A remarkably selective lactate biosensor, based on the immobilization of lactate oxidase within a rhodium-dispersed carbon paste is described. The effective catalytic action of the rhodium microparticles towards the generated hydrogen peroxide or consumed oxygen allows tuning of the operating potential to a region where unwanted reactions are prevented. Contributions from all major easily oxidizable substances are eliminated, without the need for membrane barriers or artificial electron mediators. The detection limit is 1.5 × 10−5 M and the response time is 8 s. A fast and sensitive flow injection operation is illustrated.