The heptacosapeptide amide corresponding to the entire amino acid sequence of gastrin-releasing peptide (GRP) was synthesized by assembling 7 peptide fragments followed by deprotection with a new reagent system, 1 M trifluoromethanesulfonic acid-thioanisole in trifluoroacetic acid. The deprotected peptide was purified by ion-exchange chromatography on carboxymethyl-cellulose followed by partition chromatography on Sephadex G-25. The latter removed effectively the Met (O) derivative of GRP. The highly purified synthetic GRP was as active as synthetic bombesin in dogs and rats on the molar basis. A new carboxyl-activating reagent, thiazoline-2-thione, was employed for preparation of the necessary fragments.