An antibody has been developed which recognizes opiate receptors on cells of the immune system. This antibody blocks specific binding of the radiolabeled opiate receptor ligand, 3H-dihydromorphine, to receptors on murine splenocytes. Additionally, the anti-receptor antibody competes with β-endorphin, met-enkephalin, and naloxone for the same binding site on the leukocytes. Moreover, the anti-receptor antibody possesses agonist activity similar to β-endorphin in suppressing cAMP production by lymphocytes. These results suggest the development of an antibody which recognizes classical opiate receptors on cells of the immune system. © 1988.