The carbohydrate epitope 3-fucosyl-N-acetyllactosamine (CD 15) is involved in cell-to-cell recognition processes in various tissues. In the present study the sub-cellular localization of CD15 was immunocytochemically studied in normal and pathological central nervous system fiber tracts of humans and rats. In normal human white matter of the brain, CD 15 immunoreactivity was found on the cell surface of astrocytes and within the cytoplasm of oligodendrocytes. In freshly demyelinated lesions of two human diseases (central pontine myelinolysis a d multiple sclerosis) strong cytoplasmic CD 15 staining was observed in reactive astrocytes. In normal rats CD15 immunostaining was restricted to the surface of astrocytes. In crush-induced lesions of rat optic nerves, however, astrocytes showed a cytoplasmic localization of CD15, 4 and 6 days after injury. In conclusion, abnormal localization of CD 15 in reactive astrocytes may be related to altered functional states of these cells during disease processes.