We grafted fetal thymi from wild-type mice into immunodeficient RAG-2^−/− or class II^−/−RAG-2^−/− (class II MHC⁻) recipients and followed the fate of naive CD4⁺ T cells derived from the grafts. In both types of recipients, newly generated CD4⁺ T cells proliferated to the same extent in the periphery and rapidly filled the empty T cell compartment. However, CD4⁺ T cells in class II⁻ recipients gradually decreased in number over 6 months. These results show that interactions between the TCR and class II molecules are not required for newly generated CD4⁺ T cells to survive and proliferate, but are necessary to maintain the size of the peripheral T cell pool for extended periods.