Crude or purified Aspergillus niger ß-galactosidase preparations were immobilized on chitosan (deacetylated chitin, activated with glutaraldehyde). The most active immobilized systems were obtained withcrude enzyme preparations. The immobilized enzyme hydrolyzed lactose in pure lactose solutions, ultrafiltrate whey permeates, or acid wheys at similar rates. The pH activity profiles and Km values of the chitosan-bound enzyme were not significantly altered on immobilization, and its stability on repetitive use up to 60°C was increased by reduction with NaBH4. After 8 weeks on discontinuous operation (8 h use per day), ß-galactosidase-chitosan columns were found to retain about 90, 50, or 60% of their initial activities with lactose, ultrafiltrate permeate, or acid whey solutions, respectively. The efficiency of the ß-galactosidase-chitosan conjugate appears to be comparable or greater than those of other described systems, and its stability should allow its use on an industrial scale.