Investigation of GFAP (Glial Fibrillary Acidic Protein) in 175 brain tumours showed varying amounts of fibrillary acidic protein in every glioma. In ependymal and oligodendroglial tumours a high number of positive neoplastic elements were detected, GFAP positive were also the peri-vascular cells of a so-called astroblastoma. In pilocytic astrocytomas, Rosenthal fibers were in part GFAP positive, in part negative. In giant cells gliomas, giant cells were GFAP negative or weakly positive. Intraleptomeningeal growing tumour cells presented usually a very strong positivity. In 8 recurring oligodendrogliomas, the number of GFAP positive tumour cells was the same in the primary tumour and in its recurrence. These results demonstrate that GFAP is not a specific astrocytic, but a glial-specific protein. Although GFAP is usually present in greater concentration in differentiated, slow growing gliomas, absolute reliable predictions on biological behaviour of the individual tumour are not possible, because a high GFAP content can be detected also in malignant tumours. GFAP investigation does not seem reliable for solving the pathogenetic problems of undifferentiated tumours: the results obtained in 50 medulloblastomas showed that the investigation of small tumour samples or the positivity of a single cell are inadequate data for a correct evaluation of the findings, especially bearing in mind that GFAP of degenerated astrocytes can be phagocytised by other cells, these findings giving rise to misinterpretations.