The doping of semiconducting conjugated polymers (e.g., polyaniline, PANI) can result in the elimination of the bandgap, leading to high electrical conductivities (comparable to metals). Doped PANI is totally insoluble and thus nonprocessable, which considerably limits its practical applications. Synthesis of PANI using immobilized horseradish peroxidase (HRP) as a catalyst in aqueous solutions can open up additional possibilities for applications through the direct synthesis of controlled-thickness PANI layers on various substrate surfaces. The RF plasma-enhanced surface functionalization of polyethylene and the covalent immobilization of HRP are discussed, and the polymerization of aniline initiated by immobilized HRP is presented. The nature of plasma-grafted surface functionalities on the substrate surfaces and the formation of PANI are demonstrated using X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform IR spectroscopy. The molecular weight distribution of PANI is evaluated with gel permeation chromatography, and the activity of the immobilized enzyme is monitored using UV spectroscopy. (C) 2003 Wiley Periodicals, Inc.