Converging data suggest that nitric oxide (NO) production by cytokine-induced immune cells in demyelinating lesions is involved in multiple sclerosis (MS). High levels of NO may complex to suitable amino acids, causing an immune response against the formed neo-epitopes. By testing MS sera with chemically defined nitroso-amino acids conjugated to carrier protein in ELISA, we observed a significant antibody reaction against the S-nitroso-cysteine epitope. The MS antibody response was exclusively of IgM isotype with an avidity of 8 x 10(-7) M. Sera of all clinical MS forms showed a significantly elevated antibody titer versus sera from healthy subjects or from patients affected with other neurological and autoimmune diseases. The detection of circulating antibodies to a conjugated S-nitroso-cysteine epitope provides indirect evidence for NO involvement in MS.