The article presents studies on the morphological picture of the blood of fish in a comparative aspect, depending on age and species. In recent years, the influence of anthropogenic factors on the ecosystem of water bodies has intensified. In this regard, the main direction of solving the problems of ichthy -epizootological situations is new areas in veterinary medicine, in particular, hematological recognition of shaped elements in the blood of fish. To identify and understand the state of the fish organism, which they reflect, the researcher must first of all correctly determine these forms of cells in the blood, especially this applies to white blood cells. Our review is brief and does not pretend to be completely complete literature data on this issue. All this together gives a complex morphological picture of the blood of fish, which is difficult to typify. Unlike higher vertebrates, fish lack bone marrow and lymph nodes, hematopoiesis occurs both in organs, which include reticular syncytium (gill apparatus, kidneys, lymphoid organ), and vascular endothelium of the gill apparatus and heart and spleen and, in some cases, intestinal mucosa. In bone fish, the anterior part of the kidneys is the main organ of hematopoiesis; hematopoiesis also occurs in the lymphoid organs and in the spleen. The purpose of this research is to study the morphological picture of the blood of fish in a comparative aspect, depending on age and species. 3-7 animals from each age group of fish were examined on average. To study the morphological composition of blood from fish, blood was taken from gill vessels and from the heart cavity. In the process of research, it was found that the peculiarity of fish is the presence in the blood of both mature and young red blood cells, red blood cells have nuclei. Bony fish have four types of myeloid cells at all stages of development, known in the hematology of higher vertebrates and humans. A distinctive feature of fish granulocytes is the ability to observe leukocytes in all successive stages of filling the cytoplasm with granules, and the nucleus of th ese cells is very rarely lobed. Unlike fish, in mammals it is very difficult to distinguish between the stages of development of basophils and eosinophils (myelocytes, juvenile, stab and segmented), since the segmentation of the nucleus is weakly expressed , and the number of granules does not increase as the cells mature. It can be noted that the studies initiated can contribute to determining the choice of hematological studies. Given the above factors, a detailed epidemiological classification of hematolo gical studies will be obtained. The research results make certain additions to the procedures of veterinary-hematological studies in the field of ichthyopathology. In the future, research will continue in the direction of hematological diagnosis of fish di seases and prerequisites will be created for a detailed epidemiological classification of hematological studies.