The mechanism of amyloid formation in familial amyloidotic polyneuropathy (FAP), a hereditary disorder associated with mutant transthyretin (TTR), is still unknown. It is generally believed that altered conformations exposing cryptic regions are intermediary steps in this mechanism. A TTR mutant--Y78F (transthyretin mutant with phenylalanine replacing tyrosine at position 78)--designed to destabilize the native structure has been shown to expose a cryptic epitope recognized by a monoclonal antibody that reacts only with highly amyloidogenic mutants presenting the amyloid fold or with amyloid fibrils. To test whether TTR deposition in FAP can be counteracted by antibodies for cryptic epitopes, we immunized with TTR Y78F, transgenic mice carrying the most common FAP-associated TTR mutant--V30M (transthyretin mutant with methionine replacing valine at position 30)--at selected ages that present normally with either nonfibrillar or TTR amyloid deposition. Compared to age-matched control nonimmunized mice, Y78F-immunized mice had a significant reduction in TTR deposition usually found in this strain, in particular in stomach and intestine; by contrast, animals immunized with V30M did not show differences in deposition in comparison with nonimmunized mice. Immunohistochemical analyses of tissues revealed that immunization with Y78F lead to infiltration by lymphocytes and macrophages at common deposition sites, but not in tissues such as liver, choroid plexus, and Langerhans islets, in which TTR is produced. These results suggest that Y78F induced production of an antibody that reacts specifically with deposits and leads to an immune response effective in removing/preventing TTR deposition. Therefore, TTR immunization with selected TTR mutants has potential application in immune therapy for FAP.