Epidemiologic studies and laboratory research have indicated an association between the metabolic activity of the intestinal microflora and cancer of the large bowel. It has been suggested that activation of procarcinogens could be mediated enzymatically by intestinal bacteria. The levels of incriminated colonic bacterial enzymes are increased by dietary fat and inhibited by certain dietary fibers. Organic extracts of feces contain a mutagenic substance, presumably derived from bacterial metabolism in the large bowel, that is positive in the Ames test. Whether this substance or some other organic chemical is the putative proximate carcinogen remains speculative, but the evidence continues to point to intestinal bacteria as the metabolic intermediary in colon cancer.