Biocatalysts are an attractive alternative to conventional noble metal catalysts for fuel cell cathodes, In principle. they combine the possibility of more efficient operation (lower intrinsic overpotential) with the benefits of renewability and low temperature operation. In this paper we discuss some fundamental biochemistry and electrochemistry of one cathode system that has attracted some interest in the biofuel cell community, laccase its catalyst with (ABTS) its mediator. We focus oil one laccase preparation from Trametes versicolor, and demonstrate that steady state electrocatalylic oxygen reduction currents can be modeled using the measured intrinsic properties of the enzyme (k(cat), K-M) and mediator (D-M). We then discuss some of the implications of this modeling for practical electrode design. (c) 2005 Elsevier B.V. All rights reserved.