Affinity chromatography utilizing protein A apparently covalently bound to sepharose beads was applied to isoalte pure immunoglobulin G from canine serum. Immunoglobulin M was eliminated from the sample by a single passage of serum on DEAE-cellulose prior to chromatography on protein A-sepharose. The technique is simple, inexpensive and does not require complicated equipments. The preparation proved to be highly purified when tested by immunoelectrophoresis against anti-dog whole serum. The fraction included two distinct subclasses of IgG of different electrophoretic mobility. Monospecific antiserum to purified IgG was prepared for the use in determining IgG levels in dog sera.