The effects of the 15 polyol compounds on the thermostability of penicillin G acylase (PGA) from a mutant of Escherichia coli ATCC 11105 were investigated. PGA solutions containing glucose, sucrose, mannose, adonitol, sorbitol or xylitol at 1 M concentration showed an initial activity rate (IAR) higher than 3. The IAR values of these six sugars at 50 degrees C increased with their concentrations in PGA solution. The investigations on half-life times and stabilization factor (SF) values of PGA at 45 and 50 degrees C, in the presence of these compounds, showed a tendency for the half-life and SF of the enzyme, at 50 degrees C, to increase with the OH/C ratios of polyol compounds. Sucrose and glucose were the only two compounds having higher SF values at 50 degrees C than at 45 degrees C. Sucrose showed almost twice the half-life of glucose and was selected as the best stabiliser of PGA activity against thermal inactivation. PGA showed almost the same pH and temperature activity profiles in both presence and absence of sucrose. K-m and V-m values decreased with the increasing sucrose concentrations, with a greater decrease in K-m values.