With the aid of a newly developed target unmasking fluid (TUF), p53 overexpression was visualized by immunohistochemistry on recent and archival paraffin-embedded tissue samples of colon, stomach, and pancreas neoplasms. Using monoclonal anti-p53 antibody pAb1801 as well as polyclonal antiserum to p53 CM1, TUF-mediated immunohistochemistry was fully concordant for p53 overexpression in paraffin-embedded carcinoma samples compared with freshly frozen tissue from the same tumors. Thus, prognostic and diagnostic assessment of p53 overexpression in malignant tissue, routinely fixed in formalin and embedded in paraffin wax, by TUF-mediated immunohistochemistry with monoclonal and polyclonal antibodies may be adopted as a new tool in diagnostic and research histopathology.