A reagentless amperometric glucose biosensor was constructed using glucose oxidase and hydroquinone (H2Q) coimmobilized in carbon paste. The sensor response was studied by amperometry and cyclic voltammetry in quiescent solutions, as well as in a flow injection apparatus. Studies were conducted as a function of surface activation and sensor working conditions such as glucose concentration, storage, aging, and reusability. Surface activation proved to be useful to improve the electro-chemical reversibility of the mediator and the analytical characteristics of the sensor. The harsh conditions of the surface pretreatment did not deactivate the enzyme. Results from the assay of a clinical sample gave a glucose concentration value of 76 +/- 6 mg/dL, which compared favorably with the expected range of 72-88 mg/dL.