Hereditary systemic amyloidosis caused by fibrinogen Aalpha-chain gene mutations is an autosomal dominant condition with variable penetrance, usually of late onset, and typically presents with nephropathy leading to renal failure. Amyloid deposits often develop rapidly in transplanted kidneys, and concomitant orthotopic liver transplantation has lately been performed in several patients with the hope of halting amyloid deposition. Fibrinogen is produced in vitro by hepatocytes but also by other human cell types, and although the liver is the source of plasma fibrinogen in vivo in rats, this is not known in humans. Transplantation of livers expressing wild-type fibrinogen into patients with variant fibrinogen amyloidosis provides a unique opportunity to establish the source of human plasma fibrinogen. We therefore characterized plasma fibrinogen Aalpha-chain allotypes by electrospray ionization mass spectrometry mapping of tryptic digests before and after liver transplantation. Before liver transplantation, fibrinogen amyloidosis patients with the Glu526Val Aalpha-chain variant had approximately equal proportions of peptide with the wild-type sequence TFPGFFSPMLGEFVSETESR, and with the amyloidogenic variant sequence TFPGFFSPMLGEFVSVTESR, as expected for individuals heterozygous for the mutation. After transplantation, only the wild-type sequence was detected, and the liver is thus the source of at least 98% of the circulation fibrinogen.