A 44 amino acid peptide with growth hormone-releasing activity has been isolated from a human tumor of the pancreas that had caused acromegaly. The primary structure of the tumor-derived peptide is H-Tyr-Ala-Asp-Ala-Ile-Phe-Thr-Asn-Ser-Tyr-Arg-Lys-Val-Leu-Gly-Gln-Leu-Ser-Ala- Arg-Lys-Leu-Leu-Gln-Asp-Ile-Met-Ser-Arg-Gln-Gln-Gly-Glu-Ser-Asn-Gln-Glu-Arg-Gly -Ala-Arg-Ala-Arg-Leu-NH2. The synthetic replicate has full biological activity in vitro and in vivo specifically to stimulate the secretion of immunoreactive growth hormone. The tumor-derived peptide is identical in biological activity and similar in physiochemical properties to the still uncharacterized growth hormone-releasing factor present in extracts of hypothalamic tissues.