Monoclonal antibodies (MAB) were developed which recognize a peptide, His-Glu-Ala-Pro-Ile (HEAPI), encoded by the RNA complementary to the mRNA specifying [Met]-enkephalin. One such MAB (designated 6193) exhibited a high degree of reactivity to the peptide sequence. Other characteristics of 6193 MAB include: the ability to block opioid ligand binding in a radioreceptor assay; agonist activity similar to opioid peptides in suppressing cAMP production; and the recognition of a 58 kDa protein on the surface of the neuroblastoma × glioma cell line, NG108-15. These results are consistent with a reactivity of 6193 MAB with the delta-class opioid receptor. © 1989.