Viable bacteria were not cultured from the mesenteric lymph nodes, spleens, or livers of specific-pathogen-free (SPF) mice. Viable enteric bacteria, primarily indigenous Escherichia coli and lactobacilli, were present in the mesenteric lymph nodes of gnotobiotic mice inoculated intragastrically with the whole cecal microflora from SPF mice but not in the nodes of control SPF mice similarly inoculated. These indigenous E. coli also were cultured from the mesenteric lymph nodes of 96% of gnotobiotic mice monoassociated with E. coli but from none of the mesenteric lymph nodes of SPF mice inoculated with the E. coli. Furthermore, viable E. coli were detected in the mesenteric lymph nodes of these monoassociated gnotobiotes for as long as 112 days after inoculation. Indigenous Lactobacillus acidophilus also translocated to the mesenteric lymph nodes of gnotobiotic mice monoassociated with L. acidophilus. Apparently, there are mechanisms active in SPF mice inhibiting translocation of indigenous bacteria from the gastrointestinal tract to the mesenteric lymph nodes, spleens, and livers, whereas these mechanisms are either absent or reduced in gnotobiotic mice. Indigenous E. coli maintained higher population levels in the gastrointestinal tracts of the gnotobiotes compared with their population levels in SPF mice, suggesting that high bacterial population levels might promote translocation of certain bacteria from the gastrointestinal lumen to the mesenteric lymph nodes. Gnotobiotic and SPF mice, therefore, provide experimental models for determining the nature of the mechanisms operating to confine indigenous bacteria to the gastrointestinal tract in normal, healthy animals.