A silver colloid staining technique for identifying nucleolar organizer region-associated proteins (Ag-NORs) was applied to 17 primary gliomas and 16 meningiomas. These comprised 8 glioblastomas, one pleomorphic xanthoastrocytoma, 8 benign astrocytoma, 10 nonrecurrent meningiomas and 6 recurrent meningiomas, in which the mean number of Ag-NORs' per nuclei, were 3.37, 2.34, 2.57, 2.16 and 2.80 respectively, identifying significant differences among these groups. Ag-NORs of four giant cell glioblastomas showed low numbers of Ag-NORs, suggesting better prognosis. In spite of the time consuming and complicated counting method Ag-NORs is reproductive and useful as a tool for estimating the proliferating potential of various brain tumors, and appears to widely applicable in clinical laboratories.