Modern diagnostic of oncological diseases, along with classical clinical and morphological methods, provides for the mandatory use of instrumental immunological, immunocytochemical and molecular genetic research methods. The main tasks of such a complex of diagnostic measures are aimed at monitoring oncological diseases at all stages of the diagnostic and treatment process, namely: the detection of a tumor at early stages of its development and the study of changes in metabolic processes in the body under the influence of neoplasms, morphological confirmation of the diagnosis, identification of histostructure and histogenesis of the tumor, determination the degree of its Malignancy, detection of metastatic lesion (regional and distant lymph nodes and other organs) or assessment of the risk of its occurrence. It is well know that the early stages of oncological diseases are difficult to diagnose. At the same time, early detection of the disease can save or significantly extend the life of the patient. In such cases, the determination in the blood of specific substances, which are produced by tumors of the respective organs, the so-called oncomarkers, has been successfully used by world medical practice for more than 40 years to establish the affected organ. In combination with instrumental methods (ultrasound, endoscopy, X-ray), diagnostic efficiency is sharply increasing. Successful treatment of Malignant tumors is possible under the conditions of their early detection and thorough histological diagnosis. Almost 50 % of the total number of oncologically diseased dogs has tumorous processes in the tissues of their genital organs, aggressiveness and metastasis, which often leads to lethal consequences, even after radical interventions. The aim of our research was to study the histological, cytological and immunohistochemical characteristics of transmissible venereal sarcoma. It has be en established that histologically, the tumor belongs to the low-differentiated round-cellular sarcoma of the alveolar type. Typical is tumor infiltration by lymphocytes, plasma cells, macrophages. Tumor cells are characterized by the presence of a mesenchymal marker vimentine. A positive local reaction on myogenin, cytokeratin and negative on CD31, CD34, S-100 protein and desmin was observed.