The native azurin (Az) was chemically modified with activated poly(ethylene oxide) (PEO) to solubilize in the PEO oligomers. PEO2000 (average molecular weight of 2000)-modified Az (PEO2000-Az) was dissolved without denaturation in PEG(200) (average molecular weight of 200). The electronic absorption spectrometry exhibited no detectable conformational change of PEO2000-Az in phosphate buffer. The electron paramagnetic resonance (EPR) spectrum of PEO2000-Az is also quite similar to that of native Az in water. This result confirms that the chemical modification with PEG,,,, does not cause any structural change around the copper site. Moreover, since no changes in the EPR spectrum are observed in PE200, the copper coordination geometry must be maintained even in PEO200. The redox activity of PEO2000-Az in PEO200 (containing 0.5 mol dm(-3) NaClO4) was investigated by cyclic voltammetry. When we use the PEO350-SH (alpha -methoxy-omega -mercapto-poly(ethylene oxide); PEO with average molecular weight of 350) modified gold electrode as a working electrode, the redox response was observed in PEG,,. The result of the measurements for the scan rate dependence suggests that PEO2000-Az could be immobilized on the PEO350-SH modified electrode.