An amino acid biosensor based on the immobilization of L-or D-amino acid oxidase within an iridium-dispersed carbon paste electrode is described. The strong and preferential electrocatalytic activity of iridium towards hydrogen peroxide allows rapid measurements of amino acids at very low potentials where the possibilities of interferences are minimal. The detection limits, depending on the amino acid, are in the order of 10−5M. The influence of parameters such as flow rate, working potential, iridium and enzyme loadings are discussed.