Staphylococcus aureus grew more readily in sera of children with kwashiorkor who had low concentrations of transferrin than in sera of those who had higher concentrations. The results suggested that this reduced concentration of transferrin becomes fully saturated with iron and thus gives rise to free circulating iron, which enhances bacterial growth. When purified transferrin was added to sera from patients with kwashiorkor, there was an inhibition of growth of S. aureus. It appears that the loss of the bacteriostatic effect caused by the reduced serum transferrin level may contribute to the bacteremia which is frequent in kwashiorkor, especially in the terminal stages.