The immobilization of the enzyme beta-galactosidase from Thermus sp. T2 was performed via ionic adsorption onto two different supports: a new anionic exchanger resin, based on the coating of Sepabeads internal surfaces with polyethylenimine (PEI) polymers (M-w 25,000), and conventional DEAE-agarose. Immobilization proceeded very rapidly in both cases, but the adsorption strength was much higher in the case of PEI-Sepabeads than in DEAE-supports at both pH 5 and 7 (e.g. at pH 7 and 0.4 M NaCl, less than 5% of enzyme was eluted from PEI-support while more than 70% protein was eluted from DEAE-agarose). Interestingly, the PEI-derivatives remained almost fully active at pH 5 and 7 after several weeks of incubation at 50 degreesC, conditions that allows the hydrolysis of lactose in milk coupled with the antimicrobial treatment usually performed.