Lipopolysaccharide (LPS) was extracted from cells of Centipeda periodontii ATCC 35019 by a hot phenol-water method, purified by nuclease treatment and by repeated ultracentrifugations. The LPS was composed of 21.7% hexose, 9.1% hexosamine, 10.3% fatty acid, 0.5% protein, 2.5% phosphorus, 4.3% 2-keto-3-deoxyoctonate(KDO) and 2.3% heptose. The major fatty acids of the LPS were 3OH C13:0 and C13:0. C. periodontii LPS was able to gelatinize Limulus amebocyte and to produce the local Schwartzman reaction in rabbits. LPS from C. periodontii exhibited strong immunobiological activities, mitogenicity on splenocytes of C3H/HeN mice, and stimulation of interleukin 1 (IL-1) production in peritoneal macrophages of C3H/HeN mice.