The hypothesis that satellite cells which leave denervated skeletal muscle might become circulating potential myoblasts which could participate in myogenesis in distant sites in the body has been tested. Sixteen mice had one hindlimb denervated and were given 7 daily injections of 3H-thymidine (3H-Tdr). One day later extensor digitorum longus muscle isografts from unlabelled mice were inserted into each hindlimb. As controls, the procedure was repeated in 6 non-denervated labelled mice. Fourteen days after their insertion, isografts in denervated mice contained many labelled myotubes with a labelling index of 55±4% (mean±SEM). In the control isografts in non-denervated mice, 38±4% of myotube nuclei were labelled. The results show that either labelled cells, or 3H-Tdr, had transferred from the host to isografts in both cases. The probability of 3H-Tdr reutilization was demonstrated in regenerating livers of 8 similarly labelled mice, where 34±3% hepatocytes adjacent to crush lesions were labelled after 14 days. This conclusion was reached because only 2-3% of normal hepatocytes incorporate 3H-Tdr under these conditions and this population is inadequate to provide sufficient labelled precursor cells for the large numbers of labelled regenerated hepatocytes. Therefore, it was concluded that 3H-Tdr reutilization is the most likely explanation for labelled myotube nuclei in the muscle isografts (rather than movement of labelled precursor cells), and that additional label for reutilization had been derived from breakdown of labelled cells in denervated muscle. The data do not support the hypothesis of a circulating precursor for skeletal muscle cells.