The major host defense mechanisms against bacterial overgrowth in the small bowel are the normal propulsive activity of the bowel itself and gastric acid secretion. Microbial interactions are a major factor in regulating the indigenous bacterial flora. Studies of the bacterial enzymes of the gut suggest that changes in diet may lead to marked changes in the colonic flora. Antibiotics affect the composition of the colonic microflora. The microflora also influence the degradation of mucin, the conversion of urobilin to urobilinogen, of cholesterol to coprostanol, and the production of short chain fatty acids. Current interests are focused on the bacterial flora of tropical sprue, the role of bacteria in colorectal cancer, and the involvement of intestinal microflora in the enterohepatic circulation of sex steroid hormones.