A simple kinetic method for chymotrypsin assay is described.The method is based upon the hydrolysis of N-benzoyl-L-tyrosinamide as a substrate, resulting in the production of ammonia. The initial rate of ammonia evolution is selectively measured potentiometrically with an ammonia gassensing electrode (Horiba ammonia gas-sensing electrode 5002A-06T). Optimal conditions for the assay are specified. The initial rate of ammonia evolution is proportional to the enzyme activity, and a working range of 0.08 to 0.32 U/ml chymotrypsin was obtained. The recovery of chymotrypsin was 99.2-102.5%. In order to study the applicability of the method, recovery experiments were performed by use of an ophthalmic chymotrypsin preparation.