The recovery of the primary immune response in CBA mice which have been given a potentially lethal dose of irradiation and syngeneic bone marrow therapy is very slow and incomplete if the hosts have previously been thymectomized. Restoration of the immune response can be obtained by grafting these mice with a single neonatal CBA/T6T6 or C3H thymus lobe. Mice given C57BL thymus grafts did not show such a marked restoration of immunological function. Histological investigation revealed that the CBA/T6T6 and the C3H grafts were accepted by the host but that the C57BL grafts failed to persist. Mice which had received C57BL thymus grafts were shown to be immune to C57BL skin grafts.