DNA is a structurally heterogeneous molecule that elicits antibody production in both normal and aberrant immunity. In the prototypic autoimmune disease systemic lupus erythematosus, anti-DNA antibodies occur prominently and are serological markers for diagnosis and prognosis. These antibodies target conserved sites on both single-stranded (ss) and double-stranded (ds) DNA from essentially all species. In contrast, sera of normal human subjects (NHS) contain antibodies that selectively bind to DNA from certain bacteria. The NHS antibodies differ from lupus anti-DNA in their exclusive binding to foreign DNA as well as in their isotype, light chain distribution, and mode of DNA interaction. The properties of these antibodies suggest that they arise as a specific response to bacterial DNA introduced during infection or colonization. This conclusion is supported by studies demonstrating that bacterial DNA is a potent immunogen in normal mice and can induce antibodies specific for determinants on bacterial ss and dsDNA. Furthermore, immunization of normal animals with bacterial DNA elicits antibodies that bind mammalian as well as bacterial ssDNA; this immunization can also provoke glomerulonephritis. In addition to its immunogenicity, bacterial DNA is mitogenic, stimulating polyclonal activation of murine B cells; bacterial DNA can also induce cytokine production in the mouse. The range of immunological activities of bacterial DNA suggests an important role of this molecule in stimulating host defense in normal individuals and provoking autoimmunity in individuals predisposed to SLE.