Zinc deficiency is a common nutritional problem observed both in human and in animal populations that has profound effects on host defense mechanisms. Using the young adult mouse as a model, it has been demonstrated that a moderate period of suboptimal zinc causes thymic atrophy, lymphopenia, and alterations in the proportions of the various subsets of lymphocytes and mononuclear phagocytes. As a result, antibody-mediated responses to both T cell-dependent and T cell independent antigens are significantly reduced. Cytolytic T cell responses, natural killer (NK) cell activity, and delayed-type hypersensitivity (DTH) reactions are also depressed. Suboptimal zinc during in utero development of mice causes persistent states of immunodeficiency in the offspring that can even be transferred to subsequent generations. In regard to human immunological consequences of zinc deficiency, patients with the genetic disorder of zinc absorption, acrodermatitis enteropathica, also exhibit atrophic thymuses, lymphopenia, anergic DTH responses, and reduced NK cell activity. Patients suffering from sickle cell anemia or uremia with associated deficiencies in zinc exhibit similar immune deficiencies. An additional outcome of these studies has been shown to be an essential cofactor for thymulin, one of the thymic hormones. Furthermore, addition of zinc salts to culture can polyclonally activate lymphocytes as well as augment responses to mitogens in adjuvant-like manner.