Monosymptomatic unilateral optic neuritis (ON) is a common first manifestation of multiple sclerosis (MS) in which increased numbers of autoimmune T and B cells, recognizing different myelin autoantigens including myelin basic protein (MBP) and its peptides, have been implicated in a hypothetical immunopathogenesis. Using an immunospot assay to detect specific antibodies secreted by individual cells, we analyzed the B-cell repertoire to MBP and its amino acid residues 1-20, 63-88, 110-128, and 148-165 in blood and CSF from patients with ON and MS, and from controls. There were cells secreting IgG antibodies to MBP and the four peptides in blood at mean numbers of 0.9 to 4.6 per 10(5) mononuclear cells, without differences between the three patient groups. Mostly, more than 100-fold more B cells with these specificites per 10(5) cells were found in CSF from the patients with ON and MS, without differences between these two groups but with many fewer in CSF from controls. None of the four included MBP peptides represented an immunodominant B-cell epitope in either ON or MS, and the B-cell response was not more restricted in ON than in MS. The autonomy of the autoimmune B-cell response in CSF was further supported by the pronounced asynchrony of the repertoire to the four MBP peptides in CSF compared with blood in individual patients. The large numbers of MBP- and MBP peptide-reactive B cells in CSF in early MS, as manifested by ON, could play a major role in the immunopathogenesis and perpetuation of MS. Alternatively, they could represent myelin breakdown or restoration.