The group-specific antigen of Chlamydia trachomatis serotype L2 was chemically analyzed. It is composed of typical lipopolysaccharide (LPS) components, i.e., D-glucosamine, long-chain 3-hydroxy fatty acids, 2-keto-3-deoxyoctonic acid, and phosphate in a molar ratio of approximately 2:5:3:2.6, respectively, resembling enterobacterial LPS of the Re chemotype. For the first time, 3-hydroxydocosanoic acid (3-OH C22:0) was found as an LPS constituent.