Amyloid subunit protein was isolated from familial amyloid polyneuropathy type IV (Finnish type) cardiac tissue and purified to homogeneity. N-terminal amino acid sequence analysis shows that the amyloid protein is a fragment of the inner region of human gelsolin. When compared with the predicted sequence of human plasma gelsolin, the amyloid protein contains an asparagine-for-aspartic acid substitution at position 15 corresponding to residue 187 of the secreted protein. Antibodies raised against the amyloidogenic region of gelsolin specifically stained the amyloid deposited in tissues in familial amyloidosis type IV. The results show that the subunit amyloid protein in familial amyloid polyneuropathy type IV represents a unique type of amyloid derived from a variant (Asn-187) gelsolin molecule by limited proteolysis.