Comparison of mesenchymal stromal cells of embryonic and adult rat spleen showed that splenic cells from 20-day rat fetuses exhibit the capacity for clonal growth, express surface antigens CD73, CD90, and CD106, and have weak osteogenic and adipogenic potencies, while splenic cells from adult animals are characterized by lower cloning efficiency, rapid decrease of proliferative activity during passaging, the absence of CD73 and CD90 expression, and are incapable of osteogenesis The observed changes are probably related to extinction of myelopoiesis in the spleen during the postnatal ontogeny.