KROHN1 has recently re-emphasized the role that transplantation studies can play in the elucidation of the process of ageing in organs and tissues. He has shown that skin and ovary have intrinsic life cycles, to a large extent independent of central mechanisms. These techniques have not been successfully used in the study of rapidly ageing transient tissues like the placenta. Earlier transplantation studies of placenta have always used allogeneic tissues, since the importance of genetic differences in such experiments was not appreciated2. The present experiments, to be described in detail elsewhere, were designed to demonstrate the evolution of syngeneic transplants of mouse trophoblast with age.