1. Food selenium content, selenium supply and selenium needs are presented, along with methods of evaluation of selenium status. Glutathione peroxidase, a selenium-containing enzyme, is ubiquitous in the organism. 2. Some experimental studies on animal models reported a positive relationship between selenium status and resistance against infections. 3. Only one study in humans concerned the mechanisms of immune functions in selenium deficiency. Several experimental works suggest that severe selenium deficiency compromises T-cell dependent immune functions such as the blastogenic response to mitogens, but selenium deficiency was concomitant with vitamin E deficiency in most of them. Delayed hypersensitivity response is controversial in selenium-supplemented rats and guinea-pigs. 4. Selenium deficiency in animals decreases the antibody response, especially if associated with vitamin E deficiency. Low dietary selenium supplementation of healthy animals has a positive effect upon humoral responses. 5. Despite some controversies, most experimental studies on selenium-deficient animals report normal phagocytosis and an altered bactericidal capacity of neutrophils. The decrease in glutathione peroxidase activity of polymorphonuclear cells following selenium deficiency could explain some of these alterations. 6. Splenic Natural Killer cells activity is enhanced in selenium-supplemented, healthy animals.