Tumor tissues obtained from 5 cases of malignant mesothelioma and from 5 cases of primary adenocarcinoma of the lung have been studied on staining with the sialyl SSEA-1 antibody by the PAP method. Results in all cases of a malignant mesothelioma (both Epithelioid and Sarcomatous pattern) were absolutely negative. All cases of lung cancer, however, were positive, though in differing degrees. In evaluating a malignant mesothelioma, many reports have indicated that the negativity of CEA staining is useful in achieving the differential diagnosis from lung cancer. The sensitivity of the staining by sialyl SSEA-1 was found to be far better than that of CEA in examining either malignant mesothelioma or lung cancer. Thus, we found that not only CEA but also sialyl SSEA-1 antigen staining was useful for the diagnosing of a malignant mesothelioma.