AA-protein was identified by SDS-acrylamide electrophoresis in amyloid fibrils fixed in formalin after isolation from fresh-frozen tissues obtained from patients with familial Mediterranean fever (FMF) amyloidosis and idiopathic AA-amyloidosis and, following deparaffination, rehydration and homogenization of embedded formalin-fixed tissues of old autopsy cases of the hereditary amyloidosis of FMF and amyloidosis acquired in association with tuberculosis, bronchiectasis, and rheumatoid arthritis. That AA-protein is unaltered by formalin was firmly established by agar gel diffusion using specific rabbit anti-AA serum. By contrast, AL proteins could not be demonstrated either in formalin-fixed amyloid fibrils derived from fresh-frozen tissues of a patient with presumably AL-amyloidosis dominated by cardiomegaly and one with AL-kappa amyloidosis or in blocks of cases of familial neuropathic amyloidosis, multiple myeloma, and idiopathic amyloidosis with cardiopathy. AA-protein is not denatured by formalin and retains its typical electrophoretic, chromatographic, and immunologic characteristics even 30 years after fixation and paraffin-embedding.