Transplantation tolerance to the H-Y antigen, induced in neonatal or young adult C57BL female mice, beneficially affected their immunoreactivity during the process of ageing. The tolerance acquired in 2-month-old females significantly increased their splenocyte graft-versus-host (GVH) reactivity and the production of interleukin-2 (IL-2) at the age of 18 months. The neonatally induced tolerance also resulted in a higher weight of the thymus and a significantly enhanced GVH reactivity of thymocytes in 18-month-old females compared to control mice of the same age.