Polyclonal antisera from immunized hosts are lipid extracted to improve clarity, salt fractionated, dialyzed against phosphate buffered saline containing sodium azide and freeze-dried. Antisera against whole serums are obtained by immunizing host animals with whole serum. Antisera against whole IgG molecules [i.e. Anti-IgG (H+L)] are recommended for bridging PAP to primary antibodies.-Based on immunoelectrophoresis, the antibody reacts with the heavy chain of mouse IgM but not with mouse IgG or the light chains of other mouse immunoglobulins. No antibody was detected against non-immunoglobulin serum proteins. The antibody may cross-react with IgM from other species.