Mouse monoclonal antibodies from five clones (MMA, 1G10, VIM C6, MC-1 and Tü9), elicited to human myeloid cells and reactive with the 3-fucosyl-N-acetyllactosamine sequence (also termed X-determinant), cross-react with glia cells from human and rat brain. In brain cell cultures from neonatal rats both astrocytes and oligodendrocytes express the X-determinant during the first period of the culture as shown by immunofluorescence tests. While the astrocytes, which exhibit signs of adhesion, permanently express the X-determinant, the oligodendrocytes lose the epitope nearly completely during culture. The oligodendrocytes preferentially grow on top of the astrocytic layer. After about 8-10 days of culture, numerous X-determinant-positive astrocytic membranes show oligodendrocyte-shaped negative print images underneath the galactocerebroside-positive oligodendrocytes. At this time, the oligodendrocytes are relatively firmly attached to the astrocytic layer. Since participation of the X-determinant in the compaction of the mouse 8-32 cell stage is being discussed, we conclude from our observations that the X-determinant may also play a role in the astrocyte-astrocyte and astrocyte-oligodendrocyte adhesion. It may represent the functionally active carbohydrate moiety of a heterotypic cell adhesion molecule of glia cells.