Stem cells from human fetal thymus ectomesenchyma capable of forming colonies during in vitro culturing were isolated and characterized. Selection of culturing conditions showed that the growth and phenotypical characteristics of these cultures depended on seeding density and presence of basic fibroblast growth factor in the medium. After nonspecific inhibition of proliferation clonogenic cultures of thymic mesenchymal stem cells differentiated into myoblasts, formed characteristic myotubes, and expressed specific myogenesis markers. Colonies of thymic mesenchymal stem cells differentiated into chondrogenic, osteogenic, and adipogenic lines under conditions described for bone marrow mesenchymal stem cells. Cytofluorometric analysis of surface epitopes of thymic mesenchymal stem cells showed that the majority of cells expressed mesenchymal markers Thy-1, CD44, and CD105. Testing for CD34, CD38, CD45, and HLA-DR were negative in all cases. The main cell population (70-95%) did not express MHCl antigens during long-term culturing.