To assess the human antibody response to bacterial double-stranded (ds) DNA, sera from normal human subjects (NHS) were tested by ELISA for binding to highly purified dsDNA from Micrococcus lysodeikticus (MC). Of 38 NHS tested, 19 demonstrated significant activity to MC dsDNA (defined as an OD380 > 1.0 at a 1:100 dilution of serum) without appreciable binding to dsDNA from Escherichia coli or calf thymus. In competition ELISA, antibodies in NHS to MC dsDNA showed equivalent inhibition by MC ssDNA or dsDNA. Antibody specificity was further evaluated by testing the effects of ionic strength on binding. By ELISA, antibodies in NHS had greater binding to MC dsDNA at high ionic strengths than those in systemic lupus erythematosus sera. These results indicate that NHS have antibodies which bind bacterial dsDNA and extend the range of DNA determinants that can be recognized as foreign by the normal immune system.