Spleen cells from mice receiving TLI, with or without thymus shielding, were investigated for in vitro and in vivo defects. At 4-6 weeks after irradiation spleen cells of both groups showed a normal number of Thy1 (T cells), L3T4 (CD4 positive T cells) cells, and an absence of natural suppressor cells. Splenocytes of the nonthymic shielded TLI group were not able to mount either a normal in vitro response (in MLR or PHA) or an in vivo graft-versus-host-disease reaction when injected into lethally irradiated adult allogeneic recipients or into neonatal F1 hybrids. This was in contrast to the normal immune capacity of spleen cells from the thymus shielded group that gave normal MLR and PHA tests in vitro and provoked GVHD in vivo. Thymuses recovered from mice receiving TLI with or without thymic shielding were however equally efficient in restoring the immune capacity after transplantation into neonatally thymectomized mice as measured by the PHA assay. Thymic irradiation is therefore necessary but not sufficient for creating long-lasting immune defects after TLI.