Amyloidosis was produced experimentally in guinea pigs by multiple casein injections. Amyloid fibrils were isolated and fractionated and a protein obtained that had an amino acid composition comparable with A protein, a unique nonimmunoglobulin constituent of secondary amyloid deposits. N-terminal sequence analysis demonstrated a sequence homologous with that of A proteins from human and monkey preparations but preceded by a 5-residue peptide which had an N-terminal histidine. A definite species specificity in A protein from human and guinea pig was identified on immunologic analysis.