Glial fibrillary acidic protein (GFAP) was studied in 160 cerebral tumors, mostly of neuro-epithelial nature. It was positive in astroglial tumors with an intensity proportional to the degree of cell differentiation. It was sometimes positive also in non-astroglial tumors, such as oligodendrogliomas and ependymomas, and this finding is discussed in relation to genesis and diagnostic value. In medulloblastomas, there were also positive cells, which could be reactive glia cells included in the tumors or subependymal cells. The demonstration of GFAP is very useful in gliosarcomas for identifying the glial component. It was sometimes positive in hemangioblastomas, and it is discussed in view of the nature of the stromal cells of this tumor.