Glucose oxidase (GO) (EC 1.1.3.4) was used as catalyst for oxidizing glucose into gluconic acid utilizing a 10-mL Bioengineering Enzyme Membrane Reactor or a 400-mL Millipore Stirred Ultrafiltration Cell (MSUC) coupled with a Millipore UF membrane (cutoff of 100 kDa) and operated for 12 h under an agitation of 100 rpm, pH 5.5, and 30 degrees C. The effect of feeding rate (0.10, 0.15, or 0.20 min-1), glucose (2.5 or 5.0 mM), and GO (1.0 or 2.0 mg/mL) concentrations on the catalysis were studied. A yield of about 75% was attained when the MSUC filled with 1.0 mg/mL of GO was fed with 2.5 mM glucose solution at a rate of 0.15 min-1.