A new saturation transfer difference 1D-TOCSY NMR experiment that allows the investigation of complex ligands interacting with proteins and its. application in the mapping of which portions of oligosaccharide ligands (epitope) interact with a complementary antibody are described. The interaction between trisaccharide and hexasaccharide ligands, corresponding to fragments of the cell-wall polysaccharide of Streptococcus Group A, and a monoclonal antibody directed against the polysaccharide is investigated at the molecular level. The polysaccharide consists of alternating alpha-(1 --> 2) and alpha-(1 --> 3) linked L-rhamnopyranose (Rha) residues with branching N-acetyl-D-glucopyranosylamine (GIcNAc) residues linked beta-(1-->3) to alternate rhamnopyranose rings. The epitope is proven to consist not only of the immunodominant GlcNAc sugar but also of an entire branched trisaccharide repeating unit The experimental NMR data serve to check and validate the computed models of the otigosaccharide-antibody complexes.