α-thrombin, β-thrombin, FXI, and FXIa bind with high affinity to immobilized polyP. Binding of α-thrombin, β-thrombin, FXI, or active site–inhibited FXIa to polyP was quantified using SPR, with biotinylated polyP bound to streptavidin sensorchips, over which varying protein concentrations were flowed. Panels are representative sensorgrams for 2.5 to 20nM α-thrombin (A), 5 to 80nM β-thrombin (B), 1.25 to 40nM FXI (C), and 2.5 to 20nM active site–inhibited FXIa (D). K_d values were derived as described under “Methods.”