The usefulness of immunohistochemical demonstration with anti-bromodeoxyuridine (BrdU) monoclonal antibody for estimating proliferative activity was evaluated in experimental meningeal carcinomatosis model. The experimental model was developed by inoculation of 1 x 10(4) Walker 256 carcinosarcoma cells into the cisterna magna of female Wistar rats. Every consecutive day after tumor inoculation, the rat was perfused by saline and then sacrificed 30 min after intravenous BrdU (200 mg/Kg) injection. The brain was removed, fixed in 80% ethanol and embedded in paraffin. Coronal sections of the brain 6 mu in thickness were obtained and stained immunohistochemically using the indirect immunoperoxidase (ABC) method with anti-BrdU monoclonal antibody (Becton-Dickinson). The sections were counterstained by hematoxylin. Labeling index (LI) which represented the percentage of tumor cells in synthetic phase was obtained by counting immunoreactive cells under the microscope. LI was as low as 10.8% to 16.9% in the first 3 days after tumor inoculation. Four to 6 days after tumor inoculation when tumor cells grew several layers in the subarachnoid space, LI was 24.0% to 40.1%. LI increased to reach a plateau around 40.7% to 48.2%, 7 to 9 days after tumor inoculation. Ten days after inoculation when necrosis appeared in the tumor, BrdU-positive cells declinded and LI was between 29.1% to 35.0%. It is a useful method to estimate the proliferative activity of the experimental brain tumors, design treatment modalities and evaluate the effect of chemotherapeutic agents by using immunohistochemical demonstration with anti-BrdU monoclonal antibody. Therefore, we suggest that chemotherapy against malignant leptomeningeal tumors shall be carried out in early or intermediate stage before the proliferative activity reaches its plateau stage.