Monoclonal antibodies (AE1/3, CAM 5.2 and PKK-1) and polyclonal antisera against the cytokeratin proteins were reacted with a range of astrocytic tumours, oligodendrogliomas and ependymomas. Seven of 12 cases (58%) of glioblastoma multiforme, five of eight (63%) anaplastic astrocytomas and two of five (40%) well-differentiated astrocytomas were immunoreactive with AE1/3 but not with the other anti-cytokeratin antibodies. In oligodendrogliomas, AE1/3 stained isolated astrocyte-like cells as well as scattered neoplastic oligodendrocytes in four of eight cases (50%) cases. Four ependymomas were negative for all cytokeratin markers examined. The immunostaining of astrocytomas and oligodendrogliomas with AE1/3 might represent co-expression of cytokeratin with glial fibrillary acidic protein by gliomas and calls for caution in the use of these antibodies in the differential diagnosis between gliomas and carcinomas.