Microporous polypropylene fiber bundles (Celgard), bent into U-shape and covered over most of the length with epoxy adhesive, make excellent flow-through reactors for entrapping enzymes. Such enzyme reactors containing horseradish peroxidase and glucose oxidase have been successfully used for determining peroxide in atmospheric precipitation and plasma glucose in test samples. The enzymes are stable in the reactor. This type of reactor is attractive both because no chemistry is necessary for enzyme immobilization and because orders of magnitude greater enzymatic activity can be achieved per unit reactor volume. (Copyright (c) 1987 American Chemical Society.)