Bacterial translocation is defined as the passage of indigenous bacteria from the gastrointestinal (GI) tract through the lamina propria to the mesenteric lymph nodes (MLN) and other organs. We compared the relative abilities of various aerobic, facultatively anaerobic, and obligately anaerobic bacteria to translocate from the GI tract to the MLN in gnotobiotic mice colonized with single strains of bacteria. Indigenous gram-negative enteric bacilli translocated in large numbers to the MLN, whereas gram-positive bacteria translocated at intermediate levels and obligately anaerobic bacteria at only very low levels. Our results suggest that enteric bacilli such as Escherichia coli, Proteus, and Enterobacter are associated with a higher incidence of bacteremia in debilitated patients, because these bacteria translocate more efficiently from the GI tract than do other bacteria, especially obligate anaerobes.