The nuclear phosphoprotein p53 is expressed in all normal cells and appears to function in cell cycle regulation. Abnormally high levels of the protein are found in many different types of cancer. In human cancer overexpression of p53 is associated with point mutations within highly conserved regions of p53 gene. These altered genes encode stable p53 proteins that can be detected by standard immunocytochemical techniques unable to detect rapidly degraded wild-type proteins. Using a monoclonal antibody to p53 antigen, immunocytochemical analysis of 29 squamous cell carcinomas of tongue (n=19) and tonsil (n=10) was performed. Non-tumor nuclei showed all negative reactivity. Positive reactivity was found in 4/29 (13.8%) of SCCs of tongue and tonsil. In sizes of primary tumor, cases over 4 cm showed more positive reactivity than cases under 4 cm(p<0.05). There was no statistical correlation between the reactivity and histopathologic grades, the primary sites of tumor or the presence of cervical metastasis.