The selective detection of cysteine and cystine amino acids over other standard amino acids was possible with the naked eye using silver nanoparticles (AgNPs) in a simple procedure at room temperature. The change in color and the aggregation of NPs were studied using UV-vis spectroscopy and transmission electron microscopy, respectively. It was observed that when the derivative and substructures of cysteine were employed as analytes the detection was possible only when there is free SH or S-S group present in the analytes. The method was extended to a dipeptide 'cys-gly' as a model peptide where the detection was successful due to the presence of SH moiety.