Transforming growth factor-beta (TGF-beta) has been suspected as a possible gene therapy candidate for orthopedic diseases. We demonstrated that the TGF-beta gene therapy can be applicable to orthopedic patients. After transfection of TGF-beta cDNA sequence to myoblasts [C2 (280)] and fibroblasts (NIH 3T3), stable cell lines with TGF-beta mRNA expression were selected by Northern analysis. To evaluate the possibility of clinical application of these cells to orthopedic diseases, the cells were injected into rabbit achilles tendon. Intratendinous injection was done to evaluate the viability of the cells and to determine the optimal concentration for in vivo expression. At 6 weeks after injection, the injected tendon was thickened with newly formed collagen. The results from this experiment indicates that these cells survived and stimulated matrix formation in rabbit achilles tendon. We concluded that TGF-beta cDNA transfected cells can be useful in the evaluation of TGF-beta biology in vivo.