Cell-mediated immunity was investigated in adult mice and rats treated with monosodium glutamate in their suckling period. Delayed-type hypersensitivity to xenogeneic cells and host-versus-graft reactivity to allogeneic cells were depressed in these mice. Their splenocytes showed reduced mitogen-induced blastogenesis in vitro which was restored by removal of nonadherent to glass spleen cells. Xenogeneic local graft-versus-host reaction induced by spleen cells taken from Wistar rats treated with monosodium glutamate was significantly decreased. Treatment with monosodium glutamate of neonatal mice and rats (possibly producing hypothalamic lesions) significantly reduced their cell-mediated immunity.