Oligodendrocyte-specifie protein (OSP) is a recently isolated novel protein found only in CNS laminar myelin and therefor is a good candidate as an autoantigen in patients with MS. In order to determine the humoral response in patients with MS. Westren blot analysis was performed on CSF from 6 patients with clinically stable relapsing MS (rMS) and 2 normal controls. All 6 of the CSF samples from rMS patients contained anti-OSP antibodies and none were detected in controls. Peptide mapping determined that the antigenic response was directed at a 7 amino acid peptide (OSP 114-120) which was 71% homologous with several common viral and bacterial proteins and 100% identical to the human OSP protein. ELISAs were performed using OSP 114-20 as antigen on a total 32 MS patints followed at UCLA, 53 MS patients from the National Neurological Rescarch Specimen Bank (NNRSB), and on 51 neurological control samples. Eighty % of UCLA rMS patients had an OSP ELISA reading above 0.55 OD units (median ± SD, 0.94 ± .35) while 0 of 14 CSF samples from HTLV-1 patients and normal controls had values above 0.55 units (0.28 ± .12; p<.01). Similar results were found in specimens from the NNRSB. No differences in anti-OSP titers were found in serum of MS and control patients. ELISAs performed on CSF samples using homologus viral peptieds as antigen (e.g. EBV, HSV HIV) showed a close correlation with anti-OSP 114-120 titers, and in some, the anti-viral titers far exceeded them. These data demonstrate a specific humoral response directed against a region of OSP in rMS patinets which cross reacts with several common viral peptides and suggests a possible role of molecular mimicry in the development of MS.