Abstract
When a neonatal thymus and an adult (reticuloepithelial) thymus from normal mice are simultaneously grafted to individual adult nude mice, the host‐derived T cell precursors regenerate the neonatal thymus more efficiently than the adult (reticuloepithelial) thymus. Thus, this is a direct cause for the poor restoration of the T lymphoid system of nude mice by adult and/or reticuloepithelial thymus grafts.