Translocation is defined as the passage of viable bacteria from the gastrointestinal tract to the mesenteric lymph nodes (MLN) and other organs. The extent of translocation of certain indigenous, oxygen-tolerant bacteria from the cecum to the MLN, spleen, liver, kidney, and peritoneal cavity were determined in diassociated or triassociated gnotobiotic mice. Minimal bacterial translocation occurred to the spleen, liver, kidney, or peritoneal cavity. However, most bacterial strains readily translocated to the MLN. The percentage of the total population of each bacterial strain in the ceca was compared with the percentage of the total population of that strain in the MLN. There was a direct relationship between the numbers of a particular bacterial strain populating the ceca of diassociated or triassociated mice and the numbers of viable bacteria of this strain present in the MLN. Thus, the cecal population level of a particular bacterial strain determined the numbers of viable bacteria of this strain translocating to the MLN. The translocation of these bacterial strains from the gastrointestinal tract is an important first step in the pathogenesis of infection caused by members of the normal intestinal microflora.