This specificity for N-benzyloxycarbonyl- L-phenylalanyl-L-serine methyl ester results almost entirely from the large difference between the values of Km for the two substrates. Indeed, the value of 0.17M for the bromelain-catalysed hydrolysis of N2- benzoyl-L-arginine ethyl ester is so large as to suggest that this substrate should be regarded as essentially non-specific.