A comparison of the kinetics of chymotrypsin-P catalyzed hydrolysis of N-acetyl-L-tyrosine ethyl ester and cinnamoylimidazole with those of the chymotrypsin Aα catalyzed reactions revealed the two enzymes to possess very similar catalytic properties. The deletion of the three C-terminal residues from the A-chain of chymotrypsin Aα, a feature characteristic of chymotrypsin-P, does not lead to significant changes in the function of the enzyme.