It is shown that the effect of 5-fluorouracil (5-FU) on mesenchymal stromal cells (MSCs) of the bone marrow and fetal liver of the rat increases the population of cells with a reduced proliferative potential. It is shown that, in the course of passaging, the 5-FU-resistant MSCs of the bone marrow and fetal liver lose their ability to differentiate in the osteogenic direction more and less rapidly, respectively, than the 5-FU-sensitive MSCs.