Oligodendrocytes, the myelin-forming cells of the central nervous system, can be generated from progenitor cell lines and assayed for their myelinating properties after transplantation. A growth-factor-dependent cell line of rat oligodendrocyte progenitors (CG4) was carried through 31-48 passages before being transplanted into normal newborn rat brain or the spinal cord of newborn myelin-deficient (md) rats. In md rat spinal cord, CG4 oligodendrocyte progenitors migrated up to 7 mm along the dorsal columns, where they divided and myelinated numerous axons 2 weeks after grafting. CG4 cells were transfected with the bacterial lacZ gene and selected for high beta-galactosidase expression. The cell migration and fate of these LacZ+ cells were analyzed after transplantation. In normal newborn brain, LacZ+ oligodendrocyte progenitors migrated along axonal tracts from the site of injection and integrated in the forming white matter. In md rats, extensive migration (up to 12 mm) was revealed by staining for beta-galactosidase activity of the intact spinal cord where many grafted cells had moved into the posterior columns. Similar migration and integration of grafted cells occurred in the spinal cord of normal myelinated rats and after a noninvasive grafting procedure. Thus, oligodendrocyte progenitors can maintain their ability to migrate and myelinate axons in vivo after multiple passages in vitro. Such progenitor cell lines can be used to study the molecular mechanisms underlying oligodendrocyte development and the repair of myelin in dysmyelinating diseases.