The monoclonal antibodies OX6, OX19, W3/13, OX7, OX8, and W3/25 were used to gain information about the distribution of different lymphocyte subpopulations in peripheral lymphoid organs of neonatally isogeneic and allogeneic thymus reconstituted nude rats. Splenic mitogen responsiveness, xenogeneic skin rejection, and antibody titers were also measured in the same groups of animals. The experiments showed that both allogeneic and isogeneic thymus grafting cause a significant amplification of cells in the different T lymphocyte subpopulations. The functional tests, however, indicate that the T cell response is far better following isografting. We, therefore, conclude that isogeneic thymus grafting is an easy method of reconstituting the nude rat immunologically.