Serum levels of γG, γM, γA, and total immune globulins were determined on 88 infants with the sudden death syndrome (SDS) and compared with levels on 16 infants who died from known cause and 177 normal age-matched controls. Immune globulin levels in all groups were similar except that SDS infants aged 4 to 6 months had higher mean levels of total gammaglobulin than controls. Immune globulin levels were not correlated with sex or milk agglutinin titers. Three SDS infants (3.5%) had hypogamma-globulinemia and 13 SDS infants (15%) had either elevated γM globulins (9 cases) or diffuse hyper-gammaglobulinemia (4 cases). We conclude that primary derangement of antibody proteins is not a frequent cause of SDS.