1. Barnes, A. D., and Krohn, P. L. (personal communication).
2. Billingham, R. E., Brent, L., Medawar, P. B., and Sparrow, E. M., Proc. Roy. Soc., B, 143, 43 (1954).
3. Billingham, R. E., Brent, L., and Medawar, P. B., Proc. Roy. Soc., B., 143, 58 (1954).
4. Billingham, R. E., Brent, L., and Medawar, P. B., Phil. Trans. Roy. Soc., B, 239, 357 (1956).
5. The important discovery that grafts from male mice are eventually rejected by females of the same strain—see Eichwald, E. J., and Silmser, C. R., Transplantation Bull., 2, 148 (1955)—does not affect the design of the present experiments.