organic solvent is added to the carrier solution and the solution is cooled to between 0 u00b0 and 8 u00b0 c the solution is then added to the activated hapten and coupling is allowed to proceed for 30 minutes to overnight the mixture is then dialyzed and the antigen recovered from the dialysis bag the antigens of the present invention may be utilized to induce formation of antibodies specific to methadone and related compounds in host animals by injecting the antigen in such a host preferably using an adjuvant improved titers can be obtained by repeated injections over a period of time suitable host animals for this purpose include mammals such as rabbits horses goats guinea pigs rats cows sheep etc the resulting antisera will contain antibodies which will selectively complex with methadone or an antigen prepared from a derivative thereof as described above the specific antibodies of the present invention are useful as reagents for the determination of methadone in such an assay a known amount of a radiolabeled methadone derivative such as 125 i n 2 4 hydroxyphenyl ethyl succinamic acid n methyl n 1 methyl 3 3 diphenyl 4 oxohexyl aminoethanol ester or 125 i 6 dimethylamino 4 phenyl 4 4 hydroxyphenyl heptan 3 one is mixed with the above antibody.