For the first time, the interaction of casein micelle (CM) with gold nanoparticles (GNPs) was studied using TEM, UV-visible, FTIR and fluorescence spectroscopy methods. The results show that GNPs bind to CM surface, leading to the formation of GNP-CM conjugates. GNPs exhibit similar particle diameters, and the CM structure is retained in GNP-CM conjugates. Furthermore, GNP-CM conjugates display good stability against salt concentration and pH. The combination of these measurements suggests that GNPs presumably bind to CM surfaces via complexation with the carboxylate or amine groups on CM surfaces, but not by electrostatic action. (C) 2008 Elsevier Inc. All rights reserved.