Astrocytomas are the most common brain tumors arising in the CNS and account for 65% of all primary brain tumors. Astrocytes have been shown to have the highest predisposition to malignant transformation compared to any other CNS cell type. The majority of astrocytomas are histologically malignant neoplasm. Previous studies have shown that resident astrocytes are the first cell type to react to tumors and surround them. However, the role of these astrocytes in tumor formation and progression has not been determined. In the present study, we have co-cultured astrocytes with a permanent cell line S635c15 (derived from anaplastic astrocytoma) in order to understand the cellular interactions between astrocytes and astrocytoma cells. Our studies demonstrate that astrocytes in contact with the tumor cells become reactive and fibrous with an increase in glial fibrillary acidic protein (GFAP) immunoreactivity as early as 4 days in culture. By 8 days, astrocytes formed glial boundaries around the tumor cells which grew as round colonies. The astrocytic processes surrounding the tumor cells were also intensely GFAP positive. Since the behavior of these cells observed in culture is very similar to their interaction seen in vivo, this co-culture system may serve as an in vitro model for astrocyte and astrocytoma cell line interaction and aid in our understanding of the molecular and cellular mechanisms during early stages of tumor formation and cell interactions.