Transplanted fetal tissue contains not only neurons but glial cells in the graft. Glial cells in the grafts are thought to guide the outgrowth of neuronal fibers. The present study was undertaken to investigate the migration of human astrocytes after transplantation into adult rat striatum.Thalamic, striatal and mesencephalic tissue was obtained from human fetus and cell suspensions were prepared. Two to 28 weeks after the transplantation of these cell suspensions into the striatum, immunohistochemistry using human specific GFAP (Sternberger Monoclonals, MA) antibody was performed.The results demonstrate that human astrocytic elements have a tendency to follow white matter in the rat host brain. Astrocytes originating from the striatum and thalamus exhibit extensive migration, leading contralateral migration in the corpus callosum. Migration was limited from the mesencephalic tissue, extending GFAP-positive processes into the white matter of the adjacent host striatum. Astrocytes demonstrating immature morphology were observed with all transplant types, but were most prevalent in the striatal transplanted animals.These observations suggest that long-distance migration of non-neuronal elements from human xenografts involve complex mechanisms of graft-host interactions which may be dependent on the relative development of different brain regions of the donor. The extensive migration of human fetal glial cells, especially from the striatum, suggests that these glial cells would make ideal carriers for transgenes.