Deficiency of the immunoglobulin C (IgG) in the human newborn is rare in the absence of maternal hypogammaglobulinaemia. Low concentrations of IgG in cord blood were found in 3 conditions--the donor twin in the fetofetal transfusion syndrome, hydrops fetalis and congenital hepatic disease. These were all associated with placental oedema. It is suggested that the oedema may be responsible for a disturbance in maternofetal placental transfer.