Transplantation of human bone marrow mesenchymal stem cells is considered as a promising therapeutic approach to the therapy of many diseases. However, the problem of possible alterations of the properties of mesenchymal stem cells during their expansion in in vitro cultures before transplantation is not solved. In our study, one of two hundred examined cultures of mesenchymal stem cell cultures derived from donors without bone marrow pathologies and developed under standard culturing conditions demonstrated spontaneous disturbances in morphology, proliferation, and karyotype at early passages. The cells of this abnormal culture retained immunophenotype characteristic of normal mesenchymal stem cells, but some of them (15-25%) had numerous numerical and structural chromosome aberrations.