Fluorescence correlation spectroscopy (FCS) is a valuable tool in biological research. In recent years there has been growing interest in using light scattered from metallic colloids in place of organic fluorophores. Metallic colloids display optical cross sections for scattering that are orders of magnitude brighter than fluorophores. We used the FCS method to study the scattering properties of varying sizes of gold colloids 38-100 nm in diameter. The optical cross sections of the gold colloids increase rapidly with size, as can be seen by both the G(0) value of the autocorrelation function and the scattering intensity distributions. In mixtures of different size gold colloids the autocorrelation function is dominated by the larger (brighter) colloids, even when present at a small fractional population. We show that it is possible to detect one 100 nm gold colloid in the presence of 10(3)-10(4)smaller 39 nm diameter colloids. Because the scattering cross sections of colloids will increase with aggregation, we believe that FCS can be used to detect a small number of associated bio-labeled colloids in the presence of a much larger population of non-associated colloids.