Amyloidosis was induced in mice by the simultaneous injection of sodium caseinate and heat-killed M. butyricum. Amyloid fibrils were isolated by collagenase digestion, 1 M NaCl extraction and repeated washing with 0.15 M NaCl. The amyloid fibril fraction was practically free of contaminations such as collagen, chromatin and membranes as judged by electron microscopic morphometry. The protein AA was purified from the isolated fibrils to an apparent homogeneity as judged by sodium dodecyl sulphate polyacrylamide gel electrophoresis using one step gel filtration from Sephacryl S-200 in the presence of 6 M guanidine-HCl and 50 mM dithiothreitol. The molecular weight of the peptides of the protein AA were 8,500 and 10,000.