1. Burnet, F. M., Immunological Surveillance (Pergamon Press, Oxford, 1970).
2. Burch, P. R. J., Growth, Disease and Ageing (Oliver and Boyd, Edinburgh, 1968).
3. Walford, R. L., The Immunologic Theory of Ageing (Munksgaard, Copenhagen, 1969).
4. Pantelouris, E. M., Exp. Geront., 7, 73 (1972).
5. Fabris, N., Pierpaoli, W., and Sorkin, E., Nature, 240, 557 (1972).