Chemically synthesised polyaniline was used for horseradish peroxidase (HRP) immobilisation after activation with glutaraldehyde. The immobilised enzyme showed better performance than free enzyme when submitted to concentrations of organic solvents (ethanol, acetone and acetonitrile) above 10% (v/v). The immobilised HR-P (PANIG-HRP) also presented a better performance than free enzyme when submitted to heat treatment, with a plateau of activity in the range from 30 to 60degreesC. The pH profile of the immobilised and free enzyme revealed a similar behaviour, although PANIG-HRP exhibited higher activity in the alkaline range (from pH 9.0 to 11.0) and optimum pH displacement of two units towards acidic range. In the best storage conditions the PANIG-HRP was 100% stable for 70 days. A difference in the kinetic parameters K-m, K-cat, K-m.app and K-cat.app were observed; immobilised enzyme presenting a K-m.app of 5.20 mmol 1(-1) and free enzyme 9.58 mmol 1(-1) with pyrogallol as substrate. (C) 2003 Elsevier Ltd. All rights reserved.