Infectious agents have often been implicated in the etiology of autoimmune diseases. Here we show that bacteria may also play a role in resistance to autoimmune diseases. SJL/J and (SJL/J x BALB/c)F1 mice are genetically susceptible to induction of experimental autoimmune encephalomyelitis (EAE), a murine model for human demyelinating autoimmune diseases such as multiple sclerosis. We studied the effect of several bacteria on the development of EAE and found that exposure of SJL/J or (SJL/J x BALB/c)F1 mice to Mycobacterium tuberculosis or Bordetella pertussis consistently rendered mice highly refractory to subsequent induction of the disease. Other bacteria such as Escherichia coli, Shigella and Staphylococcus aureus were found to be less effective, or were protective only if specific immunization procedures were used. Furthermore, M. tuberculosis and B. pertussis were protective irrespective of the route of administration and minute amounts (as low as 0.5 micrograms) of M. tuberculosis were sufficient to protect EAE-susceptible mice against induction of the disease. Interestingly, these bacteria, which are commonly used to promote development of EAE, conferred the highest degree of protection against the disease. The M. tuberculosis-induced protection was found to be associated with active suppression mechanisms mediated by T lymphocytes capable of transferring protection to naive syngeneic mice. These findings indicate that certain bacteria may protect against the development of autoimmune diseases. These results also suggest the potential use for still-unidentified bacterial agents in the manipulation of certain autoimmune diseases.