Bone morphogenetic protein (BMP) 12 is a recently discovered member of the human BMP family. It is the human homolog of mouse growth/differentiation factor (GDF)-7. Previously we reported that injection of mesenchymal progenitor cells transferred with the BMP-12 gene into the muscles of nude mice induced tendon-like tissue formation. In this study, we further investigated the effect of BMP-12 gene transfer on tendon cells. We observed that adenovirus mediated in vitro BMP-12 gene transfer into chicken tendon cells increased type I collagen synthesis. No change in alkaline phosphatase activity was observed following BMP-12 gene transfer. We also determined that BMP-12 gene transfer into a complete tendon laceration chicken model resulted in a two-fold increase of tensile strength and stiffness of repaired tendons, indicating improved tendon healing in vivo. We conclude that BMP-12 gene transfer is a promising procedure for improving the tendon repair process.