While the capacity of O-2A oligodendrocyte progenitors to migrate in cell culture and during in vivo myelin formation is well documented, little is known about factors that regulate the motility of these cells. Here, we report on an in vitro model that allowed us to evaluate the contribution of alpha 2–8 linked polysialic acid (PSA) to O-2A cell motility. Using explant cultures of newborn rat neurohypophysis, we observed that individual glial fibrillary acidic protein (GFAP)- positive cells rapidly disperse from the explants, and that cells of the O-2A lineage predominate in the migratory cell pool. Presumed O-2A progenitor cells had a round or bipolar morphology and presented both A2B5 and GFAP immunoreactivity. When cultured in medium containing 10% fetal calf serum, these cells differentiated into stellate-shaped, A2B5/GFAP-positive type 2 astrocytes. In serum-free medium most of them developed into O4/galactocerebroside-positive oligodendrocytes. O-2A lineage cells were found only in a specific developmental period extending from embryonic day 21 to postnatal day 3. A monoclonal antibody, which recognizes the alpha 2–8 linked PSA, characteristic of the embryonic form of NCAM, revealed immunoreactivity on the surface of O-2A progenitor cells, whereas mature oligodendrocytes, type 2, type 1 astrocytes as well as flat GFAP-negative cells were negative. Treatment of the explants with endoneuraminidase purified from phage K1, which specifically removes PSA from the surface of the cell, resulted in a complete blockade of the dispersion of O-2A lineage population from the explant. The effects of the enzymatic treatment were both selective and reversible: migration of GFAP-negative fibroblast-like cells that are normally PSA negative was not influenced, and upon removal of the enzyme, cells of the O-2A lineage were readily detectable in the migrating population. These results provide direct evidence that alpha 2–8 linked PSA contribute to the motility of O-2A, glial progenitor cells.