The cytologic distinction of carcinoma cells from reactive mesothelial cells can be difficult, especially in specimens containing abundant reactive mesotheilal cells and inflammatory cells with scant carcinoma cells. This study evaluates the usefulness of mucin and immunocytochemistry for discrimination between reactive mesotheilal cells and carcinoma cells, and sensitivity and specificity of these stains for the detection of metastatic carcinoma in serous effusions. Immunocytochemical panel including mucin cytochemistry with the periodic acid-Schiff(PAS) reaction after or without diastase digestion was undertaken on 127 serous effusion specimens with histologically confirmed diagnoses. The specimens including cell smears and cell blocks were stained with PAS and antibodies to carcinoembryonic antigen(CEA), epithelial membrane antigen(EMA), cytokeratln(CK), and vimentin. The sensitivities of these stains for metastatic carcinoma(127 cases) were 49%(46/94) in PAS, 48%(60/124) in CEA, 89%(97/109) in EMA, 88%(93/106) in CK, and 25%(20/81) in vimentin. The sensitivities of stains for reactive mesothelial cells(36 cases) were 19%(7/36) in EMA, 78%(28/36) in CK, and 75%(27/36) in vimentin. The PAS and CEA stains were not reacted with all cases of benign reactive serous effusions containing abundant reactive mesothelial cells. The specificities of stains for metastatic carcinoma(127 cases) were 100% in PAS, 100% in CEA, 81% in EMA, 22% in CK, and 25% in vimentin. The optimal combination of stains for use in a panel was PAS and CEA. Combined results from these two stains yielded an advanced sensitivity of 8% in PAS and 4% in CEA for metastatic carcinoma. EMA was also cosiderably useful for identification of carcinoma cells. CK and vimentin were not suitable for distinguishing between reactive mesothelial cells and carcinoma cells.