Lipopolysaccharides (LPS) are a constitutive part of the outer wall of gram negative bacteria. Because many of the symptoms of Lyme disease could be explained by a spirochetal LPS we have subjected Borrelia burgdorferi to standard LPS extraction techniques which yielded a LPS which accounted for 1.5-4% of the dry weight. The LPS was very similar to classical gram negative bacterial LPS both chemically and in its biological activities which included pyrogenicity, mitogenicity for lymphocytes and the induction of Interleukin 1 production by macrophages. In addition, the LPS produced an acute inflammatory reaction when injected intradermally into rabbit skin. It could also prepare a skin site for the production of the local Shwartzman reaction. These results show that the Lyme disease spirochete contains a hitherto unknown LPS that is biologically active in vitro and in vivo. It is likely that this molecule plays an important role in the pathogenesis of Lyme disease.