A series of N alpha-(arylsulfonyl)-L-arginine esters was prepared and tested as inhibitors of the clotting activity of thrombin. N alpha-Dansyl-L-arginine methyl ester was the most inhibitory of the N alpha-(arylsulfonyl)-L-arginine methyl esters. The most potent inhibitors were the n-propyl and n-butyl esters of N alpha-dansyl-L-arginine with an I50 of 2 X 10(-6) M. Esters of unsaturated straight-chain alcohols with a chain length of four carbons were also as inhibitory as the n-butyl ester. The inhibitors were hydrolyzed by thrombin and trypsin more slowly than N alpha-tosyl-L-arginine methyl ester.