1. The importance of iron on immune functions is reviewed. 2. The consequences of iron deficiency upon resistance to infection in men (adults and children) and animals are controversial. 3. Cellular immunity is often altered in iron-deficient humans and in murine species. 4. Humoral immune responses seem far less affected in iron-deficient humans than is cellular immunity, but is impaired in iron-deficient animals. Results on complement are scarce and controversial. 5. There is almost no perturbation of phagocytosis but bactericidal activity is decreased in most studies on iron-deficient subjects. 6. Natural Killer activity is decreased in iron-deficient mice. Iron deficiency also affects lymphokine production in mice and rats.