Film formation during the pitting corrosion of aluminium in human plasma is studied using conventional electrochemical techniques complemented with scanning electron microscopy as well as energy-dispersive x-ray, electron microprobe and immunoelectrophoresis analysis. The anodic polarization curve of aluminium in human plasma at 37 degrees C shows a passive region related to a low corrosion rate that extends up to -0.66 V (s.c.e.). At more positive potential values than -0.66 V, the breakdown of the oxide protective film occurs due to the chloride anions, leading to an intense localized dissolution of the metal (pitting corrosion). Scanning electron microscopy of the metal surface reveals a thick non-adherent precipitate covering pits of a crystallographic feature. Energy-dispersive x-ray and the microprobe analysis show the presence of aluminium, chloride and carbon signals. Immunoelectrophoresis of the precipitate resuspended in physiological solution shows a strong protein denaturization. Results suggest that the corrosion products are formed during pitting by the interaction of the saturated AlCl3 salt present within the pits and the adsorbed plasma proteins.