The reactivity of the antiserum is restricted to the Fc part of the IgM molecule. In immunoelectrophoresis and radial immunodiffusion, using various antiserum concentrations against normal swine serum a single precipitin line is obtained which shows a reaction of identity with the precipitin line obtained with purified IgM. No precipitation reaction is obtained with purified IgG, IgA, and IgG/Fab fragments. In precipitating techniques as immunoelectrophoresis and radial immunodiffusion to identify the presence of IgM in swine serum and other body fluids or to determine its concentration. To prepare an immunoadsorbent for the purification of swine IgM from serum or plasma.