The defined antibody specificity is directed to albumin as tested against rat 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 rat serum and the purified albumin. As reagent for the direct detection of albumin in rat cells, tissues and body fluids in immunofluorescence techniques. This immunoconjugate is not pre-diluted. The optimum working dilution of each conjugate should be established by titration before being used. Excess labelled antibody must be avoided because it may cause high unspecific background staining and interfere with the specific signal.