An amperometric biosensor was constructed on the basis of ruthenium purple-containing clay and glucose oxidase for the direct assay of glucose. These clay modified electrodes were characterized by cyclic voltammetry and flow injection analysis techniques. The electrodes were very sensitive to the presence of glucose. Linear calibration curves between 10 mu M and 15 mM with an electrode sensitivity of 3.88 mu A/M and a detection limit of 10 mu M were registered in aerated solutions (pH 5.1). Uric acid, ascorbic acid, and oxygen interfere very little with the detection of glucose. Long-term tests showed these clay electrodes were quite durable, e.g., the electrode sensitivity only decreases by ca. 25% over a period of six weeks.