Morphological examinations of the hemopoietic system in 43 patients died of different forms of leukemias revealed irregular involvement into the leukemic processes of its different parts. Differences in the spread of leukemic lesions in the spleen, lymph nodes, tonsils and other lymphoid formations in lympho- and myeloleukemias were demonstrated. They are discussed from the point of view of the concept on metastatic involvement of different parts of the hemopoietic system in leukemias and on the importance in the development of these lesions of the cellular microenvironment. Also, the importance of atrophic changes of the thymus and other lymphoid structures as manifestations of immunological insufficiency which is one of the most significant conditions for the development of leukemia is discussed.