The majority of human colon cancers express the gastrin gene, and a significant percentage bind gastrin-like peptides. However, it is not known if gastrin gene products are physiologically relevant to the growth and proliferation of human colon cancers. To investigate the functional role of gastrin gene expression, we examined the effect of gastrin antisense (AS) RNA expression on the growth and tumorigenicity of colon cancer cells. The full-length human gastrin cDNA was cloned in the AS direction in a retroviral vector under the transcriptional control of human cytomegalovirus promoter. Three representative human colon cancer cell lines that expressed negligible (Colo-205A) to significant (Colo-320 and HCT-116) levels of gastrin mRNA were transfected with either AS or control vectors and subjected to various growth studies in vitro and in vivo. The proliferative and tumorigenic potential of the AS clones from the gastrin-expressing cell lines was significantly suppressed compared to that of the control clones, whereas the growth of Colo-205A-AS cells (the negative control) was similar to that of the Colo-205A-C-cells, indicating the relative specificity of the antitumorigenic effects of AS gastrin RNA expression. We believe that this is the first evidence that supports a possible critical role of gastrin gene expression in the tumorigenicity of human colon cancers that express the gastrin gene. Because >60–80% of human colon cancers express the gastrin gene, it can be expected that the growth of a significant percentage of these cancers may be critically dependent on the expression of gastrin gene products. Therapeutic measures, such as the AS strategy used in the present study, may therefore prove to be useful in treating human colon cancers in the future.