An assay method and compositions are provided for determining the presence of an analyte in a sample. The analyte is a member of an immunological pair (mip) of immunogens--ligand and receptor. The method has two basic elements: a solid surface to which one of the members of the immunological pair is bonded and a signal producing system, which includes a catalytic member bonded to a mip, which signal producing system results in a measurable signal on said solid surface related to the amount of analyte in the medium. The signal generating compound is produced without separation of the catalyst labeled mip bound to the solid surface from the catalyst labeled mip free in solution.
In a preferred embodiment, an enzyme is bonded to a mip which acts in conjunction with a solute to produce a signal generating product which binds preferentially to the solid surface when the enzyme is bound to the surface, resulting in a signal which is readily differentiated from signal generating compound produced by the catalyst and solute in the bulk solution.