Amino-acid sequence analysis of an amyloid fibril protein from a patient with Swedish familial amyloidotic polyneuropathy showed homology with prealbumin but with heterogeneous N-terminal deletions. One-third of the molecules had the same amino acid substitution, methionine for valine in position 30, as in familial amyloidosis of Portuguese, Japanese and Swedish-American type. A protein with the same antigenic properties and size was found in the fibrils of two other patients with Swedish FAP while the amyloid fibrils in two further patients predominantly contained a smaller prealbumin-derived protein. Cyanogen bromide cleavage of this protein revealed no evidence for a methionine residue in position 30.