Protein A is a highly stable surface receptor produced by Staphylococcus aureus, which is capable of binding the Fc portion of immunoglobulins, especially IgGs, from a large number of species (Boyle and Reis, 1987). Each protein A molecule can bind 2 molecules of IgG, allowing the formation of a precipitate (Sjoholm, 1975). Protein A may be coupled to a wide variety of reporter molecules including fluorescent dyes, enzyme markers, biotin, colloidal gold and radioactive iodine without affecting the antibody binding site. These conjugates may be used to track immunoglobulins in histochemical, Western Blot and ELISA. Alternatively, Protein A may be immobilized onto a solid support to facilitate the purification and recovery of either polyclonal or monoclonal immunoglobulins. Immobilized protein A has recently been applied therapeutically to treat a variety of humoral diseases (Watson et al, 1989).