A simple procedure involving simultaneous adsorption and fixation was developed to immobilize beta-galactosidase (beta-Gal) from Aspergillus oryzae on a polysulfone hollow-fiber membrane module for converting lactose to galacto-oligosaccharides. Polyethylenimine was used as a polyelectrolyte intermediate layer to provide a positively charged character for beta-Gal adsorption. beta-Gal adsorbed on polyethylenimine layer was then fixed with cross-linking by glutaraldehyde. The concentrations of polyethylenimine, glutaraldehyde, and beta-Gal were significant parameters to achieve high activity yield. The optimum concentration of polyethylenimine and glutaraldehyde were determined as 5% (w/v) and 15% (v/v), respectively. The activity yield on the membrane surface increases as the concentration of beta-Gal used during adsorption and fixation increased. The highest activity yield obtained was 1.26 IU cm(-2) at 5 mg mL(-1) of beta-Gal concentration. The activity yield is proportionally dependent on the concentration of the beta-Gal employed. Moreover, the immobilized beta-Gal exhibited higher specific productivity and stable transgalactosylation activity compared with free beta-Gal during the lactose-catalyzed conversion reaction.