The pH dependence of the equilibrium constant KHyd for the hydrolysis of the Lys15-Ala16 reactive-site peptide bond of the bovine pancreatic trypsin inhibitor (aprotinin) was investigated over the pH range 2.3-6.5. Solutions of aprotinin, modified aprotinin with the Lys15-Ala16 peptide bond cleaved and mixtures of both species were incubated with 10 mol% porcine beta-trypsin. The state of equilibrium was determined by analytical cation-exchange HPLC. The KHyd values obtained did not exactly obey the simple equation of Dobry et al. (1952), which had to be used in an extended form with two additional parameters for a satisfactory fit. The pH-independent equilibrium constant is 0.90 and the pK values of the Lys15 carboxyl group and of the Ala16 amino group are 3.10 and 8.22, respectively. The pK of an additional group is apparently perturbed by the peptide-bond hydrolysis. It is 4.60 in the native and 4.40 in the modified aprotinin.