Although low folate status is thought to be fairly common in the older population, its implication on immunity has not been adequately investigated. Using 11-month-old and 23-month-old male rats (Fisher 344), the present study was undertaken to examine the modifying effects of feeding a control diet (NIH-07) supplemented with folate (35.7 mg/kg) for 3 weeks on the immune cells of spleen and mesenteric lymph node (MLN) origin. The serum concentrations of folate along with vitamin B(12) were elevated in response to the folate supplementation (P<.05). These results were accompanied by an improved proliferative response (stimulation index) to mitogens in both the spleen and MLNs (P<.05). The proportion of T cells in the MLNs, but not in the spleen, was significantly increased in rats fed a diet supplemented with folate. In the spleen, the folate-supplemented diet prevented the age-associated decrease (P<.05) in the production of interferon (IFN)alpha by unstimulated cells and the decrease in T-helper (Th)1/Th2-type response after stimulation with phorbol myristate acetate and ionomycin. In the MLNs, on the other hand, the folate-supplemented diet failed to influence any age-related increase in interleukin (IL)-2, tumor necrosis factor alpha and IFNgamma following stimulation but did result in a significantly increased production of IL-4 (P<.05). Overall, this study provides data suggesting that aging is associated with changes in the proportion of T cells, the ability of immune cells to proliferate and the production of cytokines after stimulation. Supplementing a folate-sufficient diet with additional folate improves proliferative response to mitogens, the distribution of T cells in the MLNs and the age-related changes in cytokine production in the spleen. These results suggest that the dietary folate requirement may be higher in the older population than in the younger population to support immune functions.