An enzyme immobilization technique employing enzyme sequestration within the porous support regions of asymmetric hollow fiber membranes is described and experimentally evaluated. Reactor conversion data over a wide range of operating conditions agrees well with predictions obtained from a mathematical model developed previously. β-galactosidase immobilized by this technique was found to retain 100% activity for 60 hours of continuous reactor operation, and for 140 days when stored at 3°C. The effects on reactor performance of (1) enzyme adsorption by the membrane, and (2) axial redistribution of enzyme accompanying radial flow of fluid through the fiber wall, have been evaluated; neither process significantly alters conversion kinetics or efficiency for the substrate/enzyme system investigated.