Lipopolysaccharides isolated from the polymyxin‐resistant Klebsiella pneumoniae 03 mutant OM‐5 and its polymyxin‐sensitive parent LEN‐1 were analyzed for chemical composition, and their lipid A portions were structurally characterized. The lipopolysaccharide of OM‐5 contained approximately five times more 4‐amino‐4‐deoxy‐l‐arabinopyranose than that of LEN‐1. Other saccharide and phosphate components exhibited no significant differences. Structural characterization, including analyses by phosphorus magnetic resonance spectroscopy and by fast atom bombardment mass spectrometry, revealed a novel type of lipid A. In the OM‐5 lipopolysaccharide, both phosphates of lipid A were almost totally present as phosphodiesters with 4‐amino‐4‐deoxy‐l‐arabinopyranose. In the sensitive‐type LEN‐1 lipid A, the extent of this substitution was much lower, especially in the glycosidically linked phosphate. Phosphate in these K. pneumoniae lipopolysaccharides was almost exclusively found in lipid A. These results show that cationic substituents of phosphates of lipid A play a decisive role in determining polymyxin reactivity. OM‐5 was also found to contain a large proportion of heptaacyl lipid A, which represented only a small fraction of lipid A in LEN‐1.