Oral senile amyloidosis in senescence accelerated mouse (SAM) was examined for two SAM sublines (P/2/Iw and R/1/Iw) and for various ages by light microscopy, immunohistochemistry, and electron microscopy. The amyloid deposition, identified by green birefrigence following Congo red stain, was observed only in P/2/Iw. In P/2/Iw, no amyloid deposition was found at age 6 months; however, frequency and extent of such deposits increased with advancing age. Distribution of amyloid deposition was as follows: along papillary layers of mucous epithelium in the tongue, the gingiva, the palate, and the buccal mucosa; foci in connective tissues; along vessels, muscles, and minor salivary glands. Immunohistochemically, the amyloid deposition was positive with anti-ASSAM serum being raised against a unique amyloid protein ASSAM which probably induced "senile systemic amyloidosis". P/2/Iw is useful as an animal model of oral senile amyloidosis.