The interaction between Ag/Pd bimetallic nanoparticles and cysteine was introduced as a new and high potential colorimetric probe for the selective, sensitive and low-cost recognition and monitoring of cysteine in aqueous samples. In the presence of NaCl, cysteine was able to induce the aggregation of Ag/Pd bimetallic nanoparticles thereby resulting in a change in yellowish-brown color of the Ag/Pd colloid to green. The presence of 6.6 mM NaCl in the samples decreases the electrostatic repulsion and accelerates the aggregation of Ag/Pd nanoparticles. The sensitivity and selectivity of the proposed sensor toward the other amino acids were studied too. This simple assay was rapid (5 min) and highly sensitive (limit of detection (LOD) = 3.3 nM)), selective and it can be used for rapid monitoring of cysteine in real pharmaceutical formulation. (C) 2014 Elsevier B.V. All rights reserved.