Abstract: The present invention is directed to a fluorescence polarization immunoassay for barbiturates, to the various components needed for preparing and carrying out such an assay, and to methods of making these components. Specifically, tracers, immunogens and antibodies are disclosed, as well as methods for preparing them and a reagent kit containing them. The tracers and the immunogens are made from substituted barbiturate compounds. A fluorescein moiety is included in the tracer, while a poly(amino acid) forms a part of the immunogen. The assay is conducted by measuring the degree of polarization retention of plane—polarized light that has been passed through a sample containing antiserum and tracer.