beta-Galactosidase was immobilized in/on poly(2-hydroxyethyl methacrylate) (pHEMA) membranes by two different methods: adsorption on Cibacron F3GA derivatized pHEMA membranes (pHEMA-CB), and entrapment in the bulk of the pHEMA membranes. The maximum beta-galactosidase adsorption on pHEMA-CB membranes was obtained as 95.6 mu g cm(-2) in 2.0 mg cm(-3) enzyme solution. The adsorption phenomena appeared to follow a typical Langmuir isotherm. In the entrapment, an increase in beta-galactosidase loading resulted in a consistent increase in membrane activity from 3.3 x 10(-2) to 17.8 x 10(-2) U cm(-2) pHEMA membranes. The K-m values for both immobilized beta-galactosidase (adsorbed 0.32 mM and entrapped 0.81 mM) were higher than that of the free enzyme (0.26 mM). The optimum reaction temperature of the adsorbed enzyme was 5 degrees C higher than that of both the free and the entrapped enzyme. The optimum reaction pH was 7.5 for free and both immobilized preparations. After 15 successive uses the retained activity of the adsorbed and the entrapped enzymes was 80% and 95%, respectively. The storage stability of the enzyme was found to increase upon immobilization.