Amyloid fibrils were isolated from the kidney of a patient with Finnish hereditary amyloidosis. After solubilization of the fibrils in guanidine-HCl, fractionation by gel filtration, and purification by reverse-phase high-performance liquid chromatography, a homogeneous amyloid protein with an apparent Mr of 9000 was obtained. The protein was subjected to enzymatic digestion by trypsin and endoproteinase Lys-C. The amino acid sequences were determined for 6 of the released peptides and they were all found to be identical to the reported, deduced primary structure of human plasma gelsoline in the region of amino acids 235-269. The results show that the amyloid fibril protein in Finnish hereditary amyloidosis represents a new type of amyloid protein that shows amino acid sequence homology with gelsoline, an actin-modulating protein.