A novel immunosensor is presented based oil a charge accumulation system using all electrode modified with a polymer containing [osmium(4,4'-dimethyl-2,2'-bipyridine)(2)-chloride](+/2+) and horseradish peroxidase (Os/HRP polymer) to detect N-1,N-12-diacetylspertnine (DiAcSprn). Glucose oxidase (GOx)-labeled antibody captured with immunoreaction on Os/HRP polymer catalyzed the production of hydrogen peroxide, which oxidizes [Os(bpy)(2)Cl](+) in polymer with HRP. [Os(bpy)(2)Cl](2+) was gradually accumulated in the polymer and reduced back to [Os(bpy)(2)Cl](+) by applying -0.1 V. This accumulation system of redox species allows enhancement of the response.