I has been known for many years1-4 that the level of the immunoglobulin G (IgG) in the cerebrospinal fluid (CSF) is elevated in 60% of cases of multiple sclerosis. When neurosyphilis or bacterial infections of the central nervous system (CNS) can be ruled out,5,6 the occurrence of an increased IgG concentration in the CSF makes the diagnosis of multiple sclerosis more certain. Although the IgG level in the CSF in multiple sclerosis has been measured in many clinical studies, the biological role and the source of the increased amounts of the protein are still largely unknown. Antibodies with known specificities account for only a small fraction of the elevated IgG. Thus cytotoxic antibody appears in the CSF of monkeys with experimental allergic encephalitis (EAE),7 but this can be absorbed with nervous tissue without significantly reducing the IgG level. We reported earlier8 that the IgG in the CSF