Protein-energy malnutrition results in an increased risk of gastrointestinal infection. This can be attributed in part to impaired immune responses. Cell-mediated immunity is decreased as judged by reduced number and function of thymus-dependent lymphocytes, impaired delayed cutaneous hypersensitivity reactions, and decreased production of lymphokines. Concentration of secretory IgA is reduced and there are fewer intraepithelial lymphocytes. Antibody responses following viral vaccine administration are reduced and there is decrease in natural killer cell activity. In addition, the number of bacteria binding to epithelial cells is increased. These changes are observed also in certain selected nutrient deficiencies, such as that of vitamin A. It is suggested that impaired systemic and mucosal immunity contributes to the increased frequency and severity of intestinal infections seen in undernourished individuals.