Complex formation between IgA and several serum proteins including albumin (1), βlipoproteins (2), haptoglobins (3), anti-hemophilic globulin (4) and in one case, to α1 glycoprotein (3) has been previously noted. However, with the exception of albumin, the binding of these proteins to IgA myeloma proteins has not been regularly observed. It has been reported (3) that 53% of IgA myeloma proteins as well as 74% of monoclonal IgM proteins complex with albumin. Subsequently, a thorough study by Mannik (5) demonstrated that the binding of albumin by both IgA and IgM proteins involved covalent disulfide bonds. Presumably, the binding site is located in the heavy chain and could involve the free SH group thought to be present in the α-chain (6), although this has not been directly demonstrated. During the course of a study attempting to localize the site of albumin binding to IgA proteins, an antiserum to albumin was employed which also contained antibodies to α1 antitrypsin (α1AT).2