A new approach to construct a reagentless hydrogen peroxide biosensor is described. Horseradish peroxidase and thionine are covalently bound to a cysteamine-assembled gold electrode using glutaraldehyde as a bifunctional reagent. Thionine immobilised in this way can shuttle electrons between the electrode and the redox activity center of the enzyme. The sensor was highly sensitive to hydrogen peroxide with a detection limit of 8.0 x 10(-7) mol l(-1) and a response time of less than 4 s. The effects of the applied potential and the pH values of the buffer solution on the response of the sensor were investigated for optimum analytical performance.