The carbohydrate epitope 3-fucosyl-N-acetyl-lactosamine (CD15) is involved, as a constituent of glycoconjugates, in cell-cell interactions and cell sorting during rodent CNS morphogenesis. The present study was designed to test whether CD15 is also involved in the development of the human CNS. Human cerebellar hemispheres and vermes from the 24th week of gestation (wg) to the 26th postnatal month (pnm) and from adults were investigated for CD15 immunoreactivity, using the monoclonal antibody MMA. Our findings establish that the carbohydrate moiety is developmentally regulated in neuronal and glial cells during their differentiation. First, the parallel fibers of granule cells are CD15+ during the epoch of synaptogenesis with Purkinje cell dendrites. Second, a subpopulation of neurons from the dentate nucleus is transiently CD15+ from the 32nd wg until the 15th pnm. Third, at the onset of myelination (around the 35th wg), CD15 immunoreactivity is discernible in the cytoplasm of young oligodendrocytes. Immunoreactivity on protoplasmic astrocytes of the inner granular layer and on fibrous astrocytes of the white matter progressively increases during fetal development. In addition, the CD15 epitope is persistently present on Bergmann glial processes and ependymal cells. Within the three subdivisions of the cerebellum, i.e., hemispheres, vermis, and flocculonodular lobe, the CD15 expression follows a different timing of morphogenesis. For example, diminution of immunoreactivity in the parallel fibers occurs first in the phylogenetically older flocculonodular lobe and vermis, and later in the phylogenetically younger hemispheres. This study shows that in the human cerebellum the distribution of CD15 undergoes marked developmental changes. This epitope may also act in cell-to-cell recognition, and perhaps could play a role in controlling CNS development.