Ten of 40 infants with a history of prolonged gestation had elevated concentrations of IgA in the cord serum. The IgA present at birth appears to be of fetal origin. IgG levels were higher for postmature than for term infants, while IgM levels were similar for the two groups. An elevated concentration of IgA at birth, in the absence of an elevated concentration of IgM, is of value for confirming an impression of prolonged gestation.