A highly sensitive and reproducible enzyme-linked immunosorbent assay for human transthyretin was developed. The method is most suitable to serial monitoring of transthyretin peaks in reverse-phase high-performance liquid chromatography for the diagnosis of familial amyloidotic polyneuropathy based on isolation of variant transthyretin. This method semimicronized the original diagnostic method, requiring plasma samples of only 0.2 ml. The present enzyme-linked immunosorbent assay method is also applicable to determine transthyretin in plasma and cerebrospinal fluid. It correlated well when compared with a commercially available single radial immunodiffusion (r = 0.951, p less than 0.001).