PH-dependence of the kinetic parameters (Vmax, Km, Vmax/Km) and constants of competitive inhibition by phenylacetic acid of penicillinamidase-catalyzed hydrolysis of benzylpenicillin is studied. There are apparently 3 equilibrium ionogenic forms of the enzyme and enzyme-substrate (or enzyme-inhibitor) complexes, i.e., acidic, neutral and alkaline, the neutral form being the only active form of the Michaelis complex. Values of pK in the ionogenic groups controlling interconversions of the free enzyme (pK1 6.1 and pK2 7.6) and of the enzyme-substrate complex .**GRAPHIC**. 6.1 and .**GRAPHIC**. 0.2) or the enzyme-inhibitor complex .**GRAPHIC**. 6.1 and .**GRAPHIC**. 9.5) were determined. From this and the previously published results, the group with pK 6.1 was shown to be involved in the catalysis, and the group with pK 10.2 was involved in the maintenance of the active conformation of the active center of penicillinamidase. The ionogenic group with pK 7.6 was apparently involved in the enzyme-substrate binding.