Determinations of the effects of iron status on the immune system are complicated by the fact that microorganisms and immune cells both utilize iron. To determine the role of iron in immune function, we utilized a model [experimental autoimmune encephalomyelitis (EAE)] in which a strong antigen-specific CD4+ T-cell response develops in the absence of infection. EAE is an autoimmune disease frequently used as a model for the human disease multiple sclerosis (MS). EAE was induced in B10.PL mice fed low iron (1 mg/kg), normal iron (10 mg/kg) or high iron (160 mg/kg) diets that were replete in all other nutrients. Liver iron measurements verified iron status, i.e., low iron mice had 1.9 micro mol/g tissue, normal iron mice, 3.27 micro mol/g tissue and high iron mice, 5.35 micro mol/g tissue. EAE symptoms were most severe in normal iron mice, and EAE did not develop in low iron mice. The incidence of EAE was 71% in normal iron mice, 62% in iron-overloaded mice and 0% in iron-deficient mice. Two of seven mice in the normal iron group developed severe EAE and were euthanized. None of the iron-overloaded mice developed severe EAE. Other measures of EAE severity were similar in the normal and iron-overloaded mice. The data suggest that iron deficiency provides protection from the development of EAE and that iron excess with its potential contribution to free radical formation was not an important factor. The mechanism of EAE inhibition in iron-deficient mice likely involves the delivery and metabolism of iron for optimal CD4+ T-cell development.