The newly recognized spirochete, Borrelia burgdorferi, the causative agent of Lyme Disease, has been examined for endotoxin-like activities as measured by the standard Farmacopea Ufficiale della Republica Italiana rabbit fever test and the Limulus amoebocyte lysate assay. The suspension of heat-killed microorganism caused a febrile response at a dose of 1 X 10(8) bacteria pro kilo. Similar results were obtained in the Limulus assay where the heat-killed spirochetes stimulated formation of solid clot until the concentration of 1 X 10(5) per ml. Both in pyrogen test and in Limulus assay heat-killed Escherichia coli exhibited a higher degree of potency. These results show that LD-Borrelia possess endotoxin-like activities which could help in understanding the pathogenesis of the clinical symptomatology of the disease.