THE skeletal muscle fibre is a long, multinucleated cell formed by fusion of mononuclear precursor cells (myoblasts) with one another1-3 and with muscle cells in various stages of maturity4,5. In subjects suffering from inherited recessive myopathies, muscle function might be restored if normal myoblasts could be made to fuse with defective muscle fibres during muscle growth or regeneration. Such a method of introducing new genetic information into muscle fibres requires that myoblasts can enter a region of growing or regenerating muscle, and that these myoblasts can participate in the formation of new muscle fibres by fusion with locally derived cells. Recently we have shown, using isoenzymes of malate dehydrogenase as markers, that host muscle cell precursors can enter muscle grafts6. Here we present evidence, based on the use of a more sensitive isoenzyme marker, that fusion can occur between host and donor muscle cell precursors.