Jejunal biopsies from 20 well nourished children (average age 12.8 months) with gastroenteritis, and 20 children (average age 20 months) with protein-energy malnutrition were examined by immunofluorescent technique for immunoglobulins A, G, M, E, and D, and for epithelial glycoprotein secretory component. Compared with previous studies on normal infants, the children with gastroenteritis showed a moderate increase in IgA-containing cells, a large increase in IgM-containing cells, and no change in IgG-containing cells. These findings are similar to previously recorded findings on adults with gastroenteritis. In contrast there was a pronounced and highly significant decrease in IgA-containing cells in the jejunal mucosa of the children with protein-energy malnutrition. No significant differences were noted between the populations of IgG-, IgM-, IgE-, and IgD-containing cells in the two groups. It is suggested that this selective deficiency in mucosal IgA results from a delay in maturation of the secretory IgA system, and the mechanisms of such a deficiency are discussed.