Immunohistochemical studies of six patients with familial amyloidosis, Finnish type, showed that their amyloid deposits did not react with polyclonal antibodies against the amyloid proteins of other, established forms of systemic or cerebral amyloidosis. However, strong immunoreactivity was observed with rabbit antiserum raised against a low molecular weight purified amyloid subunit isolated from one of the patients. This immunoreactivity was abolished by absorption with the low molecular weight amyloid fraction. The amino terminal sequence of the amyloid protein subunit was homologous to gelsolin, an actin-modulating protein, and the amyloid deposits in tissues reacted with a monoclonal antibody against gelsolin. These studies show that the amyloid protein in familial amyloidosis, Finnish type, is not related to previously identified forms of amyloid, including prealbumin (transthyretin) variants, but represents a novel amyloidogenic protein related to gelsolin, a plasma and cytoplasmic protein.