Post-mortem specimens of blood, respiratory-tract washings, bronchopulmonary tissue, spleen, and thymus were examined for respiratory viruses, immunoglobulins, and secretory component (S.C.) in eight infants with sudden-infant-death syndrome (S.I.D.S) and in eight other (control) infants with an identifiable cause of death. Serum-immunoglobulin levels were similar in infants with S.I.D.S. and in control infants. In some S.I.D.S. cases serum-IgM was slightly raised. Respiratory syncytial virus was found in the pulmonary tissues of five S.I.D.S. patients but no viruses were isolated from other subjects. However, in one control subject parainfluenza type-3 viral antigen was detected in the bronchial tissue. In all patients with S.I.D.S., immunological reactions and fluorescent antibody staining for S.C. in the broncho-pulmonary epithelium were absent or grossly reduced. The levels of IgG and IgM in bronchial washings were unremarkable. These observations suggest a possible defect in respiratory mucosal defence in patients with S.I.D.S.