For some time now, microbial agents have been implicated in the etiology of autoimmune diseases, including insulin dependent diabetes mellitus (IDDM). Recent studies, however, have revealed that exposure of genetically diabetes-susceptible animals to certain microbes or microbial agents at an early age prevents the induction and progression of disease. This suggests that microbes may act to modulate the immunological status or immune repertoire of an individual genetically programmed for IDDM away from an autoimmune response. Immunization with microbial agents at an early age may offer an important new direction for the immunotherapy of IDDM.