The biotin-avidin reaction is well studied and is often used as a prototypical interaction in the development of immunoassays. In this paper, this reaction is studied on the surface of colloidal silver and gold particles as a first step in the development of a sol-based assay. More specifically, silver and gold colloidal particles were biotinylated by self-assembly of a biotin disulfide molecule, and the reaction of the surface-modified colloidal particles with avidin molecules was followed using optical absorption spectroscopy. The specific interaction of avidin, a tetrameric protein, with biotin leads to cross-linking of the colloidal particles ("flocculation") and a consequent growth of a long wavelength absorption peak. The degree of flocculation was quantified using a semiempirical flocculation parameter, and the dependence of this parameter on the extent of biotinylation of the colloidal particle surface as well as the concentration of avidin in solution was studied to determine the optimum working conditions of the sol. The silver sol required electrostatic stabilization of the biotin-capped particles through the simultaneous incorporation of a charged bifunctional molecule, 4-carboxythiophenol, in the capping monolayer while the gold sol was stable with biotin capping. Both biotinylated silver and gold sols showed a visible color change on addition of avidin. However, changes in the optical absorption spectra were more marked for the silver sol.