Interferon production by leukocytes in culture was investigated in nine severely marasmic infants and 31 well-nourished controls. The production of interferon was induced with Newcastle disease virus and assayed in Vero cells challenged with vesicular stomatitis virus. Marasmic infants produced significantly less interferon than controls. It is suggested that the finding may be the result of a lymphocyte defect induced by malnutrition and could help to explain the increased frequency and severity of viral diseases in this condition.