The industrial application of immobilized enzymes is discussed. A model composite membrane system is described which could permit correlation of the efficiency of bioconversion with its diffusion characteristics. The diffusion and permeation characteristics of β-galactosidase (from Escherichia coli) immobilized in pure cellulose membrane (mol. wt. 7000 and 13,000) are presented. In an actual industrial scale bioreactor, using fiber-entrapped enzymes, there will be effects, in addn. to diffusional ones, which may play an important role in detg. the efficiency of the reactor, e.g., the denaturation of the enzyme, its phys. loss, and possible inhibition reactions.