Bile acids and fatty acids may promote colon cancer by inducing colonic hyperproliferation. Dietary calcium inhibits the promoting effects of bile acids and fatty acids, possibly by precipitating these surfactants and lowering their cytolytic activity. Because bile acids and fatty acids are products of fat digestion, their effects may be dependent on the type of dietary fat. The effects of the type of dietary fat (energy percentage, 40) and of CaHPO4 supplementation (25 versus 225 mumol/g diet) on the luminal solubility of surfactants, cytolytic activity, epitheliolysis, and in vivo colonic proliferation were studied in rats using Western high-risk diets. The different types of commercially available fats were butter, saturated margarine, and polyunsaturated margarine. Supplemental calcium drastically increased fecal fatty acid excretion, the effect being dependent on the type of fat, and slightly stimulated fecal bile acid excretion. Soluble surfactant concentrations were drastically decreased by calcium supplementation with all three types of dietary fat. Consequently, cytolytic activity of fecal water was decreased by supplemental calcium. These luminal effects of calcium resulted in a lower intestinal epitheliolysis. The compensatory proliferation of the colonic epithelium was decreased by supplemental CaHPO4 for the butter and saturated margarine diets. Despite CaHPO4-dependent decreases in luminal effects and epitheliolysis, no significant decrease in proliferation on the polyunsaturated margarine diet was observed. Multiple regression analysis of soluble surfactants with cytolytic activity (R = 0.76), epitheliolysis (R = 0.74), and colonic proliferation (R = 0.84) showed highly significant associations. Cytolytic activity and epitheliolysis as well as epitheliolysis and proliferation were highly correlated (r = 0.97 and r = 0.88, respectively; n = 36) for control and CaHPO4-supplemented diets, suggesting cause-and-effect relationships. It is concluded that the antiproliferative effect of dietary calcium is mediated by the precipitation of luminal surfactants and is dependent on the type of dietary fat.