Molecular chaperone prefoldin (PFD) assists protein folding by inhibition of protein aggregation in the cell. In this study, we investigated an interaction between gold nanoparticles (Au NPs) and the molecular chaperone PFD. We demonstrated that addition of PFD inhibits salt-induced aggregation of Au NPs. Interestingly, the interaction between PFD and Au NPs was observed to be Au-size dependent. As PFD has no thiol groups in the molecule, hydrophobic interaction is considered to be the primary interaction. This is the first demonstration of size-specific stabilization of Au NPs with non-covalent bonding. (C) 2010 Elsevier B. V. All rights reserved.