Antibodies specific for aggregated human and aggregated rabbit γG-globulins have been raised in guinea pigs. The antigenic determinants revealed by the aggregation process are species-specific, but similar, if not identical, determinants are revealed regardless of the means of nonspecific denaturation. These new determinants are located exclusively within the Fc portion of the γG-globulin molecule, but are found, following aggregation, in all of a series of γG myeloma proteins tested, including examples from each of the known sub-groups, γG1, γG2, γG3 and γG4. The determinants were not found however following aggregation of γA- or γM-immunoglobulins. Similar antigenic determinants were found to be revealed on the γG-globulin antibody molecule following combination with homologous antigen, suggesting similar structural changes in the Fc portion of the γG-globulin molecule on denaturation by specific and nonspecific means. These findings suggest a preferred structural form for aggregated (denatured) γG-globulin with its own characteristic antigenic determinants and biologic properties.