The expression of RAS oncogenes in benign and malignant salivary gland tumors was studied by immunohistochemistry and by immunoblotting using monoclonal antibodies recognizing the HRAS and KRAS gene products. Twenty-eight out of 29 benign pleomorphic adenomas overexpressed p21RAS, whereas only 12 out of 18 malignant salivary gland tumors expressed the p21 protein. The expression levels were also substantially higher in the adenomas than in the malignant tumors, indicating that RAS gene activation appears to be more frequent and of greater importance for benign than for malignant salivary gland tumors. Comparisons of the p21 expression levels with the karyotypes of the pleomorphic adenomas revealed a novel correlation between high p21 expression and chromosome 8 rearrangements. As a hypothesis, it is suggested that a novel gene located on the proximal long arm of chromosome 8, most likely at band q12, is involved in the regulation of RAS gene expression.