The preparation of the pure 11-leucine analog of human minigastrin I, a linear tridecapeptide amide with full gastrin activity, from the crude synthetic material obtained after deblocking of the overall protected tridecapeptide amide derivative by means of trifluoroacetic acid, is achieved by ion-exchange chromatography on DEAE-Sephadex A-25 and subsequent partition chromatography on Sephadex G-25. The isolation of a synthetic side-product and its identification as human [10-(Nin-tert-butyl-tryptophan),11-leucine]minigastrin I by means of spectroscopic (UV, fluorescence, 1H-NMR, MS [mass spectrometry]), enzymatic and chromatographic methods is described. The pure tridecapeptide amides human [Leu11]minigastrin I and human [Trp(1''-But)10,Leu11]minigastrin I are isolated in overall yields of 37.4% and 4.6%, respectively, (mol-% with regard to the overall protected tridecapeptide amide derivative).