Pleural mesotheliomas and peripheral pulmonary adenocarcinomas were evaluated by immunohistochemistry at two institutions (Yale University School of Medicine and the City of Hope National Medical Center). Twenty-three mesotheliomas, 11 with ultrastructural verification, and 24 pulmonary adenocarcinomas were studied. Antikeratin, anticarcinoembryonic antigen, anti-human milk fat globule related antigen, MC-1, B72.3, and Leu M-1 antibodies were employed. The immunohistochemistry of each case prepared at one institution was reviewed at the other. As groups, the two tumor types were distinguishable using this antibody panel. Essentially, mesotheliomas were keratin positive only. The adenocarcinomas were positive for CEA, MFG, B72.3, and Leu M-1. However, there were some ambiguities presented by the immunohistochemistry. The higher molecular weight keratins were found in most but not all mesotheliomas and in only a few adenocarcinomas. Lower molecular weight keratins were positive not only in all the mesotheliomas, but also in nearly all the adenocarcinomas, but also in a tumor determined by electron microscopy to be mesothelioma. Preabsorption of CEA decreased the sensitivity for adenocarcinoma, but did not change the positivity of the putative mesothelioma. B72.3 and Leu M-1 were specific for adenocarcinoma, but were found in only half of them. The MFG was apparently specific for adenocarcinoma, but the findings were difficult to interpret. At the level of individual cases, greater sensitivity of separation can be achieved by combining the results of two or more antibodies, but the lack of a detectable specific antigen in mesothelioma continues to make some cases difficult to evaluate by immunohistochemistry alone.