We have studied three cases of localized amyloidosis in the lower respiratory tract. Amyloid was nodularly or diffusely deposited in the lamina propria of the tracheobronchial mucosa. Its nature was confirmed by Congo red staining with green birefringence on polarized microscopy. "Tracheobronchopathia osteoplastica" also was demonstrated. Plasma cells and lymphocytes were scant in the amyloid mass. Few fibroblasts and even fewer macrophages were seen. The number of plasma cells was not increased in the bone marrow in any of our cases. Amyloid fibrils were demonstrated by electron microscopic examination. The amyloid P component was detected by immunohistochemical methods. The precursor protein of amyloidosis was shown to be amyloid L protein by the postembedding protein-A gold technique with anti-light chain antisera. The role of the plasma cells in amyloid formation, however, could not be ascertained. Based on these observations, amyloid fibril formation in tracheobronchial amyloidosis appears to be related to light chains secreted by local plasma cells, combined with amyloid P, calcium, and other factors.