Lipopolysaccharide (LPS) from Leptospira interrogans serovar copenhageni was prepared from the aqueous phase of a phenol/water extract. Electron microscopic examination of negatively stained LPS showed a mixture of ribbon-like, round and ring structures. Carbohydrate analysis of the preparations revealed pentoses, hexoses, heptoses, hexosamines, and a 2-keto-3-deoxyonic acid which was chromatographically different from authentic 2-keto-3-deoxyoctonic acid (KDO). The major fatty acids of the LPS were hydroxylauric, palmitic and oleic acids. Although the leptospiral LPS preparations did not contain KDO or hydroxymyristic acid, they were otherwise morphologically and chemically similar to the LPS of other Gram-negative bacteria.