OBJECTIVE To characterize equine bone marrow (BM)-derived mesenchymal stem cell (MSC) growth characteristics and frequency as well as their adipogenic and osteogenic differentiation potential. METHODS In vitro experimental study. METHODS Foals (n=3, age range, 17-51 days) and young horses (n=5, age range, 9 months to 5 years). METHODS Equine MSCs were harvested and isolated from sternal BM aspirates and grown up to passage 10 to determine cell-doubling (CD) characteristics. Limit dilution assays were performed on primary and passaged MSCs to determine the frequency of colony-forming units with a fibroblastic phenotype (CFU-F), and the frequency of MSC differentiation into adipocytes (CFU-Ad) and osteoblasts (CFU-Ob). RESULTS Initial MSC isolates had a lag phase with a significantly longer CD time (DT=4.9+/-1.6 days) compared with the average DT (1.4+/-0.22 days) of subsequent MSC passages. Approximately 1 in 4224+/-3265 of the total nucleated BM cells displayed fibroblast colony-forming activity. Primary MSCs differentiated in response to adipogenic and osteogenic inductive conditions and maintained their differentiation potential during subsequent passages. CONCLUSIONS The frequency, in vitro growth rate, and adipogenic and osteogenic differentiation potential of foals and young adult horses are similar to those documented for BM MSCs of other mammalian species. CONCLUSIONS The results have direct relevance to the use of BM as a potential source of adult stem cells for tissue engineering applications in equine veterinary medicine.