C S F IgM levels have been measured by radioimmunoassay in 56 patients with MS, 62 patients with other neurologic diseases, and 31 normal controls. Forty-eight percent of the patients with MS had a raised CSF IgM level compared with 5 percent of the patients with other diseases. The IgM level did not correlate with the IgG level. Forty percent of the MS patients with normal IgG levels had high IgM levels. No relationship was found between the CSF IgM level and length or severity of the MS, relapses, or steroid therapy. Attempts to identify the IgM as being anti-measles were unsuccessful.