Bone marrow stromal cells contain a population of progenitor cells which are capable of differentiation along a number of cellular pathways. They can be encouraged to fonn. bone under the correct stimulation and therefore they hold potential in tissue regeneration. The aim of this project was to develop a small animal model for the investigation ofbone marrow stromal cell therapy. A delayed union model was initially established in an externally fixated rat femoral fracture model by periosteal and endosteal stripping at the fracture site. Bone marrow stromal cells were harvested from littermates for culture expansion. Cell culture releases progenitor cells from the normal in vivo constraints and they rapidly proliferate. Cells were cultured for 6 weeks in an osteogenic medium to encourage osteogenic differentiation before re-implantation into the fracture site of animals in whom fracture repair had been frustrated by periosteal and endosteal damage. In order to re-implant the cells, a small drill hole was created at the fracture site to facilitate the introduction of cells. Animals were randomized to one of three groups - a no intervention group (to characterize the natural progression of a delayed union), a drill group with the injection of a carrier but no cells, (to determine the effect of the drilling action) and drilling with cells (to determine the effect of cells). When compared to the no intervention group neither the drill group nor the cell group significantly improved outcome in the delayed union model. Whilst it was encouraging that the injection of allogenic cultured cells did no harm; i.e the results were no worse than the no intervention group; it is disappointing that they did not improve the natural course of.a delay in union. It appears that cells alone are not sufficient for tissue regeneration in a hostile environment