The influence of Fe status on cell-mediated immunity was studied in weanling mice fed on Fe-deficient (7 mg Fe/kg), Fe-sufficient (120 mg Fe/kg) and high-Fe (3000 or 5000 mg Fe/kg) diets for 7 weeks. The contact sensitivity (CS) response to dinitrofluorobenzene (DNFB), the in vivo delayed-type hypersensitivity (DTH) response to sheep erythrocytes (SRBC) and the ability of primed spleen cells to transfer DTH response to naive normal mice were suppressed in mice consuming the Fe-deficient diet. High-Fe diets (3000 or 5000 mg Fe/kg) selectively suppressed the CS response to DNFB, but the DTH response to SRBC or the transfer of DTH response by primed spleen cells to naive normal mice remained normal. Spleen cell functions associated with the expression of class II major histocompatibility (MHC) surface antigens, concanavalin A-induced interleukin-2 (IL-2) secretion or the antigen-presenting cell (APC) ability to stimulate antigen-dependent proliferation of an SRBC-specific helper T-lymphocyte clone were not altered by Fe status. However, consistent with the suppressed DTH response in the Fe- deficient mice was the suppressed concanavalin A-induced T-lymphocyte blastogenesis and the interferon-γ (INF-γ) production by spleen cells from mice fed on the Fe-deficient diet. Spleen cells from mice fed on excess levels of Fe in the diet secreted less INF-γ than the control mice, although T- lymphocyte proliferation remained unaffected. Suppression of the cellular immune response associated with Fe deficiency may be related in part to impaired T-lymphocyte proliferation and INF-γ secretion rather than to deficits in IL-2 secretion or APC function.