presence of anti-factor antibody (▪). (D) Normal plasma and normal activated platelets (□) or platelets from factor XI-deficient patients (▵) or normal platelets in the presence of anti-factor XI antibody (▪).