The results of an immunohistochemical study of retinoblastoma protein in human non-neoplastic mesothelium (40 cases) and in malignant mesothelioma (36 cases) using the antibodies 1F8 and C15 are reported. All cases of malignant mesothelioma and non-neoplastic mesothelium were immunoreactive in more than 75% of the mesothelial cells for retinoblastoma protein with the C15 polyclonal antibody. Immunoreactivity for retinoblastoma protein was virtually absent with the 1F8 monoclonal body in neoplastic cells of all investigated mesotheliomas. In contrast, stromal cells in these mesotheliomas and all cases with non-neoplastic mesothelium were immunoreactive with the monoclonal antibody 1F8 in more than 75% of the cells. These findings may be indicative for the presence of an abnormal retinoblastoma protein in malignant mesothelioma lacking an epitope coded for between exons 21-27.