CK2 is a messenger-independent protein serine/threonine kinase that has been implicated in cell growth and proliferation. Our recent analysis of squamous cell carcinomas of the head and neck (SCCHN) revealed a significant elevation in CK2 activity in these tumor cells relative to normal mucosa of the upper aerodigestive tract and suggested a correlation with aggressive tumor behavior and poor clinical outcome. In order to further define the distribution of CK2 in these tissues, we have examined the immunohistochemical staining pattern of surgical specimens of both SCCHN tumors and normal upper aerodigestive tract mucosa using a monoclonal antibody directed against the catalytic subunit CK2-alpha of the kinase, and have compared these data with the subcellular distribution of CK2 activity in these same tissues. These measurements showed that CK2 is predominantly localized to the nuclei of the tumor cells, which agreed closely with the immunohistochemical staining pattern of CK2-alpha in tumor cells. The chiefly nuclear distribution of CK2-alpha immunostaining found consistently in SCCHN tumor cells and tumor-infiltrating lymphocytes contrasted with a relatively more predominant cytosolic staining pattern exhibited by various cellular constituents of normal oropharyngeal mucosa. The immunostaining pattern of CK2-alpha revealed that staining was observed in the cells stained for the proliferation-marker Ki-67; however, strong distinct immunostaining for CK2-alpha was also observed in large numbers of other cells in these same tumors, suggesting that CK2 elevation in these tumors is not a reflection of proliferative activity alone, but may also relate to the pathobiological behavior of the tumor.