Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) were previously isolated from bovine brain. Both peptides showed the greatest selectivity and affinity for the mu opiate receptor of any endogenous substance found to date and may serve as natural ligands for the mu-opiate receptor. We have purified them from the fronto-parietal cortex of human brain tissue by solid phase extraction and high performance liquid chromatography. Peptide content was followed by a specific and sensitive radioimmunoassay with an antibody that was generated against endomorphin-1. The isolated endomorphins showed full biological activity. The tetrapeptides were found in human brain in much higher amounts than in bovine frontal cortex.