Summary: This investigation tested the hypothesis that delivering mesenchymal stem cell-seeded implants to a lendon gap model results in significantly improved repair biomechanics. Cultured, autologous. marrowderived mesenchymal stem cells were suspended in a collagen gel delivery vehiclc; the cell-gel composite was subsequently contracted onto a pretensioned suture. The resulting tissue prosthesis was then implanted into a 1-cm-long gap defect in the rabbit Achilles lendon. Idcntical procedures were performed on the contralatera1 tendon, but only the suture material was implanted. The tendon-implant constructs were evaluated 4,8. and 12 weeks later by biomechanical and histological criteria. Significantly greater load-related structural and material properties were seen at all time intervals in the mesenchymal stem cell-treated tendons than in the contralateral, treated control repairs (p < 0.05), which contained suture alone with natural cell recruitment. The values were typically twice those for the control tissues at each time interval. Load-related material properties for the treated tissues also increascd significantly over time (p < 0.05). The treated tissues had a significantly larger cross-sectional area (p < 0.05), and their collagen fibers appcarcd to be better aligned than those in the matched controls. The results indicate that delivering mesenchymal stem cell-contracted, orgaiiized collagen implants to large tendon defects can significantly improve the biomcchanics. structure, and probably the lunction of the tendon after injury.