Application of Carcinoembryonic Antigen and Cytokeratin (PAP Method) in Differential Diagnosis of Malignant Mesothelioma and Metastatic Adenocarcinoma in pleural fluids. Carcinoembryonic antigen and cytokeratin antigens by using PAP method, are performed in the differantial diagnosis of different malignant cells exfoliated in pleural fluids. For this reason the efficacy of these immunohistochemical staining in 7 cases of malignant mesotheliomas and 16 cases of metastatic adenocarcinoma are studied. Immunoreactivity for cytokeratin is observed in 5 cases (71.5%) of malignant mesotheliomas,but it was negative in all cases of adenocarcinomas. CEA was positive in 13 cases (81.25%) of adenocarcinomas as compared with only 1 case (6.2%) of malignant mesothelioma. The significant differences between the immunoreactivity rates of mesotheliomas and metastatic adenocarcinomas for CEA and cytokeratin suggest that the use of these immunohistologic methods in pleural fluids may be value in the differantial diagnosis of above mentioned tumours.