This paper describes a novel monoclonal antibody (C1G5F2) derived from mice splenocytes immunized in vitro with a wheat germ agglutinin glycoprotein fraction isolated from bovine central nervous system (CNS) myelin. Immunohistochemical reactions with C1G5F2 were investigated on rat brain sections during the active period of myelination. From day 10 to 13 postnatally, no stained structures were observed throughout the whole brain. The first immunolabeled myelin fibers were detected within the pons at day 14, and the white matter areas in the cerebrum started to be stained some days later. White matter areas of the cerebellum were clearly immunopositive after the third week. There was a strong positive signal on myelin fibers in the cerebrum at day 30. By contrast, no immunolabeled cell bodies of oligodendrocytes were observed throughout the brain. The other neural cell types were also not labeled. This C1G5F2 monoclonal antibody bound mainly to the extracytosolic membrane surface of the processes of live cultured oligodendrocytes derived from newborn rat brain but was unreactive with live or fixed astrocytes and neurons maintained in culture. No immunostaining was detected in the peripheral nervous system or in the spleen, liver, or pancreas. The C1G5F2 epitope containing antigen may therefore be considered as a CNS myelin/oligodendrocyte specific molecule. Sodium deoxycholate-Tween 20 extracts of secondary oligodendrocyte cultures, biotinylated with biotin hydrazide, were used to attempt the purification of the antigen with C1G5F2 IgMs linked to antimouse IgM agarose. A main broad biotinylated protein band of 54-58 kDa molecular mass was noted. In a second approach, the antigen was immunopurified from cultured oligodendrocytes as an immune complex using biotinylated C1G5F2 IgMs. A distinct protein doublet of 53-56 kDa was also observed. It is postulated that this antigen may play an essential role in myelin formation and could be a possible target in diseases restricted to CNS myelin.