Fungal Cerrena maxima laccase cross-linked on graphite catalysed the reaction of direct (in the absence of any mediators) bioelectrocatalytic reduction of O-2 starting from 0.74 V versus Ag/AgCl, pH 5. Efficiency and stability of direct bioelectrocatalysis by the cross-linked laccase was enhanced by coupling laccase to carbon nanoparticles (CNPs), consistently with the increased surface concentration of the laccase molecules properly orientated for the direct electron transfer (ET) reaction within the charged CNP-laccase nanocomposite matrix. Bioelectrocatalytic activity of the laccase electrode was totally inhibited by CN-, while Cl- anions reversibly shifted the onset of bioelectrocatalysis to 0.53 V. in accordance with the different degree of inhibition of internal ET between the T1 and T2/T3 Cu centres of the laccase, which allowed the bioelectrocatalytic reduction of O-2 also in the presence of Cl-. The laccase CNP biocathode have been used for the construction of a hybrid biobattery based on the Zn anode and O-2 biocathode, providing the maximum power density of 0.16 mW cm(-2) at the cell voltage of 0.5 V, pH 5, and 0.1 mW cm(-2) in the presence of chloride. (C) 2011 Elsevier B.V. All rights reserved.