A thrombin derivative having a lowered fibrinogen activity, a process for producing thereof, an anhydrothrombin derivative having a lowered fibrinogen activity, and a process for producing thereof are provided. This invention is to provide a thrombin derivative formed by modifying carboxyl group(s) of the thrombin and an anhydrothrombin derivative formed by modifying carboxyl group(s) of the anhydrothrombin. The thrombin derivative can be utilized as a composition for inducing platelet aggregation and further as a reagent of specific aggregation for clinical testing and the like. By affinity chromatography using carboxyl group(s) bound with anhydrothrombin as a ligand, it is made possible to carry out necessary purification with high selectivity relative to the blood coagulation factor VIII. Further, the anhydrothrombin derivative which results from modifying carboxyl group(s) of anhydrothrombin produces sufficient antithrombogenic effects at a smaller amount as compared with the previous anhydrothrombin and, therefore, can be utilized as an agent for inhibiting the thrombogenesis.