The stabilisation off Escherichia coli penicillin G acylase (PGA) by dextran polymers (of molecular weight 11.5, 37.7 and 71 kDa) was studied. The inactivation of both the native and dextran-containing enzyme preparations obeyed first-order kinetics at the temperature and pH values studied. The optimal concentrations of dextran polymers of molecular weight 11.5, 37.7 and 71 kDa stabilising PGA against inactivation were 50, 20 and 7.5 mmol dm(-3) respectively. Dextran 11500 (11.5 kDa) gave 100-fold protection of PGA against thermal inactivation of enzyme above 50 degrees C. The kinetic constants of the enzyme were slightly altered, but temperature and pH profiles were not altered by the dextrans.