Twenty-seven Bence Jones proteins of immunological type K show a common set of peptides. One 6f the commnon peptides differs in three of the proteins which are the only ones classified by a serological test as Iav(a+). The difference in the peptide analyzed is caused by a valine-leucine interchange; Inv(a+) proteins have leucine, whereas Inv(a-) proteins have valine in position 189.