1. The isoenzyme pattern of phosphohexose isomerase on starch-gel electrophoresis was found to be significantly and consistently different in the muscles of the Swiss white and 129 ReJ mice. This provided a good marker for identifying donor and host muscle.
2. The isoenzyme pattern does not appear to be due to different proportions of type 1 and 2 fibres or to fibre type grouping as is found in denervated/reinnervated muscle.
3. Tibialis anterior muscles from Swiss white mice were transplanted into the legs of 129 ReJ mice and vice versa. Transplantations between mice of the same strain were used as controls.
4. The isoenzyme patterns from transplanted muscles indicate that it is the donor muscle which regenerates when the tissue is transplanted from one animal to another.