The effects of polyhydric compounds (PHC) on the stabilization of penicillin G acylase (PGA) obtained from a mutant of Escherichia coli ATCC 11105 against pH were studied. Glucose and sucrose in I M concentration and polyethylene glycols (PEG) at a molecular weight of 400, 4000 and 15 000 in 250, 150 and 50 mM concentrations, respectively were used as PNC. The inactivation mechanisms of native and PHC containing PGA are considered to obey first order inactivation kinetics during prolonged incubation time interval of enzyme solutions at different pH values in 40 degrees C. PHC containing PGA preparations showed lower inactivation rate constants than native PGA preparations at the pH values studied. Lowest inactivation rates were obtained at pH 7.0. Inactivation rates were higher when the enzyme solutions were incubated at pH values above 7.0. Among the PHC compounds used, the sucrose, PEG 400 and PEG 4000 showed the highest enhancement to the pH stability of PGA. The addition of PHC into PGA. solution did not cause any change in the pH optimum of enzyme in penicillin G (Pen G) hydrolysis reaction.