Monoclonal IgM kappa from a patient with polyneuropathy associated with paraproteinemia was found to bind to several polysialogangliosides. Binding of IgM to gangliosides was shown by enzyme-linked immunosorbent assay (ELISA) and by overlaying thin-layer chromatograms of human brain and peripheral nerve gangliosides with the patient's serum followed by radioiodinated goat antihuman IgM. The latter technique showed that the IgM paraprotein reacted with a number of gangliosides. In an ELISA the IgM paraprotein reacted strongly with GD2, GD3, GD1b, and GT1b, but not with GM1, GM3, and GD1a. Thus, the epitope for the patient's IgM paraprotein appears to involve the disialosyl configurations.