The phenomenon of non-specific adsorption, which crops up in almost every case in which biological receptor or recognition molecules are involved, is probed with reference to antibody surfaces for immunosensors. Two different types of analysis, a sandwich enzyme immunoassay and a series of direct total internal reflection fluorescence measurements, to evaluate non-specific adsorption on antibody surfaces are described. In the experiments, antibody coated substrates are exposed to solutions containing interfering agents, and the effects on the surface density of binding sites are estimated.