Children with protein calorie malnutrition have impaired immune functions. Immunoglobulin levels are low in children with severe protein deprivation, but are not affected in moderate or borderline protein malnutrition. Specific antibody response is either normal or reduced depending on the antigen used. Involution of the thymus dependent lymphatic system and severe impairment of the cell-mediated immune reactions are prominent features in children with kwashiorkor. Furthermore, in vitro granulocyte dysfunction in those children has been demonstrated as far as chemotaxis, microbiocidal activity and quantitative leucocyte iodination is concerned. There is a delayed chemotactic response in the early incubation intervals; there is decreased killing of S. aureus, E. coli and C. albicans after 60 minutes of incubation, and there is less iodination by phagocytising granulocytes of kwashiorkor children than in control experiments, indicating an impairment of the myeloperoxidase-peroxide-iodide mediated killing system. The possible clinical implications of these in vitro findings are briefly discussed.