The thymus-dependent (T-dependent) limb of the immune response in protein malnourished mice was examined by using several tests of T cell function, both in vivo within the intact animal, and after removal of lymphocytes from the protein deprived host. The capacity to provide help for IgG antibody responses and the DTH response to antigen were diminished after short-term (3 weeks) deprivation. However, both these responses were normal in mice maintained on a protein deficient diet for 11 weeks. The depressed DTH responses of protein deprived mice were due, at least in part, to a failure to mount the inflammatory phase of the response. Finally, using a graft-versus-host reaction (GvHR) as an index of T lymphocyte function, we found that spleen cells from malnourished donors were fully capable of inducing the splenic and intestinal changes associated with a GvHR in unirradiated F1 animals. Overall, these results suggest that T cell function is not irreversibly damaged by protein deprivation. However we propose that cell-mediated immune responses are influenced indirectly by the restrictive environment which interferes with cell migration, and by the impaired capacity of protein-deprived animals to mount a non-specific inflammatory reaction.