The carboxyl groups of purified beta-glucosidase from Aspergillus niger NIAB280 were modified by water soluble 1-ethyl-3(3-dimethylaminopropyl) carbodiimide (EDC) in the presence of glycinamide and ethylene-diaminedihydrochloride for 60 min. The neutralization (GAM) and reversal (EDAM) of negative charges had a dramatic effect on the thermostability of modified beta-glucosidases. The half-lives of GAM and EDAM at low temperatures (55 and 60 degrees C) were reduced, whereas at higher temperatures (64 and 67 degrees C) half-lives were enhanced as compared with the native beta-glucosidases. At 70 degrees C the half-life of GAM became equal to the native whereas that of EDAM was increased. The denaturation activation energies of of native, GAM and EDAM were 389, 279 and 175 kT mol(-1) respectively. Native, GAM and EDAM beta-glucosidases showed compensation effect at 70 degrees C. Both enthalpies (Delta H*) and entropies of activation (Delta S*) for denaturation of GAM and EDAM were decreased compared with native enzyme. The effect of subtilisin on native, GAM and EDAM beta-glucosidases were tetraphasic with periodic loss and gain of enzyme activity. Denaturation of all three beta-glucosidases in 4 M urea for 150 min also showed a periodic activation effect. A possible explanation for the thermal inactivation of native and increased thermal stability of GAM and EDAM at higher temperatures is also discussed.