Enhanced sensitivity of hydrogen peroxide was performed by accumulating the charge through oxidation of [Os(bpy)(2)](2+) (bpy = bipyridyl) into [Os(bpy)](3+) in a peroxidase-containing polymer layer on an electrode under the open-circuit condition and then reduced by applying negative potential. The accumulation resulted in a large current response, and the lower detection limit for hydrogen peroxide and that for glucose oxidase, a hydrogen peroxide-producing enzyme, were 0.1 nM and 2 x 10(-4) U L-1, respectively.