The aminoacid sequence of the "hinge" region of two human IgA(1) myeloma proteins was found to be identical. A 30-residue fragment contains 3 cysteines, 12 prolines, and carbohydrate, and the sequence of a small segment of the hinge region is repeated at least twice. Since a deletion of 12-13 residues was found in the same region of an IgA(2) myeloma protein, it is possible that IgA(1) molecules are the result of the insertion of a partially duplicated gene segment in the immunoglobulin genes.