The characteristics of a novel cathode on which O-2 is electroreduced to water under physiological conditions (pH 7.4, 0.15 M NaCl, 37.5 degreesC) are reported. The cathode was made by electrically connecting ("wiring") redox centers of bilirubin oxidase (BOD) from Trachyderma tsunodae (Tt) to carbon electrodes through a redox polymer. The cathode was more stable than that made by wiring BOD from Myrothecium verrucaria (Mv) (N. Mano et al., J. Am. Chem. Soc. 2002, 124, 6480-6486) (5% loss of current per day of operation vs 10%), and its operating potential at 3 mA cm(-2) current density was -140 mV vs the potential of the reversible O-2/H2O electrode, 50 mV higher than that of the Mv-BOD cathode. The improved stability is attributed to stronger electrostatic bonding in the adduct of Tt-BOD with the "wiring" redox polymer, and the higher operating potential to the replacement of methionine by phenylalanine in the axial position of the type 1 Cu+/2+ center, which upshifts the center's potential.