Non-specific adsorption of serum protein components to a direct immunosensor constructed from a silvered diffraction grating coated with goat immunoglobulin G has been determined. Non-specific adsorption to a 'simplistic' sensor interface was found to be significant and would be expected to limit the sensitivity of reported immunosensors when used for the estimation of specific analytes in serum. This work highlights the need for complex sensor interfaces comprised of suitable chemical modifications and controlled, oriented, immobilisation of immunological components. © 1990.