Normal 7S human γ-globulin was found to contain two fundamental antigenic groups of molecules. The group 1 molecules of normal γglobulin correspond antigenically to group 1 multiple myeloma proteins and Bence Jones proteins; and group 2 molecules of normal γ-globulin correspond antigenically to group 2 multiple myeloma proteins and Bence-Jones proteins. Among pooled human Fr II and several individual γ-globulin preparations, approximately 60 per cent of molecules belong to group 1 and approximately 30 per cent of molecules to group 2 in this classification. The possible existence of a third minor antigenic group, constituting about 10 per cent, is discussed. Antisera to Bence Jones proteins of antigenic group 1 and group 2, in conjunction with I-131-labeled 7S γ-globulin proved to be the most useful system for defining the antigenic groups of normal γ-globulin. The group-specific antigenic determinants of normal 7S γ-globulin molecules were located on the S fragments of these proteins.

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