Lipopolysaccharides (LPSs) were isolated from Treponema denticola (T. denticola) and Treponema vincentii (T. vincentii) by the phenol/water (PW) and the phenol/chloroform/petroleum-ether (PCP) procedures. 1) T. denticola PW-LPS (LPS isolated by the PW procedure), PCP-sup-LPS (LPS isolated by the PCP procedure in the supernates of ultracentrifugation), PCP-ppt-LPS (precipitated LPS isolated by the PCP procedure, obtained after ultracentrifugation), and T. vincentii PCP-ppt-LPS were composed of carbohydrate, hexosamine, protein, fatty acid, and phosphorus. T. vincentii PW-LPS was contained major amount of carbohydrates and small amount of fatty acids. 2-keto-3-deoxyoctonic acid (KDD) was not detected in these LPSs. 2) The major fatty acids of T. denticola PW-LPS and PCP-sup-LPS were palmitic, stearic, oleic, and linoleic acids. The major fatty acids of T. vincentii PCP-ppt-LPS were palmitic, stearic, myristic, oleic, and linoleic acids. Hydroxy fatty acids were not detected. 3) Glucose, galactose, and mannose were comprised in T. denticola PW-LPS. Glucose, galactose, and arabinose were comprised in T. denticola PCP-sup-LPS. Glucose and galactose were comprised in T. vincentii PCP-ppt-LPS. 4) The Limulus amoebocyte lysate (LAL) clotting activity of T. denticola PW-LPS was 1/10, as compared with that of Escherichia coli (E. coli) UKT-B LPS standard. The LAL clotting activities of T. denticola PCP-sup-LPS, T. vincentii PW-LPS, and T. vincentii PCP-ppt-LPS were 1/100, as compared with that of E. coli LPS standard. 5) Five hundred micrograms/kg of T. denticola PCP-sup-LPS was pyrogenic in rabbits. Two thousand micrograms/kg of T. vincentii PCP-ppt-LPS was pyrogenic in rabbits. 6) T. denticola PCP-sup-LPS and T. vincentii PCP-ppt-LPS were capable of increasing or decreasing the release of lysosomal enzymes from human polymorphonuclear leukocytes.