It has been shown that hypoxia (5% 02) and fibroblast growth factor bFGF reduce the doubling time of bone marrow mesenchymal stem cells under their cultivation in vitro that indicates an increase in cell culture proliferation. It has been found out that low concentrations of O2 and factor bFGF added to the cell culture medium increase an expression of abcg2 gene and its gene protein, ABCG2 transport gene, in mesenchymal stem cells. These events potentiate the effects of hypoxia observed in mesenchymal stem cells. We revealed that blocking of ABCG2 protein functional activity led to increased generation of reactive oxygen species in mesenchymal stem cells. The effect of hypoxia and/or bFGF on protein profile of mesenchymal stem cells was studied. The results represented in this work together with previous data proved a link between ABCG2 protein expression, its activity and maintenance of viability and proliferative activity of mesenchymal stem cells cultivated under hypoxia. ABCG2 acts as protector.