The human small intestine is normally sterile in nearly one half of North American subjects. In this study the duodenum, jejunum, and ileum were sterile in 82, 69, and 55 per cent of the cases, respectively. Gram-positive cocci were the most frequent finding. E. coli, Enterobacter, and Klebsiella were present in the small bowel in nearly 7, 15, and 35 per cent of duodenal, jejunal, and ileal samples, respecatively. They were present in significant numbers (greater than 1 X 10(5)/ml) in the mid-jejunum in two patients and in the mid-ileum in seven patients (23 per cent). Even with modern anaerobic techniques, anaerobes are scarce in the small bowel; 4 to 6 per cent of persons may have aerotolerant anaerobes like clostridia, but strict anaerobes like bacteroides are rare. Our study provides baseline data for use in interpreting the intestinal bacterial overgrowth associated with certain postoperative disorders.