The presence of p53 protein in non-neoplastic pleural mesothelium (40 cases) and in human malignant mesothelioma (36 cases) was assessed immunohistochemically using the antibodies DO7, CM-1, and PAb240. In a quarter of the malignant mesotheliomas, there was nuclear immunoreactivity for p53 protein with both the DO7 and CM-1 antibodies. There were no statistically significant differences between the various mesothelioma subtypes (P > 0.05). No immunoreactivity was found with the PAb240 antibody, suggesting absence of mutant-type p53 protein. Nonneoplastic mesothelium was not immunoreactive with any of the antibodies. We conclude that there is immunoreactivity for p53 protein in some mesotheliomas. p53 protein immunoreactivity could be used to discriminate between neoplastic and reactive mesothelium.