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 rat serum, as tested by immunoelectrophoresis and double radial immunodiffusion. In immunoelectrophoresis and double radial immunodiffusion to identify the presence of IgG; as secondary antibody to precipitate the immunoglobulin in normal rat serum; to prepare an immunoadsorbent for the production of Ig-free rat serum, plasma or non-Ig preparations. This antiserum is not intended for use in non-precipitating antibody-binding or other highly sensitive assays. For such uses labelled and unlabelled cytochemical and immunoassay reagents meeting the necessary additional specificity and performance requirements are available.