Objective To compare the biological properties in vitro of bone marrow stromal cells(BMSC) derived from fetal and adult rats.Methods\ The second passage cells primarily cultured from 3 weeks and 3 months old rat femoral marrow were induced by osteogenesis medium and compared with respects to cellular growth observed by phase-contrast microscopy.The proliferation capacity was determined by MTT method.Mineralization in vitro was assessed by Von Kossa and Alizarin red staining.Total protein level,expression of mRNA of collagen I was detected by RT-PCR and ultrastructure was examined by transmission electron microscopy.Results\ Cells from fetal rats grew faster than those from adults,their doubling times were 4.8 d and 5.4 d,respectively.After seven days many colonies were formed among fetal cells,which could not be observed among adult cells.Strong positive cells stained by von Kossa and Alizarin red and total protein level of the former were more frequently observed than those of the latter.Expressions of mRNA of Collagen I were demonstrated by agarose electrophoresis by Day7 and many intra-cellular reticula and precollagen extracellular matrices were showed by electron microscopic examination.Conclusion\ Both cells can be transformed into osteoblasts in vitro,but considering the proliferation capacity and osteogenic potency,BMSCs derived from fetal rat are most suitable for studies on cell /biomaterial interaction.