Mesenchymal Stem Cells (MSCs) possessing the capacity to differentiate into various cell types such as osteoblasts, chondrocytes, myoblasts, and adipocytes have been previously isolated from the marrow and periosteum of human, murine, lapine, and avian species. This study documents the existence of similar multipotential stem cells in canine marrow. The cells were isolated from marrow aspirates using a modification of techniques previously established for human MSCs (hMSCs), and found to possess similar growth and morphological characteristics, as well as osteochondrogenic potential in vivo and in vitro. On the basis of these results, the multipotential cells that were isolated and culture expanded are considered to be canine MSCs (cMSCs). The occurrence of cMSCs in the marrow was determined to be one per 2.5 x 10(4) nucleated cells. After enrichment of the cMSCs by centrifugation on a Percoll cushion, the cells were cultivated in selected lots of serum. Like the hMSCs, cMSCs grew as colonies in primary culture and on replating, grew as a monolayer culture with very uniform spindle morphology. The population doubling time for these cMSCs was approximately 2 days. The morphology and the growth kinetics of the cMSCs were retained following repeated passaging. The osteogenic phenotype could be induced in the cMSC cultures by the addition of a synthetic glucocorticoid, dexamethasone. In these osteogenic cultures, alkaline phosphatase activity was elevated up to 10-fold, and mineralized matrix production was evident. When cMSCs were loaded onto porous ceramics and implanted in autologous canine or athymic murine hosts, copious amounts of bone and cartilage were formed in the pores of the implants. The MSC-mediated osteogenesis obtained by the implantation of the various MSC-loaded matrix combinations is the first evidence of osteogenesis in a canine model by implantation of culture expanded autologous stem cells. The identification and isolation of cMSCs now makes it feasible to pursue preclinical models of bone and cartilage regeneration in canine hosts.