Résumé dans la langue de publication, traduction non disponible New thrombin inhibiting N**alpha -arylsulfonyl-p-guanidinophenylalanine amides of the formula wherein Ar is a substituted aryl and R1 and R2 are an alkyl group having 1-5 carbon atoms or together with the amine nitrogen form an heterocyclic ring, in racemate form as well as in form of optical active antipodes, theirs pharmaceutically acceptable salts, methods for their preparation, pharmaceutical composition and diagnostical preparation containing these compunds, use of the compounds in treatment of thrombosis and methods of treatment of thrombosis as well as methods for determination of thrombin concentration in blood.