A modulator of the enzymatic activity of human thrombin, designated LY254603, was identified that enhances the thrombin-catalyzed generation of the anticoagulant factor activated protein C, yet inhibits thrombin-dependent fibrinogen clotting, By means of mutant substrates, it was shown that LY254603 mediates the change in enzymatic substrate specificity through an alteration in thrombin’s S3 substrate recognition site, a mechanism that appeared to be independent of allosteric changes induced by either sodium ions or by thrombomodulin. This compound may represent the prototype of a class of agents that specifically modulates the balance between thrombin’s procoagulant and anticoagulant functions.