Cell suspensions from 14-day-gestation rat spinal cord, which had previously been soaked for 1 hr in a 2 micrograms/ml solution of Phaseolus vulgaris leucoagglutinin (PHAL), were cultured on collagen gels containing laminin for 2 weeks. Pieces of the gel and attached cells were then transplanted into the dorsal column of adult host thoracic spinal cord. At 1, 2, and 3 months postimplantation (MPI), animals were sacrificed, and the spinal cords were removed, embedded in paraffin, and sectioned at 8 micron for immunohistochemistry at the light microscopic level. Sections were double labeled for PHAL and utilized as a marker for transplant-derived cells and glial fibrillary acidic protein (GFAP), a specific marker for astrocytes. Transplant-derived astrocytes (PHAL-GFAP positive cells) migrated from the transplantation site in both rostral and caudal directions and were observed within the host dorsal column ipsilateral to the transplantation site. At 2 months, lateral migration into the contralateral dorsal column and ipsilateral dorsal horn was observed. At 3 MPI transplant-derived astrocytes were observed in host medulla (nucleus gracilis). Transplant-derived astrocytes were also observed on the glial limitans as far as nucleus gracilis. A migration rate of 0.72 mm/day was calculated, assuming a 14-day delay in the initiation of migration. The ramifications of such extensive migration are discussed with regard to return of function and amelioration of lesion-induced deficits.