Thymomas are characterized by significant variability in the cytological pattern and each type of thymomas (A, AB, B1, B2, and B3) requires a different spectrum of differential diagnosis. The thymomas must be differentiated from lymphomas and mesenchymal, neuroendocrine, and germ-cell tumors. The criteria for the cytological diagnosis of thymic epithelial tumors and the possibilities of using an immunocytochemical (ICC) assay to specify the diagnosis have been little studied. This leads to misinterpretation of the microscopic pattern of these neoplasms and their misdiagnosis. Objective — to study the cytological and immunophenotypic signs of thymic epithelial tumors and to determine an optimal antibody kit for the ICC diagnosis of thymomas. Material and methods. The investigators conducted 226 ICC assays of 43 mediastinal neoplasms, including thymomas (n=18) (type A (n=2), type AB (n=5), type B1 (n=4), type B2 (n=3), and type B3 (n=4), thymic carcinomas (n=2), neuroendocrine tumors (n=2), lymphomas (n=8), solitary fibrous tumors (n=2), metastatic tumors (n=11) (lung adenocarcinomas (n=3), small cell lung cancer (n=3), lung carcinoid tumors (n=2), thyroid cancer (n=2), and breast cancer (n=1). Results and conclusion. The immunohistochemical and ICC signs of thymic epithelial tumors (thymomas, neuroendocrine tumors, and squamous cell carcinomas) were comparatively studied; an optimal antibody kit has been proposed for the ICC diagnosis of thymomas, which will be able to more effectively make a cytological diagnosis of these tumors.