Enzymatic hydrolysis of lactose by beta-galactosidase (Lactozym) has been studied in a jacketed stirred-tank reactor (JSTR) and in a recirculation hollow-fibre bioreactor (RHFB). The hollow-fibre module was manufactured with modified cellulose-acetate membrane (Cuprophan). The variables tested were: temperature (25-40degreesC), lactose concentration (0.0278-0.208 M) and enzyme concentration (0.100-4.51 g l(-1)). Within the experimental range analysed, the conversions in the RHFB, were lower than in the JSTR. A kinetic model is proposed for enzymatic hydrolysis of lactose in the RHFB, which considers competitive inhibition by the product (galactose) and reversible adsorption of the enzyme-galactose complex on the hollow-fibre membrane. The adsorption enthalpy of the enzyme-galactose complex onto the membrane was determined as being 775 kJ mol(-1). (C) 2003 Elsevier Ltd. All rights reserved.