The finding of individuals homozygous for FAP I (familial amyloidotic polyneuropathy, transthyretin TTRMet30) with amyloid deposits in the vitreous body, gave us access to a unique material lacking wild type transthyretin and contaminating proteins. Amyloid TTR is modified in several ways. Besides the full-length protein and its dimer form, two smaller bands were identified by SDS-PAGE and protein sequencing. One corresponded to a peptide starting at amino acid Thr49, the other was a mixture of two peptides starting at positions 1 and 3 in a 3:1 ratio. Upon reduction the amount of the TTR dimer decreased, the monomer amount increased, and the resulting monomers became available for carboxymethylation. Moreover, the mobility of the small band, which includes Cys10, increased upon reduction. This cysteine seemed to be involved in an interchain disulfide bridge both between intact TTR molecules and between small fragments. The same pattern was found in heterozygous fibril material although smaller amounts of the truncated peptides were found. Fibrils were formed both from normal and mutated TTR in heterozygotes. The significance of our results for amyloid formation is discussed.