Direct electron transfer of hemoglobin (Hb) molecules was found at bare indium-tin-oxide (ITO) electrodes without promoters and mediators or modifications of electrodes by cyclic voltammetry. The quantity of electroactive Hb molecules was close to that of monolayer deposition on the electrode surface. The peak current, indicating redox reaction of a heme Fe-III/Fe-II couple, changed in proportion to sweep rate.