Osteogenic activity of canine and rabbit bone marrow and marrow stromal fibroblasts (MSF) derived from marrow cultured in vitro was evaluated using diffusion chambers. Marrow from young dogs and rabbits grown in cell culture produced confluent layers of MSF. Diffusion chambers containing 0.18-7.6 x 10(6) allogeneic MSF were inserted into the peritoneal cavities of 5 dogs and 6 rabbits. Chambers recovered from the dogs (15/16) contained only loose fibrous tissue while chambers from rabbits (9/13) contained membranous bone and cartilage. Diffusion chambers implanted with 1.0-32.4 x 10(7) fresh allogeneic marrow cells suspended in cell culture medium were inserted into the peritoneal cavities of 11 dogs and 9 rabbits for 3-8 weeks, and after recovery examined histologically. Membranous bone was formed in 4/40 chambers containing canine marrow while bone and hyaline cartilage was formed in 21/27 chambers containing rabbit marrow. This apparent species difference in incidence of bone marrow osteogenesis may relate to a lower concentration of osteogenic precursor cells in canine marrow, a failure of osteogenic precursor cells to differentiate to osteoblasts in a somewhat artificial environment in vivo (viz diffusion chambers), a lack of cell-matrix interaction to stimulate cell differentiation, inappropriately short diffusion chamber implantation times post grafting, or a difference in ontogenetic stage of development of marrow donors with rabbit cells being physiologically younger.