The presence of tumor within the serosal cavities, often connected with accumulation of serosal effusion, is a quite common problem in the small animal veterinary medicine. The first step in diagnosis of such cases is cytopathological examination. The aim of the present study was to evaluate the usefulness of cytology and immunocytochemistry, using commercially available antibodies (anti-cytokeratin, anti-vimentin, and anti-desmin), in differential diagnosis of malignant tumors located within serosal cavities in dogs. The final cytological diagnosis of carcinoma/adenocarcinoma, sarcoma, and mesothelioma was obtained on the basis of routine cytopathology and immunocytochemistry, and then confirmed by histopathology and immunohistochemistry. Cytoplasmic immunoreactivitiy of normal mesothelid cells and cytoplasmic immunoreactivity of hyperplastic mesothelial cells revealed constant and strong expression of all examined intermediate filaments: cytokeratin, vimentin and desmin. Application of routine cytopathology and immunocytochemistry allowed 32 neoplastic tumors to be detected: 19 cases of carcinomas/adenocarcinomas, 6 cases of sarcomas, 7 cases of mesotheliomas. Immunostaining of cytopathological samples with chosen set of antibodies: anti-cytokeratin, anti-vimentin, anti-desmin is a useful, and low invasive test for differentiation between mesotheliomas and carcinomas/adenocarcinomas in dogs.