Hemopoietic cells at different stages of differentiation were found in the liver imprints of adult thymus-less mutant mice BALB/c nu/nu, thus suggesting the presence of extramedullary hemopoiesis. During the postnatal development the intensity of leucopoiesis in the liver of mutant animals increased reliably and that of erythropoiesis suffered no marked changes. In the liver of adult BALB/c nu/nu mice, a high content of stem hemopoietic cells was found. The subcutaneous implantation of thymus to 2 weeks old BALB/c nu/nu mice resulted in the reliable decrease of leuco- and erythropoiesis in the liver. The neonatal thymectomy of the control mice BALB/c+/+ did not prevent the switching embryonic hemopoiesis in the liver off. A question of regulating influence of thymus on the process of switching hemopoiesis in the mouse liver off is discussed.