After transplantation into the adult CNS the immortalized neural stem cell lines ST14A and HiB5 differentiate preferentially into glia-like cells. After lesions of the host brain, which activate resting glial populations, the grafted cells responded with up-regulation of glial markers and a change in morphology towards a reactive state. The protein expression followed the same pattern as in the host glial population. The results show that the astrocytes formed by the grafted neural stem cells become functionally integrated in the host brain and that they may take an active part in the reactive gliosis caused by brain damage.