Twenty-four cases of classical medulloblastoma and one case of desmoplastic medulloblastoma were examined for glial fibrillary acidic protein (GFAP) using the immunoperoxidase method to assess astrocytic differentiation. In 16 cases of classical medulloblastoma GFAP-positive cells were present in variable numbers. These cells were classified as three different types according to size and shape. The type 1 cell was morphologically identical to the ordinary tumor cell, with a hyperchromatic nucleus and a scanty cytoplasm. The type 2 cell had a fairly rich cytoplasm with short cytoplasmic processes. The type 3 cell was characterized by a relatively large nucleus with sparse chromatin and well-developed cytoplasmic processes, and was considered a reactive astrocyte. The type 1 and some of the type 2 cells seemed to be neoplastic, displaying astrocytic differentiation. The remaining type 2 cells may have been reactive astrocytes. In one case of desmoplastic medulloblastoma, the majority of GFAP-positive cells were arranged in "islands", and had delicate fibrillated processes. GFAP-positive cells were also observed outside these "islands", though they were less numerous. Most of them were regarded as type 3 cells, but some were type 2. This may be interpreted as meaning that the glial character of the tumor was expressed more within than outside these "islands".