1. Amyloid isolated from the liver of a domestic short-haired (DSH) cat was dissolved and purified by gel filtration for amino acid sequence analysis. 2. Sequences of two major peptides corresponding to positions 18-23 and 25-75 of human amyloid protein AA were obtained when cyanogen bromide-cleaved protein was applied to an amino acid sequenator. 3. Comparison of these regions of amyloid protein from the Abyssinian cat (high incidence of AA amyloidosis) and DSH cat (low incidence of AA amyloidosis) revealed three amino acid differences, two of which occurred within regions that are completely conserved in the Abyssinian cat and all other species. 4. Secondary prediction plots showed less potential for amyloidogenicity (i.e., less beta-sheet conformation) in protein AA of the DSH cat as compared to the Abyssinian cat and other animal species. 5. These differences in protein AA of the DSH cat may, therefore, be linked to the comparatively uncommon occurrence of AA amyloidosis in the DSH cat as compared to the Abyssinian cat and other animals species.