The second harmonic generation response from protein-mediated gold nanoparticles assemblies in solution has been studied by the technique of hyper Rayleigh scattering (HRS). It is found that the HRS intensity from biotinylated bovine serum albumin coated gold nanoparticles is enhanced when StreptAvidin is added into the solution. This increase in intensity is attributed to the aggregation of the gold nanoparticles through the binding of biotin and StreptAvidin. Comparison with photo-absorption spectroscopy indicates that the technique of HRS is a potential tool in detecting small levels of particle aggregation in liquid samples. (c) 2007 Elsevier B.V. All rights reserved.