Until recently, the standard approach of most laboratories in distinguishing epithelioid pleural mesothelioma from metastatic adenocarcinoma has been a negative result from a panel of adenocarcinoma-associated antibodies. However, several "mesothelium-associated" antibodies have been proposed as useful in this situation, and we have applied four of these putative mesothelioma markers--thrombomodulin, cytokeratin 5/6, calretinin, and CD44H--to a series of 61 epithelioid pleural mesotheliomas and 63 metastatic adenocarcinomas with known primary sites (lung = 19; breast = 21; ovary = 6; colon = 10; kidney = 4; uterus, epididymis, pancreas = 1 case each). Of the mesotheliomas, 55 of 61 (90%) stained for thrombomodulin, 56 of 61 (92%) for cytokeratin 5/6, 47 of 51 cases (92%) were positive for calretinin, and 39 of 43 (91%) were positive for CD44H. Of the metastatic adenocarcinomas, 12 of 63 (19%) cases were positive for thrombomodulin, 9 of 63 (14%) were positive for CK5/6, and 27 of 60 (45%) were positive for CD44H. With calretinin, only 1 case of 59 (2%) showed positive nuclear staining. All four antibodies stained reactive mesothelium; thrombomodulin also stained endothelium; and CD44H variably stained lymphocytes, macrophages, and fibroblasts. We conclude that all four antibodies show high sensitivity for epithelioid mesothelioma, but only calretinin (98%), cytokeratin 5/6 (86%), and thrombomodulin (81%) show sufficient specificity for practical use in this situation.