In this work, the direct electron transfer of ferritin adsorbed on bare gold is investigated. Clean gold electrodes were exposed to purified horse spleen ferritin in phosphate buffer at controlled potential. Cyclic voltammetry was the principal method used to investigate the reactivity of the adsorbed layer. Ferritin adsorbs at fairly negative potentials and exhibits reasonably well-defined current-potential curves. The packing density increases with ionic strength, indicating a hydrophobic interaction between adsorbed ferritin and the gold electrode surface. Potential dependent studies suggest that an electroactive layer is formed only with reduced ferritin. An adsorption time dependence study shows that a limiting packing density is reached within 90 s exposure time. (C) 2000 Elsevier Science B.V. All rights reserved.