The carbohydrate-binding protein (lectin) CSL is an antigen involved in the stabilization of the myelin structure by interacting with the carbohydrate moiety of myelin glycoproteins. Since anti-CSL Fab fragments were able to produce destruction of CNS myelin in vitro, CSL was considered as a potential immunological target in multiple sclerosis. The presence of anti-CSL antibodies has been examined in the cerebrospinal fluid of 1388 different patients with various neurological diseases. It is concluded that the presence of anti-CSL antibodies in the cerebrospinal fluid of patients less than 50 years old constitutes a very sensitive and specific test for multiple sclerosis.