Possible significance of enkephalin, an endogenous ligand of opiate receptors Hans W Kosterlitz and John Hughes, 'Possible significance of enkephalin, an endogenous ligand of opiate receptors' reprinted from John J Bonica and D G Albe - Fessard, eds., 'Advances in Pain Research and Therapy' (vol. 1., New York, 1976), 641 - 645. Photocopy of original article. 1976