A cancer-associated antigen, sialyl-Le a oligosaccharide, was isolated from human milk using a monoclonal antibody recognizing carbohydrate moieties of mucin-type glycoproteins. The structure was identified asbased on 500-MHz 1 H-NMR spectroscopy. This oligosaccharide comprises 0.07% of sialyloligosaccharides in human milk. The NMR spectra of two fellow oligosaccharides, Le a oligosaccharide (or lacto-N-fucopentaose II) and LS-tetrasaccharide a are also given. © 1988 COPYRIGHT 1988 BY THE JOURNAL OF BIOCHEMISTRY.