The capsular polysaccharide (CPS) produced by the non-pathogenic Klebsiella strain I-714, selected for its immunomodulating activity, has been purified and almost completely detoxified in a previous study [Adam, O., Vercellone, A., Paul, F., Monsan, P. F. & Puzo, G. (1995) Anal. Biochem. 225, 321-327]. The present report concerns the structural elucidation of this CPS by several one-dimensional and two-dimensional 1H-NMR and 13C-NMR experiments performed on the native molecule. It was found to be a high molecular-mass branched polymer constituted by a hexasaccharide repeating unit of following structure: 4)-alpha-L-Rhap-(1-->3)-beta-D-Galp-(1-->2)-alpha-L-Rhap-(1- ->4)-beta-D- GlcpA-(1-->3)-¿alpha-L-Rhap-(1-->2)-¿-alpha-D-Galp-(1. The presence of two glycosidic substitutions on the Galp residue resulted in a strong overlapping of its proton signals, preventing direct assignment of both proton and carbon resonances. However, assignment could be achieved using the two-bond and three-bond 1H-13C heteronuclear coupling observed in the 1H-13C heteronuclear multiple-bond correlation (HMBC) spectrum.