Lipopolysaccharides from phenol-water extraction of cells of Campylobacter jejuni serotype O:19 were separated into a water-soluble gel of low M(r) and a water-soluble component of high M(r). Acetic acid hydrolysis of the ketosidic linkages to lipid A furnished respectively a core oligosaccharide, the structure of which is reported herein, and an O antigenic polysaccharide. Structural investigations were performed on the O-deacetylated lipopolysaccharide of low M(r), the liberated core oligosaccharide and the various products from removal of neuraminic acid and phosphate residues, and from the Smith degradation. It is concluded that the lipopolysaccharide from the serostrain has a core region with two types of closely related oligosaccharide chains showing striking homologies with gangliosides, the first with a single N-acetylneuraminic acid residue in an outer chain resembling GM1 and the second with two N-acetyl-neuraminic acid residues with a terminal region resembling GD1a. Similar experiments were carried out on lipopolysaccharides of low M(r) from bacterial isolates OH 4384 and OH 4382 serotyped as O:19 that had been obtained from two patients who subsequently developed the Guillain-Barré syndrome. The core oligosaccharide region of lipopolysaccharide from the former isolate differed only slightly from that of the serostrain, whereas that from the latter isolate was distinctly shorter.