The properties of the oxygen electrode active layer with laccase immobilized on highly dispersed colloid graphite (HCG) or carbon black AD 100 are studied. It was assumed that such a composite material provides the optimum conditions for bioelectrocatalytic oxygen reduction and allows enhancing the measured current values referred to the electrode geometric surface. These expectations were not wholly justified: the employment of HCG as a carrier allowed enhancing the specific activity per laccase molecule up to 29.8 mu A/pmol of the enzyme, which is five times higher than the specific activity of laccase applied to AD 100 (5.9 mu A/pmol of the enzyme); however no success was achieved in forming the active layers of any considerable thickness both in the first and in the second case, as the activity decreased drastically at the increase of the layer thickness. The nature of these problems can be caused by a number of reasons: the physico-chemical and structural characteristics of the carbon materials determining, in their turn, the adsorption value and the orientation of the enzyme molecules towards the carrier and the regularities of the active layer structure consisting of the fractal clusters of carbon particles. With the help of computer modeling, the fractal and percolation properties of the active layer structure are studied. The models of the active layer structure are suggested, in which carbon particles form agglomerates of fractal cluster type. This allows to suggest a number of considerations that can help to explain the fractal and percolation effects and the effect of the carbon carrier particle size on the immobilized enzyme coverage. (C) 2005 Elsevier Ltd. All rights reserved.