Methods Assay of Protein C Activation-Initial rates of protein c activation with purified thrombomodulin and thrombin were determined by stopping the activation reaction and measuring activated protein C formed in the reaction mixture at selected time points. All activations were performed in 0.1 M NaCl, 0.02 M Tris-HC1, lO mg/ml of bovine serum albumin, pH 7.5, at 37" C. Gelatin (0.18) replaced the bovine serum albumin in those cases where the effect of calcium was being determined. In these experiments, aU proteins were dialyzed against Chelex-treated buffer (Bio-Rad) before use and the buffer was also