The expression of transferrin receptor (Tr) was investigated by means of immunohistochemistry in 101 tumours of the human central and peripheral nervous system. The results were compared with the proliferative activity of the tumours, determined by immunostaining for the proliferation-associated antigen Ki-67. In addition to immunostaining of normal and proliferated blood vessel endothelium and of a fraction of tumour infiltrating lymphocytes, we observed staining for Tr in a variable fraction of neoplastic cells of all histological tumour types. Immunoreactivity in the majority of tumour cells was found only in anaplastic tumours such as glioblastomas. Furthermore, a positive correlation between Tr expression and the Ki-67 growth fraction was established for gliomas. Non-glial tumours strongly expressing Tr included one metastatic rhabdomyosarcoma, one intracerebral malignant lymphoma, two of four plasmocytomas and seven of nine metastatic carcinomas. Our results indicate that immunohistochemistry for Tr and Ki-67 can provide additional information about the biological behaviour of nervous system tumours, thus complementing conventional histopathological criteria for anaplasia.