Electrocatalytic activity towards oxygen reduction of fungal laccase entrapped in poly-o-phenylenediamine (POPDA) matrix on glassy carbon electrodes was studied. Cyclic voltammetry and amperometric responses to dissolved oxygen were investigated in pH range from 2 to 6. POPDA reveals a unique ability to serve as a redox mediator for laccase and immobilizing matrix at the same time. The entrapped enzyme efficiently catalyzes reduction of molecular oxygen without any additional mediators. The electrocatalytic current reaches 0. 1 mA/cm(-2) per 1 mu g of immobilized enzyme on cyclic voltammograms recorded at I mV/s in a not stirred electrolyte. Current density values are comparable with those revealed by dissolved laccase (60 mu g/ml) mediated by hydroquinone and greatly higher than by the mediator less laccase/glassy carbon system. The potential of oxygen reduction is determined by the polymer redox couple. Consequently, the onset of the oxygen reduction shifts from -0. 15 V versus Ag/AgCl in pH 6 to +0.05 V versus Ag/AgCl in pH 2. The laccase-POPDA layers immersed in the deaerated solution show fast amperometric responses to addition of the oxygen containing solution. The observed current values depend linearly on the oxygen concentration. Factors affecting the electrocatalytic activity of the laccase-POPDA system, including the layer thickness and the pH value, are studied. The electrodeposited laccase-POPDA films are characterized by infrared spectra. The results prove that the enzyme secondary structure remains unchanged during the entrapment procedure. POPDA matrix structure consists of the phenazine-type polymer according to infrared spectra. (c) 2007 Elsevier Ltd. All rights reserved.