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The distinction between pleural mesothelioma and peripheral pulmonary adenocarcinoma involving the pleura continues to be a diagnostic problem in surgical pathology. In recent years, the use of various immunohistochemical markers to facilitate this differential diagnosis has become common. In this study, the value of monoclonal antibodies 44-3A6, SM3, HBME-1, and thrombomodulin is compared in the differentiation of these conditions. Fifteen (68.2%) of 22, and 10 (52.6%) of 19 mesotheliomas stained positively with 44-3A6 and SM3, respectively, whereas all 23 (100%) adenocarcinomas reacted with both antibodies. Sixteen (80%) of 20 mesotheliomas and 14 (63.6%) of 22 lung adenocarcinomas reacted with HBME-1, whereas 16 (80%) of 20 mesotheliomas and only three (11.1%) of 27 adenocarcinomas were positive for thrombomodulin. Because thrombomodulin was expressed in most mesotheliomas but in only a few lung adenocarcinomas, this marker may have some diagnostic value when it is included in the standard immunohistochemical panel of markers used in the evaluation of mesotheliomas, especially when a positive marker for mesothelioma is needed. Antibodies 44-3A6, SM3, and HBME-1 have no practical value in discriminating epithelial pleural mesothelioma from lung adenocarcinoma.