Regeneration of mature skeletal muscle fibres involves the formation of new multinucleate muscle fibres by the fusion together of mononucleate muscle precursor cells. Such precursor cells appear to be largely or entirely derived from satellite cells, located between the basement membrane and the sarcolemma of the muscle fibre. We have previously presented evidence that precursor cells which contribute to regenerating muscle in a region of muscle damage are not all locally derived but that some migrate in from exogenous sources. The present study examines the possibility that a regenerating muscle might receive muscle precursor cells from neighbouring muscles. To do this we have made whole muscle allografts in the mouse and used the two murine isoenzyme allotypes of the dimeric enzyme Glucose-6-Phosphate Isomerase (GPI) as markers to demonstrate whether there is movement of muscle precursor cells between these allografts and adjacent host muscles. In host muscles adjacent to some allografts, a "hybrid" form of GPI was detected, each molecule consisting of one donor and one host GPI subunit. Such heterodimers can form only where host and donor nuclei share a common cytoplasm: in muscles this means that mosaic host/donor muscle fibres are present. The presence of such fibres implies that muscle precursor cells must have migrated into the host muscle from the neighbouring allograft.