We generated a panel of monoclonal antibodies (mAbs) selected to recognize components from a Triton X-100 extract of ovine oligodendrocytes. One of these Abs, mAb H, recognizes an O-linked N-acetyl glucosamine residue in a specific conformation and/or environment. mAb H stained, weakly, two bands with Mr x 10(-3) of 209 and 62 in lysates of cultured rat astrocytes, suggesting antigens of low abundance. We have employed immunohistochemistry to investigate the cell and tissue distribution of the mAb H antigen(s). In normal rat and human brains, the sparse reaction products detected were confined, mostly, to fibrous astrocytes. In sharp contrast, when pathological specimens from a variety of brain lesions, including anisomorphic and isomorphic gliosis, were examined, a strong reaction with mAb H was in evidence in all reactive astrocytes, independent of the origin or nature of the lesions. This we interpret as meaning that the gene product(s) recognized by this mAb is (are) upregulated or induced following injury to the brain. Hence, epitope H represents a new addition to the list of molecules that are affected by brain injury. Structural and functional identification of the antigen(s) should shed light on its (their) relevance to the pathophysiology of the disease process.