Amyloid fibril protein has been isolated from the tissues of a patient of Swedish ancestry with autosomal dominant heredofamilial amyloidosis. After solubilization in guanidine HCl, a significant amount of the protein was contained in a homogeneous low molecular weight fraction. Molecular weight of approximately 14,000, amino acid analysis, double immunodiffusion analysis and immunoelectrophoresis all supported this material being a prealbumin-related protein. Automated sequence analysis gave a mixture of amino acids at each step, suggesting an heterogeneous NH2-terminus. After cleavage of the protein with cyanogen bromide, a homogeneous peptide was obtained with the sequence Val-Val-Val-Leu-Asp-Ala-Val-Arg-Gly-Thr-Pro- corresponding in 9 of the 11 positions analyzed with the known sequence of human prealbumin, starting with position 14. Antiserum raised to the amyloid protein reacted with normal human prealbumin. After absorption with normal human serum, this antiserum continued to detect a determinant in the amyloid patient's serum, suggesting an abnormal serum prealbumin, which may be the precursor of the fibril protein in this type of heredo-familial amyloidosis. Indirect immunohistochemical studies on kidney tissue from the patient with amyloidosis showed marked staining with anti-prealbumin and anti-heredofamilial amyloid protein, but not with anti-AA or anti-kappa antisera. No genetic association between this family and amyloidosis and Portuguese families with familial amyloid polyneuropathy is known.