The specific cyanate reagentN-acetyl-p-cyanato-L-phenylalanine ethyl ester (compound 1) was synthesized in an attempt to selectively modify the binding pocket of chymotrypsin (EC 3.4.21.1) while leaving the catalytic residues un touched. The reagent reacts with chymotrypsin to yield chiefly an inactive derivative 3a, with the active site Ser-195 carbamylated and the alpha-amino group, with the reagent 1 yields a modified enzyme(compound 4) with an additional carbamyl group on Ser-195.Neither derivative 3a nor 4reacts with diisopropylfluorophosphate under conditions where chymotrypsinogen is modified, indicating that Ser-195 is altered. Both derivatives 3a and 4 are retained on a 4-phenylbutylamine affinity colomn demonstrating that the substrate binding pocket is intact in both derivatives. The results indicate the potential value of aryl cyanates as protein reagents for the selective modification of nucleophilic sites. However, it is apparent that reaction at unreactive residues in the binding pocket of chymotrypsin with cyanates or similar reagents will require blockage of the more nucleophilic catalytic residues.