Amyloidosis was induced in mice by 25 subcutaneous injections of casein. The splenic amyloid fibrils were identified by electron microscopy to be closely associated with reticular cells. After isolation of the fibrils by simple physical techniques, their ultrastructure revealed single filaments of 80 to 100 A width, which were rigid, nonbranching, and of indeterminate length. This is comparable to previous studies on human preparations. The amyloid fibrils were dissociated by solution in guanidine and chromatography. The resultant amyloid fibril protein was characterized as to its molecular weight, amino acid analysis, and amino-terminal sequence. It was thus definitely identified as protein AA, the major component of secondary amyloidosis. An antibody to this protein, murine AA, identified a cross-reacting mouse serum protein SAA and indicated a species specificity when tested against human preparations. A comparison is made with the AA protein in another murine model as well as AA proteins from human, guinea pig, monkey, and mink amyloidosis.