This study has established the presence of IgM against S-nitrosylated proteins in cerebrospinal fluid (CSF) of multiple sclerosis (MS) patients using S-nitrosocysteine epitope (anti-SNOcys) as previously shown in serum. Anti-SNOcys IgM increased significantly in CSF during relapsing-remitting MS compared to milder neurological conditions. Evidence from albumin, IgG and IgM suggest that the production of anti-SNOcys IgM is intrathecal rather than the result of ingress from serum. Two correlations during relapse: between CSF level of anti-SNOcys IgM and time elapsed since relapse onset; and between CSF and serum anti-SNOcys IgM levels, suggest that this antibody may have potential as a biomarker.