Highly active synthetic secretin was obtained after deprotection of completely protected secretin with hidrogen fluoride/anisole followed by a simple two-step purification, involving ion-exchange chromatography and preparative reverse-phase high performance liquid chromatography (HPLC). The homogeneity of synthetic secretin was assessed by thin layer chromatography, HPLC, disc electrophoresis and other physicochemical methods. The synthetic secretin was found on HPLC to contain none of the diastereoisomers which might be produced during fragment condensation. Its biological activity was 5750 c.u./mg in stimulating exocrine pancreatic juice in the anesthetized rat, and this is the highest value among those of various natural and synthetic secretins (4000 clinical unit/mg). Secretin was found to be unexpectedly stable throughout the purification procedures. Our synthetic method is suitable for the large-scale production of secretin for clinical use.