Sciatic nerve chronic constriction injury (CCI) in rodents produces nerve demyelination via proteolysis of myelin basic protein (MBP)， the major component of myelin sheath. Proteolysis releases the cryptic MBP epitope， a demyelination marker， which is hidden in the native MBP fold. It has never been established if the proteolytic release of this cryptic MBP autoantigen stimulates the post-injury increase in the respective circulating autoantibodies. To measure these autoantibodies， we developed the ELISA that employed the cryptic 84-104 MBP sequence (MBP84-104) as bait. This allowed us， for the first time， to quantify the circulating anti-MBP84-104 autoantibodies in rat serum post-CCI. The circulating IgM (but not IgG) autoantibodies were detectable as soon as day 7 post-CCI. The IgM autoantibody level continually increased between days 7 and 28 post-injury. Using the rat serum samples， we established that the ELISA intra-assay (precision) and inter-assay (repeatability) variability parameters were 2.87% and 4.58%， respectively. We also demonstrated the ELISA specificity by recording the autoantibodies to the liberated MBP84-104 epitope alone， but not to intact MBP in which the 84-104 region is hidden. Because the 84-104 sequence is conserved among mammals， we tested if the ELISA was applicable to detect demyelination and quantify the respective autoantibodies in humans. Our limited pilot study that involved 16 female multiple sclerosis and fibromyalgia syndrome patients demonstrated that the ELISA was efficient in measuring both the circulating IgG- and IgM-type autoantibodies in patients exhibiting demyelination. We believe that the ELISA measurements of the circulating autoantibodies against the pathogenic MBP84-104 peptide may facilitate the identification of demyelination in both experimental and clinical settings. In clinic， these measurements may assist neurologists to recognize patients with painful neuropathy and demyelinating diseases， and as a result， to personalize their treatment regimens.