Poly(ethylene oxide) (PEO)-modified myoglobin (PEO-Mb) was immobilized on a carbon electrode with the aid of polyelectrolytes, and the redox response was successively analyzed in salt-containing PEO oligomers. Complexation of PEG-Mb with poly-(L)-lysine was found to be quite effective in maintaining a constant peak current in cyclic voltammograms for more than 20 h, but the peak current gradually decreased at 50 degreesC. Addition of poly-(L)-glutamic acid or their polyion complex was less effective to fix the PEG-Mb on the electrode. PEG-Mb was fixed through electrostatic interaction between PEG-Mb and poly-(L)-lysine. In order to achieve more stable redox activity of Mb in PEO oligomers, covalent immobilization of Mb to electrode surface was attempted. In this work, the maleic anhydride unit in the PEO-maleic anhydride copolymer was initially used to fix the Mb, followed by an ester formation between newly generated carboxylic groups and hydroxyl groups on the electrode surface. A stable redox response was obtained, and this effective immobilization of Mb on the electrode has yielded thermally stable redox response for 30 h even at 80 degreesC without denaturation nor elution of Mb into PEO oligomers.