PolyP inhibitors reverse the ability of platelet releasates to accelerate factor XI activation by thrombin. Initial rates of activation of 30nM human factor XI by 20nM human α-thrombin were determined in the presence of releasate prepared from TRAP-stimulated human platelets as described,¹¹  normalized to the rate of factor XI activation without any added polyP inhibitor. Percent inhibition is plotted versus inhibitor concentration for the following: low MW polyethyleneimine (▴); generation 1.0 dendrimer (■); spermine (▵); PPXbd (●); or polymyxin B (□). Data are mean ± SE (n = 4). IC₅₀ values calculated from these curves are given in the text. In the second stage of the assay, factor XIa levels were quantified, as previously described,¹¹  by monitoring the rate of cleavage of the chromogenic substrate, L-Pyr-Pro-Arg-p-nitroanilide. At the concentrations used, none of the inhibitors altered the rate of hydrolysis of this substrate by factor XIa.