Effect of blocking GPIb receptors on fibrin-enhanced platelet procoagulant activity. Thrombin generation was measured as in Fig 1. (□) Control; (▪) three fibrin clots added at t = 0; (∗) PRP preincubated with antibody 6D1 (anti-GPIb, 20 μg/mL); (▴) preincubation with 6D1 and fibrin clots added.