Red meat intake is associated with an increased risk of colorectal cancer. We have previously shown that haemin, Hb and red meat promote carcinogen-induced preneoplastic lesions, aberrant crypt foci (ACF), in the colon of rats. We have also shown that dietary calcium phosphate inhibits haemin-induced promotion and normalises faecal lipoperoxides and cytotoxicity. Unexpectedly, high-calcium phosphate control diet-fed rats had more preneoplastic lesions in the colon than low-Ca control diet-fed rats. The present study was designed to find a Ca supplementation with no adverse effect, by testing several doses and types of Ca salts. One in vitro study and two short-term studies in rats identified calcium carbonate as the most effective Ca salt to bind haem in vitro and to decrease faecal biomarkers previously associated with increased carcinogenesis: faecal water cytotoxicity and thiobarbituric acid-reactive substances. A long-term carcinogenesis study in dimethylhydrazine-injected rats demonstrated that a diet containing 100 μmol/g calcium carbonate did not promote ACF, in contrast with a previously tested calcium phosphate diet. The results suggest that calcium carbonate, and not calcium phosphate, should be used to reduce haem-associated colorectal cancer risk in meat eaters. They support the concept that the nature of the associated anion to a protective metal ion is important for chemoprevention.