Some chemically synthesized conductive polymers, based on polyaniline (PANI), were used as supports for Horseradish Peroxidase (HRP) immobilization. The polymers were first activated with glutaraldehyde (PANIG) and then HRP was successfully immobilized. Among the tested polymers, those with intermediate oxidation state (PANIG(1), PANIG(2) and PANIG(3)) were more suitable for HRP immobilization than that predominately reduced (PANIG(2)) or oxidized (PANIG(5)). PANIG(1) presented the best performance for HRP immobilization, and the analysis of the protein versus active enzyme retention showed that immobilized HRP maintained 100% native activity. The effects of enzyme concentration, reaction time and pH on the immobilization efficiency were investigated. (C) 2003 Elsevier Science Ltd. All rights reserved.