Abstract A simplified procedure for the preparation of pepsin from porcine pepsinogen has been developed. Chromatographic and kinetic data indicate that this procedure yields a pepsin product identical with that obtained by the conventional method. The K m and k cat values for the hydrolysis of Ac-Phe-Tyr-OMe by commercial and freshly prepared pepsins were determined. All pepsins studied were found to have the same value for k cat . The parameter which kinetically distinguished each of the pepsins was K m .