SummaryA comparative study has been made of the growth and repopulation of thymus grafts in the subcapsular space of the kidney and in the subcutaneous tissues of mice. In both sites thymus grafts underwent initial necrosis followed by regeneration, and in both sites the lymphoid population was replaced by host cells. In the kidney capsule site, thymus graft growth was six times more rapid and lymphoid differentiation and proliferation occurred earlier than in the subcutaneous sites. Kidney tissue did not potentiate the growth of thymus tissue in mixed grafts of thymus and kidney tissue. The initial delay in lymphoid differentiation of subcutaneous thymus grafts may be significant in determining the susceptibility to neoplastic change in these thymus grafts.