The potential of dietary glutathione to alter immune response in aging mice was studied. Four (young), 17 (mature) and 24 (old) month old C57BL/6Nia male mice were fed semi-purified, nutritionally adequate diets containing 0 (control) to 1.0% of reduced glutathione (GSH) for 4 weeks. Concanavalin A (Con A) stimulated proliferation of splenocytes was assessed by [3H]thymidine incorporation. Delayed-type hypersensitivity (DTH) to dinitrofluorobenzene (DNFB) was measured by a radioisotopic method. Spleen GSH and splenocyte thiol (-SH) levels were determined by HPLC and N-ethyl[14C]maleimide binding, respectively. In the control fed group, mature and old mice showed 67% and 72% reductions (P less than 0.05) in Con A stimulated [3H]thymidine incorporation compared to young mice. Dietary GSH supplementation partially, but significantly (P less than 0.05) reversed this age-associated decline in mature and old mice. DTH assays revealed that the in vivo T-cell-mediated immune function is depressed with age and that dietary GSH supplementation reverses this depression. Spleens from control-fed mature and old mice contained 12 and 19% less GSH, respectively, than young mice (P less than 0.05). This decline was also reversed (P less than 0.05) by dietary GSH supplementation. Splenocyte -SH content after incubation with Con A and responsiveness to this mitogen were positively correlated in old mice and were greater (P less than 0.05) in GSH supplemented animals. Thus, dietary GSH supplementation improves the splenic status of this tripeptide and enhances T-cell mediated immune responses in aging mice.