the present invention is directed to novel methadone derivatives which are synthesized for the covalent attachment to antigens for the preparation of antibodies or receptors to methadone and methadone metabolites . the resulting novel antigens may be used for the production of antibodies or receptors using standard methods . once generated , the antibodies or receptors and the novel derivatives which are covalently attached to proteins , polypeptides or labels may be used in the immunoassay process .