Cytologic differantiation of reactive mesothelial proliferation from malignant mesothelioma in plevral effusions, using epithelial membrane antigen, alpha-1-antitrypsin and alpha-1-anti-chymotripsin. The present work comprised 7 cases preciously diagnosed as malignant mesothelioma and 20 cases as reactive mesothelial proliferations. For the purpose of cytologic differentiation, peroxidase-labeled antibodies epithelial membrane antigen (EMA), alpha-1-antitrypsin (AAT) and alpha- 1-antichymotrypsin (ACT) are performed. Three of the cases of malignant mesothelioma and 1 case of benign mesothelial proliferation, positive staining with anti-EMA is demonstrated. Immunoreactivity for AAT in 1 of the malignant mesothelioma and in 10 cases of reactive proliferations is observed. By the use of anti-ACT, positive immunostaining in 1 of the malignant mesothelioma and 6 of benign proliferations is also observed. The results obtained by the use of these antibodies in the aim of cytologic differantiation of the benign and malignant mesothelial lesions is discussed and the findings are compared with those in the literature.