Strains of indigenous Escherichia coli, Bacteroides, and Lactobacillus were isolated from the gastrointestinal tracts of specific pathogen-free (SPF) mice. Nonvaccinated SPF mice exhibited in their spleens low numbers of plaque-forming cells (PFC) and rosette-forming cells reacting with antigens of these andigenous bacteria. PFC reacting with these bacterial antigens were not detected in infant SPF mice until 7 days after birth. Compared with nonvaccinated controls, SPF mice vaccinated parenterally with indigenous E. coli or Bacteroides produced a moderate increase in the numbers of specific PFC. Thus, the SPF mouse is capable of responding immunologically after vaccination with microbes indigenous to its intestinal tract. However, more PFC reacting with homologous vaccine antigens were detected after parenteral vaccination of SPF mice with nonindigenous E. coli O127:B8, E.coli O14, or B. fragilis than after parenteral vaccination with indigenous E. coli or Bacteroides. Gnotobiotic mice orally monoassociated with these nonindigenous bacteria exhibited greater immune responses to antigens of the bacteria used to monoassociation than did gnotobiotes monoassociated with the indigenous microbes. The results are consistent with the hypothesis that mice are more responsive immunologically to antigens of nonindigenous bacteria than they are to antigens of certain microbes indigenous to their gastrointestinal tracts.