One of the major limitations of myoblast implantation as a therapy for muscular disease is that multiple injections by intramuscular implantation may be required for widespread delivery of cells. Also, some sites (eg, the diaphragm) are relatively inaccessible to injection. As an alternative, we have undertaken intra-arterial administration of myoblasts. For these experiments, we used donor cell myoblasts from the immortal L6 cell line labeled with lacZ via the beta-gal-at-gal retrovirus. In our model, target rat skeletal muscle (tibialis anterior [TA]) was injured using 0.5 ml of 0.5% bupivacaine and 15 IU of hyaluronidase; saline was injected into the contralateral side as a control. We infused 3 x 10(6) lacZ-positive cells into the abdominal aorta of previously injured, immunosuppressed (cyclosporine A) rats. At 7, 14, and 28 days, TA, liver, heart, lung, and spleen were examined for lacZ staining. In both the injured and control muscles, a few differentiated, lacZ-positive muscle cells were present, both singly and in groups, at each time point. These studies demonstrate that genetically labeled, transformed myoblasts may migrate from the arterial circulation to muscle and fuse there to form differentiated muscle cells. It is conceivable that intra-arterial delivery of myoblasts may have a role in the therapy of selected diseases of skeletal muscle.