Motor syndrome patients were screened for anti- GM1 IgM-antibodies showing differences with normally occurring ones. Depending on the disease, between 34 and 60 percent of the patients had antibodies with higher affinity or different fine specificity. These different antibodies were not detected in other autoimmune diseases. We further characterized both normal and disease-associated antibodies using HPTLC-immunostaining of GM1 and its related glycolipids GA1 and GD1b, binding resistance to a chaotropic agent, soluble antigen binding inhibition, and GM1 affinity columns.  Normal low-affinity anti-GM1 antibodies cross-react with GA1 and/or GD1b.  In motor syndrome patients, different populations of antibodies characterized by their affinity and cross-reactivity were detected.  Although one population is shared by almost all samples (low-affinity, not cross-reacting with GA1 and GD1b), there are remarkably few sera having the same set of populations.  These results suggest that the appearance of the new antibody populations is a random process. Structural analysis of the glycolipid antigens indicates that binding variability of the different populations reflect slight differences in the area of the GM1 molecule required for binding. We hypothesize that disease-associated antibodies originate by spontaneous mutation of normal occurring antibodies