Abstract Straight, coiled, beaded, and packed-bed reactors containing immobilized glucose oxidase or l-(+)-lactate dehydrogenase were compared in terms of reaction rate, sensitivity, sample throughput, and sample dispersion. The enzymes were covalently attached to the inside walls of 5.0-cm long, 1.12-mm i.d. nylon tubes, and the resulting reactors were tested in a flow-injection system. The beaded enzymatically-active reactors (BEARs) were filled with solid glass beads of 0.5-mm or 1.0-mm diameter. Reactors with the larger beads had 2–4 times the activity, twice the sensitivity, and better throughput than the open reactors; they also minimized the physical and chemical contributions to dispersion. Packed-bed reactors were superior in the lactate determination, but the beaded reactors were better for the determination of glucose. With BEARs containing 1.0-mm beads, glucose was determined in the range 10–800 μM with a conversion efficiency of 0.056 mol of product per mole of substrate; for lactate, the range was 8–64 μM with a conversion efficiency of 0.13 mol mol −1.