The presence and distribution of glial fibrillary acidic protein in fixed, paraffin embedded tissue were studied in 85 human intracranial neoplasms, using the peroxidase-anti-peroxidase method. In some cases, indirect immunofluorescence of frozen sections was used as well. In normal tissue, only the cell processes and perikarya of fibrous astrocytes were stained. Immunostaining was also observed in the following glial neoplasms: astrocytomas (all varieties), astroblastoma, subependymal giant cell astrocytoma, subependymoma, glioblastoma multiforme and ependymoma. The astrocytic elements of mixed gliomas and of medulloblastomas undergoing glial differentiation were likewise strongly stained. In contrast, oligodendrogliomas, meningiomas, pituitary adenomas, sarcomas, lymphomas and metastatic carcinomas were negative. Either a perikaryal or a diffuse fibrillary staining pattern was observed. Combination of the two patterns occasionally occurred. The perikaryal staining was prominent in gemistocytic astrocytomas and in astroblastomas. A distinct negative correlation existed between the degree of anaplasia and the intensity of immunostaining.