Abstract: The present invention is directed to a fluorescence polarization assay for opiate alkaloids and their metabolites, 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 making them. The tracers and the immunogens are made from substituted opiate alkaloids. A fluorescein moiety is included in the tracers, while a poly(amino acid) forms a part of the immunogens. The assay is conducted by measuring the degree of polarization retention of the fluorescence resulting when a sample mixed with antiserum and tracer is irradiated with plane-polarized light.