Transgenic mice that overexpress human progastrin (hGAS) are more susceptible than wild-type mice (FVB/N) to the induction of colonic aberrant crypt foci (ACF) and adenomas by the chemical carcinogen azoxymethane. We have previously shown significantly increased levels of colonic mitosis in hGAS compared with FVB/N mice following -radiation. In order to investigate whether the effects of progastrin observed in hGAS colon require the presence of other forms of circulating gastrin, we have crossed hGAS (hg^+/+) with gastrin knockout (G^-/-) mice to generate mice that express human progastrin and no murine gastrin (G^-/-hg^+/+). Following azoxymethane, G^-/-hg^+/+ mice developed significantly more ACF than control G^-/-hg^-/- mice (which do not express any forms of gastrin). G^-/-hg^+/+ mice also exhibited significantly increased colonic mitosis both before and after exposure to 8 Gy -radiation or 50mg/kg azoxymethane compared with G^-/-hg^-/-. Treatment of G^-/-hg^-/- mice with synthetic progastrin (residues 21-101 of human preprogastrin) or G17 extended at its COOH terminus corresponding to the C-terminal 26 amino acid residues of human preprogastrin (residues 76-101,G17-CFP), resulted in continued colonic epithelial mitosis following -radiation, whereas glycine-extended gastrin-17 and the C-terminal tryptic fragment of progastrin (human preprogastrin 96-101,) had no effect. Immunoneutralization with an antibody against G17-CFP prior to -radiation significantly decreased colonic mitosis in G^-/-hg^+/+ mice to levels similar to G^-/-hg^-/-. We conclude that progastrin does not require the presence of other forms of gastrin to exert proliferative effects on colonic epithelia and that the portion of the peptide responsible for these effects is contained within amino acid residues 76-101 of human preprogastrin.