The reactivity of the antiserum is directed to the Fc and Fab subunits of the IgG molecule. It includes a certain degree of reactivity with other immunoglobulins via the common Fab portion. It does not react with any non-Ig protein in cat serum, as tested by immunoelectrophoresis and double radial immunodiffusion. In precipitating techniques as immunoelectrophoresis and double radial immunodiffusion to identify the presence of IgG; as secondary antibody to precipitate the immunoglobulin in normal cat serum; to prepare an immunoadsorbent for the production of Ig-free cat serum, plasma or non-Ig preparations.