The core oligosaccharides obtained from the lipopolysaccharides of Neisseriameningitidis serotypes 1 and 6 were isolated by mild acid hydrolysis, and their structures elucidated by combined chemical, biochemical, and physical techniques. Use was made of 500 MHz 1H nuclear magnetic resonance measurements in both one-dimensional and two-dimensional modes as well as of nuclear Overhauser enhancement experiments. To assist in the structural assignments, the purified oligosaccharides were also degraded by chemical and enzymatic procedures to smaller fragments. The L1 oligosaccharide is a biantennary partially O-acetylated heptasaccharide in which the larger antenna terminates in a α-D-Galp1 → 4β-D-Galp1 → 4β-D-Glcp unit, whereas the smallest disaccharide antenna carries phosphorylethanolamine groups at O-3 of its heptosyl moiety. The L6 oligosaccharide is also a biantennary partially O-acetylated heptasaccharide but, unlike the L1 core, is terminated by a β-D-GlcpNAcl → 3β-D-Galp1 → 4β-D-Glcp unit in its larger antenna. The heptosyl moiety in the small disaccharide antenna bears a phosphorylethanolamine group at O-7, unlike O-3 for the L1 determinant. Keywords: lipopolysaccharides, Neisseriameningitidis, oligosaccharide epitopes, structure, NMR spectroscopy.