Amyloid protein AA consists of several subspecies which mainly arise through proteolytic cleavage at various sites of the precursor, serum protein AA. The most common protein AA subspecies (the protein AA prototype) contains 76 amino-acid residues. In previous studies we have shown that distinctive patterns of amyloid infiltration occur in AA-amyloidosis. The amyloid in different patterns of infiltration seems to consist of distinctive protein AA subspecies. In the present study we have analysed protein AA in three patients with a form of AA-amyloidosis with heavy vascular infiltration and show that the amyloid fibrils contain two groups of protein AA subspecies. One, quantitatively predominating, group contains large subspecies of up to 94 amino-acid residues and a second group of protein AA-molecules contains around 50 amino-acid residues. The AA molecules lack the N-terminal arginine residue. It is concluded that AA-amyloidosis with massive vascular infiltration is a distinctive subform with typical clinical and histological appearance and with fibrils containing characteristic protein AA subspecies.