Synthesis of an analog of a biosyntheticprecursor of lipid A containing four (S)-3-hydroxytetradecanoic acids was effected via an improved synthetic route to investigate the relationship between the bioactivity and the chirality of the 3-hydroxy fatty acid residues in lipid A. The S-analog inhibited endotoxin-induced interleukin-6 (IL-6) production from human peripheral whole blood cells more strongly