Polo-Like Kinase(PLK) is a cell cycle-regulated, cyclin-independent serine/threonine protein kinase. Recent reports have shown a critical role for PLK during tumorigenesis. To explore whether PLK plays a general role as a tumor marker of oral squamous cell carcinomas, we examined the expression of PLK mRNA and protein in oral squamous cell carcinoma cells and immortalized normal oral keratinocytes(INOK). We also investigated that PLK mRNA was expressed in specimens from 4NQO-induced SD rat tongue carcinomas using in situ RT-PCR methods. Immunocytochemically, most of the PLK was highly expressed in the nucleus of carcinoma cells, but not INOK. RT-PCR revealed PLK1 mRNA was detected in the FaDu and Hep2 cancer cells, but no detected in the INOK. In situ RT-PCR revealed PLK1 mRNA expression increased sequentially from hyperplasia to dysplasia, and squamous cell carcinoma during the malignant progression. PLK1 expression could reflect the degree of malignancy and proliferation in oral squamous cell carcinomas. Thus, in addition to being of diagnostic value, modulation of PLK1 activity in the tumors by chemotherapeutic agents or gene therapy may prove to be of therapeutic value.