The detection of amino acids using aminoacyl-tRNA synthetases (ARSs) as the molecular recognition element was proposed, and the binding activity and specificity of ARSs were evaluated. Using this rapid and easy method, from 15 to 50 microM tyrosine could be measured specifically. The method suggested in this article could be realized without an amino acid labeling process or a large volume of organic solvents, and the time for measurement was reasonable.