Five cases of solitary (localized) fibrous mesothelioma were studied immunohistochemically, utilizing monoclonal antibodies against high and low molecular weight cytokeratin, vimentin, carcinoembryonic antigen, epithelial membrane antigen and factor VIII related antigen. In contrast to normal mesothelium and contrary to previous reports from malignant mesotheliomas all the tumours were negative for keratin. They were strongly positive for vimentin. Two cases were studied electron microscopically; no ultrastructural evidence of mesothelial cell differentiation was identified. Our immunohistochemical and ultrastructural findings argue against a mesothelial cell origin, whereas the presence of vimentin suggests a mesenchymal histogenesis. It is postulated that these tumours arise from sub-mesothelial mesenchymal connective tissue.