The reactivity of the antiserum is restricted to hemopexin as tested in immunoelectrophoresis and radial immunodiffusion. A characteristic single precipitin line is obtained with normal serum which shows a reaction of identity with the precipitin line with purified hemopexin. In precipitating techniques as immunoelectrophoresis and single and double radial immunodiffusion to identify the presence hemopexin in human serum or other body fluids and to determine its concentration. This antiserum has not been tested in more sensitive assays as nephelometry, ELISA or use in immunohistology, but this does not exclude such use if proper controls are included.