The dynamics of the O-antigen part of the lipopolysaccharide from the enteroherriorrhagic Escherichia coli O91 has been determined in solution using C-13 NMR relaxation measurements at two magnetic field strengths, 9.4 and 14.1 T, thereby facilitating the testing of several dynamical models. The biological repeating unit, consisting of five sugar residues and substituents, could be determined by spectral analysis of different H-1. C-13 correlations and corroborated by the relaxation data. The site specifically C-13-labeled material was shown to have similar to10 repeating units with a narrow distribution. A model-free analysis of the relaxation data revealed a complex dynamical behavior where the sugar residues could be described by a global correlation time (T-m = 5.4 ns), generalized order parameters (S-2 approximate to 0.63), and different cot-relation times for internal motions related to their position in the repeating unit along the polymer (tau(e) approximate to 360-520 ps). One of the sugar residues showed, in addition, a chemical exchange contribution. Furthermore, a substituent on another sugar residue was described by two order parameters (S-f(2) = 0.51 and S-s(2) = 0.21). The solution dynamics of the polysaccharide are thus described by highly intricate motions, both in amplitude and time scales. These results are of significance in the general description of polysaccharides surrounding bacterial cell surfaces and in the presentation of antigenic epitopes to the immune system of an invaded host.