We tested monoclonal IgM anti‐GM1 and asialo‐GM1 antibodies from 6 patients with chronic motor neuropathies for binding to lipopolysccharides (LPS) from three strains of Campylobacter jejuni. Four of the 6 patients showed strong reactivity with LPS from at least one of the three C. jejuni strains tested as shown by enzyme‐linked immunosorbent assay or western blot. Preabsorption with GM1 or asialo‐GM1, or blocking with cholera toxin, prevented antibody binding to LPS. These studies indicate that human anti‐GM1 or anti–asialo‐GM1 antibodies cross‐react with LPS from certain strains of C. jejuni, and that bacterial LPS might provide antigenic stimuli for the activation of B cells expressing anti‐GM1 antibodies.