125I-FXIa (2 nM) was incubated with platelets (1–2 × 108 platelets/ml) and TRAP (25 μM) in the presence of CaCl2 (2 mM) and ZnCl2 (25 μM). Increasing concentrations of non-radiolabeled peptides were added to each reaction. The reaction mixture was incubated for 30 min. prior to centrifugation through silicone oil to separate the platelets with bound peptide from the free peptide. A, Peptide 1 (●), peptide 2 (■), peptide 3 (◆), peptide 5 (▲), and peptide 6 (▼) were unable to displace 125I-FXIa from the activated platelet surface. B, Peptide 7 (●), the cyclic heparin binding peptide was able to displace 125I-FXIa from the activated platelet surface (Ki =5.8 ± 0.78 nM), whereas peptide 8 (■), a scrambled peptide of identical composition was unable to displace 125I-FXIa. Values in 6B represent the mean ± standard deviation of three experiments repeated in triplicate.