Malignant mesothelioma is a highly aggressive tumor of the serosal cavity that arises from the mesothelial cells of the pleura, peritoneum, or pericardium. The immunohistochemical diagnosis of epithelioid mesothelioma from biopsy or surgically resected specimens has been actively pursued, using markers such as mesothelin. Several markers have indeed been helpful for confirming the diagnosis of mesothelioma and distinguishing between mesothelioma and adenocarcinoma. The authors have developed a novel mAb to human C-ERC/mesothelin, which performed well when used in western blotting, fluorescence-activated cell sorting, immunocytochemistry and immunohistochemistry, and which therefore will be useful in studying the molecular biology of mesothelin, in addition to improving the diagnosis and therapy of mesothelin-expressing cancers.