MUCH information about the structure of the human immunoglobulins has derived from investigations of the antigenic characteristics of Bence Jones proteins. The two major antigenic groups of Bence Jones proteins have immunological counterparts in the light (L) chains of the normal population of γ2-, γ1A- and γ1M-globulins1. Using group specific rabbit antisera, we have shown that Bence Jones proteins of the same immunological group from different patients contain antigenic determinants not overtly present in pooled human 7S γ-globulin from normal individuals2. It is probable that some degree of the antigenic uniqueness of Bence Jones proteins is related to the individual antigenic specificity of single immunizing proteins2. Another variable determining at least a part of the immunological difference between Bence Jones protein and normal unreduced γ-globulin is related to the unmasking of hidden antigenic determinants of L chains when they are either not incorporated into or cleaved from the parent γ-globulin molecule3. It has been suggested that certain potential antigenic sites of L-polypeptide chains are inaccessible in the intact γ-globulin molecule due to steric hindrance of adjacent H chain4.