Enzyme electrodes based on cross-linking bilirubin oxidase or a Streptomyces coelicolor laccase (SLac) and osmium redox complexes possessing an amine-terminated molecular tether at graphite electrodes can produce current for oxygen reduction under pseudo-physiological conditions. Here we report on enzyme electrodes for oxygen reduction by co-immobilization of SLac, polyallylamine (PAA) and an [osmium(2,2'-bipyridine)(2)(4-aminomethylpyridine)Cl](+) complex. Enzyme electrodes prepared by incorporation of added multi-walled carbon nanotubes as support produce oxygen reduction current densities of 0.8 mA cm(-2) in oxygen saturated, 150 mM NaCl, 50 mM phosphate buffer solution at 37 degrees C, a 3-fold increase in oxygen reduction current densities over those prepared without multi-walled carbon nanotubes. Membrane-less glucose-O-2 fully enzymatic fuel cells are assembled by combination of the SLac-based enzyme electrode as a cathode, with glucose-oxidizing anodes, based on either a pyrroloquinoline quinone (PQQ) or FAD-dependent glucose dehydrogenase (GDH) and tetherable osmium redox complexes. The fuel cell based on selection of PQQGDH enzyme electrode as anode provides a maximum power density of 66 mu W cm(-2) in 5 mM glucose, 150 mM NaCl, phosphate buffer solution at 37 degrees C, dropping to 37 mu W cm(-2) in human serum, the highest reported power density to date for an enzymatic fuel cell operating in serum. (C) 2013 The Electrochemical Society.