quantity to yield a number of reaction sites well in excess of the number of analyte molecules or moieties to be assayed for a competition assay the coated cell is contacted with a mixture of the sample that may contain the analyte being assayed and a known quantity of the same analyte tagged with a second label for the well known sandwich assay the coated cell would also be contacted with the sample that may contain untagged analyte being assayed and also with a reagent containing free ligands of that specified type but tagged with a second label for a fluorescent assay the tags contains fluorophors in particular compounds such as fluorescein rhodamine rare earth chelates and the like methods for linking fluorescent tags to a wide variety of analytes particularly proteinaceous materials are very well known and many of the commercially available fluorescent compounds need not be functionalized for attachment because they already have linking groups for example a large number of dyes and procedures for coupling same to ligands such as antibodies are described in u s pat no 4 166 105 issued aug 28 1979 to tomas hirschfeld similarly methods for coupling known luminescent or radioactive tags to analytes are also well known to those skilled in the art and need not be incorporated here signals obtained from the tagged captor ligands immobilized on the substrate and from the immobilized analyte whether from a.