A glycopeptide isolated in relatively large amounts from human pituitary glands was completely purified, and its sequence was determined. The primary sequence represents the NH2-terminal 76 amino acid residues of pro-opiomelanocortin (POMC). This important secretory product of POMC was shown to possess an interesting aldosterone-stimulating activity on a human adrenal aldosteronoma. It is O-glycosylated at Thr-45 and N-glycosylated at Asn-65. Only one sequence variation with the human genomic DNA was found. Furthermore, comparison with the other preferred cleavage sites of human POMC reveals that the pair of basic residues Lys-Arg represents the major sites of enzymatic maturation of this precursor molecule. This predicts a highly specific type of enzyme involved in the maturation of POMC in the anterior lobe of the human pituitary.