The effect of dietary calcium on the cholic acid-induced colon cell proliferative response was investigated with C57BL/6J mice. The control diets were formulated on the basis of the AIN-76 diet with the levels of calcium set at 0.1, 0.5 or 1.0% by weight and the ratio of Ca:P at approximately 1:1. Cholic acid was added at 0.1% and 0.25% by weight. The cytokinetic parameters of the colons were evaluated after 2 weeks of feeding by the metaphase arrest technique and by autoradiography. Mitotic activity was found to monotonically decrease with increasing calcium level from 0.1 to 1.0% for both the cholic acid and control groups, cholic acid-fed groups exhibiting significantly higher mitotic activity than the control groups. The effect of calcium (0.1 versus 1.0%) was evident throughout the 24-h period with both the control and cholic acid-supplemented diets. Autoradiographic analyses of the colons yielded similar results. Among the controls, the 0.1% Ca group exhibited significantly higher (P less than or equal to 0.05) crypt height, higher whole crypt labelling index and higher labelling indices at various positions along the crypt height than the 1.0% Ca group. Cholic acid-fed groups had higher values for crypt height, whole crypt labelling indices and the position of the uppermost labelled cells than the control groups. The level of calcium affected the bile acid-induced cell proliferation in the colonic epithelium; the groups fed 0.1% Ca + cholic acid had a markedly higher whole crypt labelling index, a higher position of the uppermost labelled cells and higher labelling indices at various cell positions along the crypt column compared with the 1.0% Ca + cholic acid group. The present study thus demonstrated that increased dietary calcium alleviates the mitogenic effect of a cholic acid supplemented diet on the colonic epithelium.