SUMMARY1. Neonatally thymectomized mice can be immunologically reconstituted by injection i.p. of as few as 1 million allogeneic adult spleen cells. These animals are tolerant of skin grafts from the same strain as the cell donors, but will reject third-party skin. Their peripheral immunologie competence is of donor type in graft-versus-host assays. 2. Partially thymectomized mice, deprived of as much as Vs of their thymic tissue at birth, are capable of normal allograft rejection, are not susceptible to tolerance induction by allogeneic cells, have a normal life span, and display significant graft-versus-host activity.