Introduction: In the orthopaedic surgery, whether it is trauma, reconstructive surgery or alloplastic surgery, the need for a fast and strong bone regeneration is often essential. In order to obtain this autologue or allogenic bonegraft is widely used. This, however, posses a number of limitations. The risk of infection from the donor, when the graft is allogenic, and the limitation in the amount of graft that can be harvested when it’s auotologue. As an alternative, or as a combination, the interest in using mesenchymal stem cells (MSC) in reconstruction of a bone defect has increased drastically the later years. Severel studies have shown that these multipotential cells are capable of inducing an efficient bone reconstruction1 since they have the capability of differentiate into osteblasts, as well as other cell lines. That the MSC can easily be grown in vitro from a bone marrow aspirate and that cryopreserving have no effect on either their growth or ostegenic differentation2 makes them easy to handle in the laboratory. However most of the studies shows the effect using autologue MSC. In the clinical aspect this posses a number of limitations. The bone marrow needs to be harvested from the patient ahead of the surgery involving the risk of infection and discomfort for the patient, and, furthermore, this limits the use to scheduled surgery only. Materials and Methods: Eight adult female sheep were chosen for this study. Bone marrow was harvested and the MSC were isolated and cultured in a medium consisting MEM+10%Fetal Bovine Serum (FBS)+1%pen/strep. Each sheep had four implants inserted in 10mm drilled bone defects. A lateral and a medial in each distal femur condyle. The implants were cylindrical, 12mm in height. Between the two end caps, there was a tantalum core with a structure which resembles that of trabecular bone. The diameter of the core was 8mm which made a 1mm critical gap from the surrounding bone to the trabecular metal. The implants were divided into four groups (table 2), and each sheep had one from each group inserted, hence each sheep was its own control. The sheep were paired two by two, donating the allogene MSC to the other. The site of the implant group were rotating from one individual to the next, to avoid biases from the various surgical positions. Eight weeks after surgery the sheep were euthanized and the implants including the surrounding bone were removed. A destructive push-out test was performed (DATA) to measure the Shear Failure Energy (SFE) which is the energy needed to detach the actual implant from the surrounding bone, and thereby measuring the degree of bone in growth (bone fixation) into the actual implant. The results were analyzed statistically using one-way ANOVA, and a p-value <0.05 was chosen as significant. Results: The destructive test showed, that all four groups had a degree of bone fixation across the critical gap. As the implant group #4 was only added medium and no MSC, this one is chosen as the control. Compared to the control, only the group with allogenic MSC (group#3) proved to have a significant higher mean SFE (Fisher's LSD-test). The other groups (#1 and #2) had a slightly higher mean SFE (Table 2). Discussion and Conclusion: There are shown two interesting things in this minor pilot-study. There is a trend showing, that the use of MSC has a positive influence on bone regenerating in a critical sized bone defect. Furthermore, it is shown, that the use of allogenic MSC can efficiently be used, and in this study is proven better than that of autologue MSC. Further studies are of course needed, especially in the use of allogenic versus autologue MSC since the clinical aspects in using allogenic MSC are huge.