Gelsolin-related amyloidosis, also called familial amyloidosis, Finnish type (FAF) is an autosomal dominantly inherited disorder characterized by progressive polyneuropathy and corneal lattice dystrophy. All the analyzed patients are found to carry a nucleotide substitution of A or T for G654 in their gelsolin gene, which at the protein level results in the conversion of the 187 amino acid residue, aspartic acid, to asparagine or tyrosine, respectively. In this study, we transfected mammalian mesenchymal COS-1 cells with a derivative of the expression vector pCD-X containing cDNA coding for the wild-type (D187) and mutant forms (N187 and Y187) of plasma gelsolin. Both disease-associated mutant forms of gelsolin were found to be abnormally processed, which led to the secretion of an aberrant 68 kDa gelsolin fragment into the culture media. This fragment most probably represents a carboxy-terminal part of the protein and contains the suggested amyloid-forming sequence. Initial data were also obtained for involvement of a metalloendoprotease in the pathologic processing. This aberrant proteolysis is likely to represent a crucial initiator step in the cascade resulting in amyloid accumulation in patients' tissues.