Abstract
Two antigenic types of naturally occurring proteins related to the papain produced F(c) fragment of gammaG-globulin have been studied. Most normal and myeloma gammaG-globulins are related to one or the other of these proteins, based on antigenic and structural characteristics of their heavy chain. Molecules bearing the determinant "Cr" are approximately 10 times as abundant as those bearing the determinant termed "Zu" in pools of normal gammaG-globulin and among a large group of gammaG-myeloma globulins. Both populations have properties of typical gammaG-globulin antibodies. Splenic cells, stained for these two populations with specific fluorescent antibody preparations, are found to contain one or the other of the two populations of gammaG-globulin, but not both. Approximately 10 times as many cells contain the Cr determinant as contain the Zu determinant.