The purpose of this study is to evaluate the value of cytokeratin (CK) MNF116 and vimentin in the differential diagnosis of malignant pleural effusions. There were evaluated smears from 30 patients with pleural effusions stained with May-Grünwald Giemsa and Papanicolaou techniques for the routine cytological diagnosis. Additional smears were immunostained with CK MNF116 and vimentin using LSAB2 technique. Two independent observers evaluated all smears. Smears were classified first by cytological examination in seven cases (23.33%) as benign, and in 23 cases (76.67%) as malignant pleural effusions. Mesothelial cells expressed CK MNF116 in 96.67% (29/30) of cases and vimentin in 33.33% (10/30) of cases. Malignant cells expressed CK MNF116 in 52.17% (12/23) of cases and vimentin in 30.43% (7/23) of cases. The pattern of immunostaining was diffuse cytoplasmic. In conclusion, CK MNF116 and vimentin may be used as a part of the panel of antibodies for differential diagnosis of malignant pleural effusions with primary unknown.