The purpose of this study was to develop a quantitative method for variant transthyretin (also called prealbumin) in amyloid-laden tissues of familial amyloidotic polyneuropathy (FAP). The variant transthyretin was extracted by using 70% formic acid solution in which the protein was solubilized completely and rapidly. The quantity of the variant transthyretin was measured by a radioimmunoassay (RIA). An immunoreactive species detected in the tissues of FAP was confirmed to be the variant transthyretin by reverse-phase high-performance liquid chromatography. The quantity of the variant transthyretin in the thyroid gland of 2 FAP patients was 9.90 and 9.76 micrograms/mg tissue, and that of the kidney was 4.03 and 6.27 micrograms/mg tissue, respectively, while its quantity in the liver was only 0.15 and 0.12 microgram/mg tissue. We present here a quantitative analysis of the distribution and predominance of amyloid deposits in systemic organs of FAP.