The well-defined redox reaction of ferritin was observed at an indium oxide electrode with a fully hydrophilic surface at a low concentration of ferritin solution (0-7 mumol dm(-3)). This was a diffusion-controlled electrode reaction process. Conversely, at a higher concentration of ferritin (7 mumol dm(-3)<), ferritin was weakly adsorbed on the electrode surface. The electrochemical redox response of ferritin was strongly affected by the number of iron atoms at the core of the ferritin molecule.