The non-selective gastrin/eholeeystokinin receptor antagonists proglumide and benzotript inhibit colon carcinoma cell proliferation by binding to the 78 kDa gastrin-binding protein (GBP) (Baldwin, Proc. Natl. Acad. Sci. USA, 91 (1994) 7593-7597). However, although most colon carcinoma cell lines synthesize progastrin, production of mature amidated gastrin~7 has not been observed. In order to define the structural requirements for the binding of gastrin to the GBP the affinities of various fragments of amidated amd C-terminally extended gastrin~7 for the GBP have been measured. The results indicate that the GBP recognizes both N-and C-termini of gastrin~7. Moreover since C-terminal amidation is not a prerequisite for binding of gastrin to the GBP, the GBP is a potential target for the autocrine effects of progastrin.