Five biopsy materials of human amyloidosis and experimental amyloidosis in mice induced by‐, casein injection were studied by an electron microscope, and the following results were obtained. 1. Amyloid consisted of amyloid fibrils with an approximate width of 100 Å. These fibrils were found in both biopsy materials of human cases and experimental amyloidosis in mice. Amyloid was produced by the reticulo‐endothelial cells which were functionally‐disturbed. 3. The process of amyloid production in the reticuro‐endothelial cells was essentially similar to that of collagen synthesis in the fibroblast. The protein synthesized in the granular reticulum was transported to the Golgi complex, and polymerization occurred in the lysosome, and finally complete amyloid fibrils were released from