Novel compounds of the formula: IMAGE wherein R is a ligand-analog having a single reactive primary or secondary amino group which is attached to a carbonyl carbon atom of a carboxy- fluorescein moiety and having a molecular weight within the range 100 to 2000; wherein said ligand- analog has at least one common epitope with a ligand so as to be specifically recognisable by a common antibody, are useful as tracer compounds in a fluorescence polarization immunoassay procedure. IMAGE
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