The defined antibody specificity is directed to transthyretin as tested against human sera. In immunoelectrophoresis and double radial immunodiffusion (Ouchterlony), using various antiserum concentrations against appropriate concentrations of the immunogen, a single characteristic precipitin line is obtained which shows a reaction of identity with the precipitin lines obtained against human serum and the purified transthyretin. As precipitating antiserum to identify or measure human transthyretin by a variety of immunodiffusion techniques, including immunoelectrophoresis, single and double radial immunodiffusion (Mancini, Ouchterlony) and electroimmunodiffusion (Laurell). It has not been tested for use in nephelometry, ELISA or immunochemistry, but this does not exclude such use if proper controls are included.