This study examines some characteristics of the lymphohaemopoietic system of heterochronically parabiosed CBA mice. It was found that a decline of the primary immune response in a young mouse sutured with an old one is accompanied with the diminution of: bone marrow and thymus cellularity; thymus weight; haemopoietic stem cells (CFC-S) and granulocyte-macrophage precursor cells (CFC-C) contents in the femur, and stromal precursor cell (CFC-F) contents both in the femur and thymus. All the indices tested, including the immune response level, in old parabionts vs. old single mice remained unchanged. In the spleen, the considerable variations of splenic weights in the mice of different control and experimental groups have been found. At the same time, no significant changes in the cellularity and CFC-F number in the spleen of all examined groups of animals have been observed. The findings strongly indicate the considerable rearrangement of lymphohaemopoietic and stromal tissues in the central organs of the immune system, namely, bone marrow and thymus of young mice due to the parabiosis with old ones.