Human α-thrombin is a potent chemoattractant for human monocytes, with optimum activity occurring at about 10 nanomoles per liter. A variety of thrombins that were chemically modified to alter procoagulant or esterolytic functions showed a similar optimum activity, but complexes of prothrombin or α-thrombin with either antithrombin III or hirudin did not. These findings indicate that the regions in thrombin responsible for monocyte chemotaxis are proximate to those involved in certain protein recognition interactions of α-thrombin (for example, hirudin binding) but are distinct from the catalytic site and from certain exosites required for clotting.