The effect of colloidal Au nanoparticle immobilization on surface plasmon resonance (SPR) reflectivity is reported. Immobilization of 25 nm diameter colloidal Au on to an evaporated Au film results in a large shift in plasmon angle, a broadened plasmon resonance and an increase in minimum reflectance. This results in increased SPR sensitivity, demonstrated in several ways, including a sandwich immunoassay for human IgG, enlargement of a 1.4 nm diameter Au cluster, and detection of displacement of surface-bound biotinylated colloidal Au by free biocytin in solution. Similarly large changes in reflectivity are realized upon binding of colloidal Au to electrolessly deposited Au films prepared entirely by wet-chemical methods. These results represent potentially significant advances in the generality and sensitivity of SPR.