The presence of GFA protein, an astrocyte specific antigen, was investigated in surgical biopsies of ninety-five gliomas using an immunoperoxidase detection method. In some cases electron microscopy was added and also in one tumour immunoelectron microscopy. The results were correlated with the histological classification and grading of the tumours. In astrocytomas, the number of GFA positive cells decreased with increasing malignancy. Some malignant oligodendrogliomas surprisingly showed GFA in oligodendroglial and gemistocytic tumour cells. Thus, transition from oligodendroglial into astrocytic tumour cells seems to occur. In the mixed astrocytoma/oligodendrogliomas, theoli godendroglioma areas were negative. Subependymomas were strongly GFA positive, whereas ependymomas and medulloblastomas were negative. The presence or absence of GFA seems to contribute a 'hard' criterion for classifying gliomas and may be helpful in grading them according to the degree of malignancy.