Lactobacilli belong to the normal oropharyngeal and intestinal microflora in humans. These microorganisms contribute to the stabilization of the microflora and maintain the colonization resistance against pathogens. Lactobacilli have been used as dietary supplements in order to prevent gastrointestinal disturbances. Claims have been made that certain strains of lactobacilli possibly exert anticarcinogenic activities. The activity of bacterial enzymes, implicated in colon carcinogenesis may be elevated by a high meat, Western-type diet. Supplements of Lactobacillus acidophilus decreased these levels in both rats and humans. Colon cancer patients given L. acidophilus fermented milk showed a significant increase both in numbers of intestinal lactobacilli and dietary calcium intake, while decreasing trends in levels of both soluble faecal bile acids and faecal bacterial enzymes, two risk makers for colon cancer, were observed. In vitro studies have revealed that lactobacilli and other lactic acid bacteria have the ability to absorb cooked food mutagens. Recent studies in humans have shown that intake of L. acidophilus significantly reduced the mutagen excretion after consumption of fried meat. Several mechanisms by which lactobacilli might exert anticarcinogenic effects are discussed. Thus, certain strains of lactobacilli might lower the colon cancer risk in humans.