A major sialyl-saccharide alditol isolated from mucins of human amniotic fluid was purified by high-performance liquid chromatography followed by high-performance thin-layer chromatography of the methylated derivative and subjected to structural analyses which comprised fast atom bombardment and electron impact-mass spectrometry, methylation analysis, exoglycosidase digestion and CrO3 oxidation. The structure of a disialylated pentasaccharide characterized by a sialyl-X antigenic determinant was established: (formula; see text)