We report the electroreduction of O-2 to water under physiological conditions (pH 7.4, 0.15 M NaCl, 37.5 degreesC) at a current density of 5 mA cm(-2) and at a potential only 0.18 V reducing versus that of the reversible O-2/H2O electrode at pH 7.4. The immobilized electrocatalyst enabling the reduction is the electrostatic adduct of bilirubin oxidase from Myrothecium verrucaria, a polyanion at pH >4.1, and the polycationic redox copolymer of polyacrylamide and poly (N-vinylimidazole) complexed with [Os (4,4'-dichloro-2,2'-bipyridine)(2)Cl](+/2+), cross-linked on carbon cloth. The current density of the rotating electrodes was O-2 transport limited up to 8.8 mA cm(-2); their kinetic limit was reached at 9.1 mA cm(-2). The operational life of the electrodes depended on their angular velocity, which defined not only the current density but also the mechanical shear stress stripping the electrocatalyst. When the electrodes were rotated at 300 rpm and were poised at -256 mV versus the potential of the reversible O-2/H2O electrode, their 2.4 mA cm(-2) initial current density decreased to 1.3 mA cm(-2) after 6 days of continuous operation at 37.5 degreesC.