A novel myelin antigen has been defined by a mouse monoclonal antibody (8-18C5) raised against rat cerebellar glycoproteins. By immunohistochemistry, the antigen was detected in white matter tracts of the central nervous system, but was absent from peripheral myelin. Western blotting followed by immunostaining revealed that the antigen was a glycoprotein of Mr 51 000 daltons which was degraded on storage to cross-reacting products of 20-26 000 daltons. Antigen was detected in central but not peripheral nervous system samples. Products of identical Mr were obtained by affinity chromatography of iodinated cerebellar glycoproteins on a column of immobilised 8-18C5.