Nucleolar organizer regions (NORs) are loops of DNA which occur in the nucleoli of cells and which possess ribosomal RNA (rRNA) genes. The numbers and/or configurations of NORs have been thought to be related to cellular activities. To assess the applicability of NORs associated protein (Ag-NORs) in the field of diagnostic histopathology, a silver staining was done in paraffin sections of malignant lymphomas, tonsils and reactive lymph nodes and the numbers of Ag-NORs in the nuclei of low-grade and those of high-grade lymphomas were compared. A significant difference was found between the numbers of Ag-NORs in the nuclei of low-grade lymphoma (a mean of 1.3 per nucleus) and those of high-grade lymphomas (a mean of 4.2 to 8.3 per nucleus). The Ag-NORs were often observed in nuclei in areas where nucleoli themselves were not visible in H and E stain. It is suggested that this method would be of great value in the field of tumor histopathology.