A biosensor for the measurement of glucose in serum has been developed, based on a screen-printed carbon electrode modified with Meldola's Blue-Reinecke salt, coated with the enzyme glucose dehydrogenase (from Bacillus sp.), and nicotinamide adenine dinucleotide coenzyme (NAD+). A cellulose acetate layer was deposited on top of the device to act as a permselective membrane. The biosensor was incorporated into a commercially available, thin-layer, amperometric flow cell operated at a potential of only +0.05 V versus Ag/AgCl. The mobile phase consisted of 0.2 M phosphate buffer (pH 7.0) containing 0.1 M potassium chloride solution, and a flow rate of 0.8 ml min(-1) was used throughout the investigation. The biosensor response was linear over the range of 0.075-30 mM glucose, with the former representing the detection limit. The precision of the system was determined by carrying out 20 repeat injections of a 5-mM glucose standard, and the calculated coefficient of variation was 3.9%. It was demonstrated that this biosensor system could be applied to the direct measurement of glucose in serum without pretreatment. Therefore, this would allow high-throughput analysis, at low cost, for this clinically important analyte.