Salivary gland tumors present diverse histopathological aspects and show various biological behavior. At present, the immunohistological method using anti-BrdU monoclonal antibodies, which was originated in the field neurosurgery, has been tried extensively in various fields of medicine. Using this method, the authors have attempted to detect biological characteristics of salivary gland tumor tissues, especially those of pleomorphic adenomas. The labelling index (L.I.) is defined as the percentage of the labelled cells (proliferating cells) in the tumor tissue. In order to confirm the usefulness of the L.I. for measuring the proliferating activity of tumor tissue, an investigation was conducted with respect to squamous cell carcinomas of the head and neck, excluding salivary gland tumors. The results indicate that the L.I. is highly correlated with the differentiation of these carcinomas. In salivary gland tumors, values of the L.I. are also higher for benign tumors than for normal or inflammatory tissues and the values were even higher for malignant tumors. Values of the L.I. are low for pleomorphic adenomas, which are the majority of salivary gland tumors, as well as for other benign tumors. However considerable individual variabilities, are bound for pleom. ad. and the values of the L.I. are markedly high in some cases. These high values are particularly common in patients with whom strong positivity in Ga scintigraphy, or primary growth in the submandibular glands are found. The localization of proliferating cells shows a characteristic pattern in each type of tumor tissue. In pleomorphic adenomas, proliferating cells are sporadically distributed but the main growth is located at the site of the epithelioid proliferative region, excluding the duct-forming locus. However, the overall proliferating activity of myxochondromatous tumors is not generally low, and the proliferating activity is not particularly high in the region of infiltrating is the capsule.