Thymectomy in neonatal mice leads to the rapid emergence of a new antigen in the tissues which, when injected into guinea pigs, results in their lymphocytes showing marked sensitization to scrapie mouse brain as compared with normal brain. The same anti-genie changes occur though much more slowly in normal ageing animals so that thymectomy appears to have accelerated an ageing process. Lymphocyte sensitization was determined by the macrophageelectrophoresis migration (MEM) test. The relation of scrapie to normal ageing is discussed.