The lipopolysaccharide (LPS) ofChlamydia trachomatis serotype E was isolated from tissue culture-grown element-ary bodies and analyzed structurally by mass spectrometry and 1 H, 13 Cand31 P nuclear magnetic resonance. The LPS is composed of the same pentasaccharide bisphosphate aKdo-(2–8)-aKdo-(2–4)-aKdo-(2–6)-bGlcN-4P-(1–6)-aGlcN-1P (Kdo is 3-deoxy-a-D-manno-oct-2-ulosonic acid) as reported forC. trachomatisserotype L2[Rund,S.,Lind-ner,B.,Brade,H. and Holst,O. (1999)J. Biol. , 16819–16824]. The glucosamine disaccharide backbone is substituted with a complex mixture of fatty acids with ester or amide linkage whereby no ester-linked hydroxy fatty acids were found. .