The blockage of the carboxyl groups of a pancreatic trypsin inhibitor (inhibitor of Kunitz) has been performed with a water-soluble carbodiimide and glycine ethyl ester. The inhibitor has five carboxyl groups located on residues Asp 3 , Glu 7 , Glu 4 9 , Asp 5 0 and Ala 5 8 . When the reaction is carried out on native inhibitor or on inhibitor treated with 8 M urea, approximately 3 moles of glycine ethyl ester are introduced but 4, 3 residues can be incorporated into performic acid oxidized inhibitor. Fully active derivatives with a complete substitution of the C-terminal residue can be obtained. It may be concluded that the C-terminal carboxyl group is not involved in the binding with trypsin.