Prealbumin, immunoglobulin G, A, and M concentrations in serum and cerebrospinal fluid from patients with familial amyloid polyneuropathy (Japanese type) were determined and compared with those of asymptomatic relatives or controls. Prealbumin concentrations were not significantly different between them. Using polyacrylamide disc-gel electrophoresis, no qualitative difference in the serum prealbumin was found between them. Among the serum immunoglobulins, only the immunoglobulin A concentration was significantly elevated in the patients as compared with asymptomatic relatives.