Abstract Twelve hybridoma cell lines producing MAbs against morphine were established by using morphine hemisuccinate-conjugated bovine serum albumin as an immunogen. The MAbs belonged to the IgG1 subclass with κ- or λ-chains. The association constants of the antibodies ranged from 4.6 × 10 8 to 4.7 × 10 10 ( M −1). These antibodies revealed slightly different cross-reactivities with various agonistic opiates and antagonists. In general, the antibodies were strongly cross-reactive with the opiate agonists, codeine, ethylmorphine, dihydromorphine and dihydrocodeine, while their cross-reactivities with norcodeine and the opiate antagonists, naloxone and naltrexone, were weak. The cross-reactivities with dihydromorphinone, dihydrocodeinone, naloxone, naltrexone, dextromethorphan and homatropine varied from clone to clone. Interestingly, certain MAbs displayed weak but significant cross-reactivities with the synthetic opiate, meperidine. However, none of the antibodies was cross-reactive with the opioid peptides, β-endorphin, Met-enkephalin, and d-Ala 2- d-Leu 5-enkephalinamide. Radioimmunoassay for morphine using one of the antibodies (MOR 131.5.13) was shown to be sufficiently sensitive (IC 50 = 0.1 n M) for the purposes of forensic analysis of morphine. This set of monoclonal anti-opiate antibodies is assumed to be suitable for analyzing the structure-function relationship in the hapten-antibody interaction, since the antibodies revealed similar but not identical cross-reactivities with various morphine related compounds.