High pressure (>200 Mpa) or high temperature (>45 degrees C) can both induce an irreversible inactivation of the beta-galactosidases of Aspergillus oryzae, Kluyveromyces lactis and Escherichia coli. Moderate pressures (50 MPa-250 MPa) exerted a protective effect against thermal inactivation for the three beta-galactosidases investigated. High pressure could thus be used to carry out beta-galactosidase catalysed reactions such as lactose hydrolysis, at higher temperatures.