SEVERAL groups of workers have recently demonstrated the existence of naturally-occurring peptides which show agonist activity at the opiate receptor1-4, and Hughes et al.5 have shown that the opiate activity isolated by them is due to a mixture of two peptides H-Tyr-Gly-Gly-Phe-Met-OH and H-Tyr-Gly-Gly-Phe-Leu-OH termed methionine-enkephalin and leucine-enkephalin respectively.