Patients with systemic amyloidosis frequently show gastrointestinal (GI) tract and adrenal gland involvement with amyloid. However, the evolution of amyloid-related pathological changes in humans is unknown. Conversely, although GI amyloidosis has been described in casein-stimulated mice, the chemical nature of the GI amyloid have not been carried out. Here we report studies on these and related aspects using alveolar hydatid cyst (AHC)-infected mouse model of reactive amyloidosis. Paraffin sections from the adrenal gland, stomach, small and large intestine were obtained from AHC-mice at different time period post-infection (p.i.) and stained with Congo red or immunohistochemically with antibodies against mouse AA amyloid (RAA) or bovine ubiquitin (RABU). The GI-amyloid was purified by column chromatography and analyzed. In the adrenal gland, amyloid deposits were first detected at 4 weeks p.i. in the cortical-medullary junction which then extended into the adrenal parenchyma. In the GI tract, submucosal blood vessels, the first site of amyloid deposition, became amyloidotic at 1 week p.i. With time, the amyloid deposits extended into the lamina propria of the mucosa. Ileum was the most severely affected region in the GI tract. Both RAA and RABU reacted specifically with the GI amyloid. Amyloid was purified by exclusion chromatography in 4M guanidine. The purified amyloid on Western blotting reacted with RAA and on N-terminal amino acid sequence analysis revealed homology with murine serum amyloid A$ sb2$ (SAA$ sb2$) indicating its derivation from SAA$ sb2.$ Both the similarities and the differences between the GI amyloid-related pathological changes in humans and mice are discussed.