Gastrin and its derivatives are becoming important targets for immunotherapy of pancreatic, gastric and colorectal tumors. This study was conducted to design antibodies able to block gastrin binding to the gastrin/cholecystokinin-2 (CCK-2) receptor in order to delay tumor growth. The authors have used different gastrin molecules, combined with the diphtheria toxoid, to generate and select human single chain variable fragments (scFvs) as well as mouse monoclonal antibodies and scFvs against different regions of gastrin. There was a remarkable conservation in the antibody repertoire against gastrin, independently of the approach and the species. The germlines most frequently used in gastrin antibody formation were identified. Three different epitopes were identified in the gastrin molecule. The resulting mouse monoclonal antibodies and scFvs were analyzed for gastrin neutralization using Colo 320 WT cells, which overexpress the CCK-2 receptor. The gastrin neutralizing activity assay showed that N-terminal specific mouse monoclonal antibodies were more efficient to inhibit proliferation of Colo 320 WT cells than the anti-C terminal antibodies. Moreover, the human antigastrin scFvs obtained in this study inhibited significantly the proliferation of Colo 320 tumoral cells. These findings should contribute to a more rational design of antibody-based antigastrin therapies in cancer, including passive administration of human antibodies with blocking activity. © 2008 Wiley-Liss, Inc.