The number of cells containing immunoglobulins A, G and M in the respiratory tract mucosa of pigs from birth to maturity was assessed using the immunoperoxidase technique. Immunoglobulin-containing cells first appeared at 6-7 days of age and rose rapidly to reach levels at 3-4 weeks similar to those in the adult. IgA-containing cells predominated at all sites in all age groups, although there were significant proportions of cells containing IgM and IgG. Our findings suggest that IgA is transported into secretions via the mixed mucoserous glands of the nasal and tracheobronchial mucosa, and that this route is also operative for colostral IgA absorbed from the gut in the baby pig.