Lipopolysaccharides (LPS) prepared from N. catarrhalis cells were separated into a chloroform-soluble fraction (26%) and a chloroform-insoluble fraction (74%). Both LPS fractions contained D-glucose, D-galactose, D-glucosamine, galactosamine, lipid A, ethanolamine, fatty acids, acetyl, phosphate, and protein in approximately equal proportions. The lipid A moieties prepared from the two LPS fractions were also similar in composition to each other. The fatty acids and galactosamine of the LPS fractions were recovered quantitatively in their lipid A fractions. The major fatty acid component was β-hydroxylauric acid in contrast with β-hydroxymyristic acid, which is the major fatty acid component of the lipid A of N. perflava and other Gram-negative bacteria. The lipid A of N. catarrhalis also contained a considerable amount of D-glucose and D-galactose, which are not normal constituents of lipid A fractions. The presence of amino acids (ca. 2%) in all fractions suggested that proteins were an integral part of the LPS molecules. The absence of heptose and 3-deoxyoctulosonic acid (KDO) from the N. catarrhalis LPS shows that it lacks a lipopolysaccharide "core" structure similar to that present in the LPS of N. perflava; the polysaccharide part of the LPS molecule is also compositionally different from that of N. perflava. These differences may provide additional evidence to that already accumulated from other sources that N. catarrhalis is taxonomically a "false neisseria".