In rats, goblet cells were found in the bronchiole epithelium surrounding enlarged lymphoid noduli. The goblet cell number increased with age. Old rats had goblet cells in the columnar epithelium. Immunization with aerosolized antigen increased goblet cell hyperplasia in bronchioli with a luminal diameter of 0.5-1.0 mm. In mice, goblet cell differentiation in the respiratory epithelium was caused by local delayed-type hypersensitivity reactions. Manipulations of the delayed hypersensitivity reactions with immune suppression and depletion of helper T cells eliminated the increase in goblet cell number.