This paper describes the use of Nile Red-adsorbed gold nanoparticles (NRAuNPs) as selective probes and matrixes for the determination of aminothiols through surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). The binding of three aminothiols-glutathione (GSH), cysteine (Cys), and homocysteine (HCys)-to the surfaces of these NRAuNPs induces their aggregation, which causes asubsequent changes in their color and fluorescence. Because arginine-a non-thiol amino acid-does not induce such aggregation, it is a straightforward process to use the NRAuNPs to selectively concentrate the aminothiols from a solution containing all four of these analytes; we were able to identify the three aminothiols in the precipitate, and arginine in the supernatant, directly through SALDI-MS measurements. Without using this preconcentration approach, the limits of detection (LODs) at a signal-to-noise ratio of 3 were 1.0, 2.0, and 1.3 microM for GSH, Cys, and HCys, respectively. In comparison, selective concentration using the NRAuNPs provided LODs of 25, 54, and 34 nM, for the determinations of GSH, Cys, and HCys, respectively. NRAuNP matrixes provide a number of advantages over the use of conventional organic matrixes (e.g., 2,5-dihydroxybenzoic acid), such as ease of preparation, selectivity, sensitivity, and repeatability. We validated the applicability of our method through the analyses of GSH in red blood cells and of Cys in plasma; we believe that this approach has great potential for diagnosis.